The acceleration of a car is determined by measuring the time it takes for the car to accelerate from 0 to 100 . The more powerful the engine is, the more time the car needs to accelerate.
If we want to shorten the time of doing the work, we must do the work with more power.
Two machines worked for one hour. If the first machine did more work during this time than the second, then, the first machine worked with more power.
A machine that works at a higher power always does more work than one that works at a lower power.
If two locomotives pull wagons with the same amount of force, the locomotive with higher power will have a higher speed.
An elevator with more power can lift a heavier load.

Example 2.

Solve the following problems without calculation, only by reasoning:

If we have two identical cars with different engine powers. The first has a maximum power of 100 kW and the second has a maximum power of 180 kW. Which of them can accelerate faster from 0 to 100 ?
If we have two cranes at a construction site. The first lifts a load of 2 tons while the second lifts a load of 3 tons. Which of the cranes will lift the load to the top of the building faster if they work with the same power?
Two machines can do the same job. The first machine does the job in 3 hours and the second machine does the same job in 4 hours. Which machine works with more power?

Which of the following statements about work is true?

The work done by a body is greater when more time is expended.
The force acting in the opposite direction to the movement of a body does not do any work.
When an electric drill is used to make a hole in a wall, work is done by the electric drill.
A groom lifts his bride from the ground to a height and then walks a distance ahead. He only does work when he is lifting the bride.
A force is used to stop the downward movement of a trolley. Work is done on the trolley.
We applied a push to a cabinet, but it does not move. Our work is zero.
A car that moves faster does more work than a car that moves slower on the same route, considering air resistance.
Satellite orbits the Earth above the equator at an altitude of 36,000 km. During orbit, the attractive force (centripetal force) between the Earth and the satellite does the work.
Current water from a dam falls on a power plant's turbines. In doing so, it does the work.

The kinetic energy of a body with a mass of is . What is its velocity?

Calculate the power of a device when it does 900 J of work in 20 seconds.

An electric drill performs 270 kJ of work in 15 minutes while drilling. Calculate the power with which the drill works.

How much work does a motor do in 1 hour if it runs at 7.5 kW?

How long does a 420 W machine have to work to do 630 kJ of work?

A formula 1 driver wants to accelerate his car so that its kinetic energy will increase by 1.5 MJ. At least how long does he have to accelerate to do this if the engine is running at 750 kW?

10.

It takes a crane 18 seconds to lift a load to a certain height. Calculate the change in potential energy of the load if the crane lifted the load with a power of 25 kW.

11.

A car drives uphill and accelerates while driving. In doing so, its potential energy increases by 660 kJ in 20 seconds, and its kinetic energy by 100 kJ. At what average power did the engine run for those 20 seconds?

12.

A student pushes a cart with a force of 28 N for 20 seconds. The cart moves uniformly and travels a distance of 15 m. Calculate the power used by the student.

13.

When we chop cooked vegetables with an electric blender, the blender is set to the first level and does 5 kJ of work in 20 seconds. When chopping hazelnuts, the electric blender is set to the maximum speed at which the power is three times as much as that in the first level.

With how much power does the blender chop the cooked vegetables?
How long will it take to chop the hazelnuts if the blender has to do 90 kJ of work?

14.

Dad pulls a sled up a 45 m long slope. If he pulls it with a power of 30 W, he will pull the sled to the top in half a minute.

How much work does dad do?
With how much force does dad pull the sled to the top?

15.

A trolley on a smooth horizontal surface is pushed with a force of 20 N. Calculate the work done if the trolley covers a distance of 100 m.

16.

A swimmer swam for 4.5 minutes with a power of 60 W. How far did he swim if he pushed the water with an average force of 540 N?

17.

Tom slides down a slope into a horizontal plane. If the plane has a frictional force of 48 N and Tom covers a distance of 20 m on the plane before stopping, how much work is done to stop Tom on the plane, and what does the work?

18.

How long does it take a car to cover a distance of 6 km if it moves uniformly in a straight line with a power of 50 kW? 2.5 kN of frictional force is applied to the car while driving.

19.

A log is acted upon by a force of 40 N at an angle of , as shown in the figure. If the log moves a distance of 4 m due to the force. How much work is done?

20.

An elevator can lift a load of 1.4 t through a height of 12 m.

By how much, in kJ, does the potential energy of the load increase during lifting?
How long does the elevator lift the load when operating at an average power of 4.2 kW?

21.

An elevator, having a mass of 800 kg, is uniformly lifted to a height of 18 m. How much work is done by the elevator motor?

22.

An electric car of mass 256 g accelerates from rest to a speed of 9 in 4 s.

By how much does the car's kinetic energy increase during acceleration?
What is the average power of the car's engine?

23.

A boy weighing 40 kg climbs a pole. How high does he climb in 28 seconds if he climbs with an average power of 50 W?

24.

Determine the correctness of the statement:

An electric machine that does the same job in less time has more power.
The unit of electrical power is joules per second [J/s].
An electric drill that has more power will do more work.
Two bulbs are connected to the same voltage. The current through the first bulb is twice as much as the current through the second bulb, so the first bulb has twice the power than the second.
The power of the electric furnace is greater with a longer heating wire.

25.

The car travelled in a straight line for 6 minutes. At what speed was the car travelling if the braking force was 2.1 kN at all times and the engine power was 42 kW?

26.

A car with a mass of 1400 kg accelerates at . How much work is done by the car engine after moving a distance of 100 m?

27.

The locomotive of a train pulls it with a force of 8 kN. If the train is moving evenly at a speed of . How much work does the locomotive do in one hour?

28.

When a force F is exerted on a log and moved a distance s, a work W is done. How much work will be done when half of the force is exerted and the log is moved twice the distance?

29.

A cyclist rides his bike down a slope. At the highest point of the slope, he has potential energy equal to 3.6 kJ. As he wants to further increase its speed, he drives by pedaling while driving. How much work does he have to do in driving to have kinetic energy that is 5.7 kJ higher at the lowest point than he had at the top of the slope?

30.

A parachutist descends evenly towards the ground from a height of 800 m. The mass of the parachute is 65 kg. How much work does the force of air resistance do on the parachute?

31.

A paper plate is lifted off the ground so that its potential energy relative to the ground is 180 mJ. When the plate is dropped, an air resistance force acts on it during the fall, which inhibits its movement. During the fall, the resistance force performs 140 mJ of work. Calculate the kinetic energy of the plate just before it falls to the ground.

32.

Three identical crates, each weighing 60 kg and 40 cm high, lie on the ground. We need to stack them on top of each other. How much work will we do?

33.

A car drives up a slope. At the bottom of the slope, its kinetic energy is equal to 245 kJ, and at the top of the slope, its kinetic energy is equal to 192 kJ. While driving uphill, its potential energy increases by 136 kJ.

What is the change in the car's kinetic energy?
Does the car receive or transmit work while driving?
Calculate the work that the car exchanges with its surroundings while driving uphill.

34.

2000 litres of water fall from a height of 16 m. How much work is done by the water?

The specific gravity of the water is .

35.

A child pushes a car up the slide. Initially, the kinetic energy of the car is 3.8 J. As the child moves up the slide, the car's potential energy increases by 2.5 J. How much kinetic energy does the car have at the top of the slide? The frictional forces in this task can be neglected.

36.

An electric stove is connected to a voltage of 230 V. A current of 6 A flows through the stove.

How much power does the electric stove take from the voltage source?
What does this power turn into?

37.

An apple weighing 350 g falls from a tree at a height of 2.8 m. By how much joules does the apple's kinetic energy increase during the fall? Air resistance can be ignored.

38.

A trolley weighs 200 g. It is pushed along the horizontal ground to a slope. Just before it starts to climb, it has a speed of 18 km / h. As it is driven up the slope,the speed decreases. Frictional forces performing 1.2 J of work act on the trolley during the ascent.

Draw a sketch and describe the energy changes of the trolley during the ascent.
By how much does the kinetic energy of the trolley change during the ascent (from the moment it starts to climb to the moment it reaches its highest position)?
Calculate the change in potential energy of the trolley during the ascent.
How high does the trolley climb?

39.

A car weighing 1400 kg drives uniformly uphill. In doing so, it travels a distance of 800 m uphill and a vertical distance of 200 m. How much work does the car engine do?

40.

A skier weighing 80 kg descends a gentle slope. He drives during the descent so that at the moment he descends 8.3 m lower, he has a speed of 14 m / s. How much work does the skier do while driving?

41.

A child stands on the balcony and drops a stone to fall freely. His friend stands under the balcony and catches the stone. In doing so, the stone acts on his arm with force and performs 8.8 J of work. The mass of the stone is 160 g, and the friend catches it at a height of 120 cm from the ground. Air resistance can be ignored. At what height does the child drop the stone?

42.

A small metal plate weighing 84 g is placed on an icy slope. The plate is stationary at first. By how much does the potential energy of the plate have to be reduced in order to have a speed of 6.2 m / s in the end.

43.

A car with a mass of 1.4 t descends a slope. At the top of the slope, the car's speed is 18 km / h. It drops by 12 m while driving downhill and has a speed of 54 km / h at the end. Calculate the work that the car exchanges with its surroundings while driving downhill. We ignore air resistance.

44.

A stone initially at rest is pushed on the icy ground with a force of 100 N. After travelling a distance of 4 m, the stone is allowed to move freely until it stops. At what distance, from the moment it was released, does the stone stop if it is braked by a frictional force of 10 N from the beginning of the motion?

45.

A child weighs 50 kg together with a sled. He drives the sled towards a slope 1.5 m high. What is the minimum kinetic energy the child must have at the base of the slope to reach the top of the slope without propulsion?

46.

A flowing water acts with a force of 100 N perpendicular to the blades of the mill wheel with a radius of 2 m. How much work does the water do when the wheel turns 10 times?

47.

A trolley weighing 2.5 kg is pushed up a slope at an initial speed of 6 m/s. A frictional force of 3 N acts on the trolley during motion. What is the speed of the trolley at the moment when it rises by 80 cm?

A trolley moving up a slope

48.

A skier weighing 84 kg descends a steep slope. He does not stop or brake during the descent. How many meters must he descend to have a velocity of 72 km / h in the end if he does not have kinetic energy at the beginning?

49.

Three forces act on the wooden log, as shown in the figure below. The applied force is 14 N, the frictional force is 10 N and the vertical force is 16 N. Calculate the work of each force and the total work done by the forces in moving the log through a distance of 6 m.

50.

An archer fires an arrow weighing 22 g vertically upwards. As it moves upwards, the arrow's potential energy increases by 4.4 J. How fast is the arrow as it flies away from the bow? Air resistance can be ignored.

51.

Dave pushes a trolley with a force of 12 N at a distance of 20 m.

How much work did he do?
How far does the trolley have to move with the same force in order to do 1 kJ of work?

52.

Express the unit of work (J) in base units of measurement (m, kg, s).

53.

A 6 kg body is acted upon by a force of 3 N for 10 s. How much work is done if the initial velocity of the body is zero?

54.

Two bodies have the same kinetic energy. The first body has a mass of and a velocity of . The second body weighs . What is the velocity of the second body?

55.

A lady is cooking lunch. She uses a 3 kW stove for two hours and a 800 W stove for four hours. What will be the total cost of the electrical energy needed to prepare lunch if 1 kWh costs 6 cents?

56.

An electric saw was used on a board. The teeth of the saw acts on the board with a force of 1600 N. The board moves under the saw at a speed of 0.1 m / s. What is the power of the saw motor?

57.

A child weighing 40 kg slides down the slide. At the top of the slide, the child has a height of 1.9 m, and at the bottom of the slide, his height is 30 cm. During the descent on the slide, the frictional force does 180 J of work. Calculate the change in the child's kinetic energy during the descent down the slide.

58.

A bulb has a power of 80 W at a voltage of 230 V. What is its resistance?

59.

What is the maximum number of identical bulbs with a voltage of 10 V that we need to connect in series to a voltage source of 220 V?
What will be the total power of the bulbs if the power of each bulb is 4 W?

60.

A household has a dishwasher with a power of 3 kW and an electric stove with a power of 2 kW connected to the same fuse. The mains voltage is 230 V. We have 10, 15, 20 and 25 A fuses available. Which should we use to protect the circuit if both devices can be connected at the same time?

61.

An elevator transports passengers to a height of 40 m in 20 s. If the total mass of the passengers on the lift is 800 kg, what is the power of the elevator's electric motor?

62.

There is a liter of water in a container. When an immersion electric heater is put in the water, the water heats up by 40 K in 10 minutes. What is the electric power of the heater given that the specific heat of the water is ?

63.

A car battery has a voltage of 12 V and a charge of 60 Ah. How long will it take to discharge if we leave a 6 W light bulb on?

64.

Which of the following objects possess potential energy?

A bag lying on the floor.
An apple hanging on a branch.
Pencil in a student's raised hand.
An airplane flying on the runway.
A bee flying through the air.

65.

How did the objects listed below get their potential energy?

A child on a tree.
Skier at the top of the ski slope.
Box on top of a cupboard.
A ball high in the air.
A mountaineer at the top of a mountain.

66.

A building has a ground floor, 5 floors, and 2 basement floors. Each floor is 2.7 meters high. For zero height, choose the floor on the ground floor.

Calculate the floor height of the top floor.
What is the height (or depth) of the floor in the lower basement?

67.

A basketball court has a flat ceiling. An observer, standing on the parquet floor under the basket, has a ceiling 9.5 meters above his head. How many meters above the head of the same observer will the ceiling be when he climbs to the podium which is 2.3 meters higher than the parquet?

68.

On one side of an overhanging swing, 3 meters from the axle, sits a girl Nina weighing 21 kg. On the other side, her sister Jane with a mass of 30 kg occupies the same position. Due to her weight, Jane drops 70 cm to the ground with a swing in a slow squat, and her sister lifts the swing into the air. By how much does each of them increase or decrease in potential energy?

69.

The top of a Hotel is located at an altitude of 1853 metres. A skier descends a height of 253 metres from the top to the base of the Hotel. At what height is the base of the hotel?

70.

A child weighing 45 kg climbed to the top of a slide 1.8 m high. Calculate the potential energy of the child at the top of the slide.

71.

A pot has a mass of 80 dag and is placed on a window sill that is 1.1 m high. Mom moves the pot to a cupboard that is 1.7 m high. Calculate the change in potential energy of the pot.

72.

How high do we have to lift an iron ball weighing 4.8 kg to increase its potential energy by 36 J?

73.

A crane lifts a load from the ground to a terrace, which is at a height of 7.5 m. Calculate the mass of the load if the potential energy of the load on the terrace is 28.5 kJ.

74.

A mountaineer descends from the top of a mountain to a hut. Together with the backpack, the mountaineer weighs 95 kg. Calculate the change in potential energy of the mountaineer if the top of the mountain and the hut are 2864 m and 2401 m respectively to the sea level.

75.

A man weighing 73 kg rises from a supine position in which he has his centre of gravity at a height of 10 cm to an upright position where his centre of gravity is at a height of 1.1 m. By how much does his potential energy increase?

76.

A miner descends down the shaft into the mine. The weight of the miner together with the equipment is 85 kg. By how many kJ does the potential energy of the miner change when the miner descends to a depth of 380 m below the surface? Does the potential energy increase or decrease?

77.

A high-rise has a ground floor and 14 floors. Each floor has a height of 2.8 m. To which level does a man with a mass of 86 kg have to climb in order to have approximately the same potential energy as a car with a mass of 1200 kg in a parking house at a height of 2.6 m? In this task, the height of the car is measured from the ground of the ground floor of the tower.

78.

A wooden cube with a volume of is raised from the ground by , and an aluminum cube is raised from the ground by . The specific weight of the wood from which the wooden cube is made is , and the specific weight of aluminum is .

Calculate the weight of the wooden cube.
Calculate the potential energy of the wooden cube.
What is the mass of the aluminum cube if the cubes have the same potential energy with respect to the ground?

79.

An apple weighing 23 dag hangs on a tree at a height of 3.3 m. Another apple with a mass of 24 dag is at a height of 3.2 m. Which apple has the highest potential energy?

80.

One day, a father weighing 80 kg and a mother weighing 65 kg go to the mountains. Together they arrive at a hut, which is at an altitude of 640 m. The father decides to rest, so he puts down his backpack and lies down on a lounger. The mother walks to a vantage point, which lies 45 m above the hut and is 100 m away from the hut.

Calculate the potential energy of the father on the lounger.
Calculate the potential energy of the mother at the vantage point.
What is the mass of the mother's backpack for the mother with her backpack at the vantage point and the father on the lounger to have the same potential energy when viewed from sea level?

81.

A wooden board lies on the floor, on the largest surface. Its length is 100 cm, width 50 cm, and height 8.0 cm. The density of the wood from which the board is made is . We do not lift the board, but just tip it from the largest surface (the lying position) to the smallest surface (in an upright position). What is the least amount of work we have to do?

82.

We have six identical wooden shelves with a length of 50 cm, a width of 10 cm and a height of 4 cm. All shelves initially lie on the floor, on the largest surface. The density of the wood from which the shelves are made is .

The six shelves are stacked on top of each other so that they all lie on the largest surface. Calculate the change in potential energy for the top shelf.
Calculate the change in potential energy for all six shelves together. Note that at the beginning of the observation all the shelves lie on the floor, and at the end they are stacked on top of each other.

83.

A worker pushes a cart weighing 15 kg up a slope. The cart moves straight evenly, and the braking forces are neglected. The worker pushes with a force of 107 N, which is parallel to the direction of movement of the cart, and in doing so performs 375 J of work.

Force applied on the cart and the height change

How far does the cart travel?
Calculate the height change of the cart.

84.

Describe the energy conversions for the given examples:

A tile falls from a roof.
A ball rolled down a slope.
A jumper jumps off a bridge while he is tied to an elastic rope (Bungee jumping).
An archer fires an arrow vertically upwards.
A weight attached to a rope is displaced slightly so that it swings on the rope.
A brake is applied on a car until it stops.

85.

At the point of release, a ball has a potential energy of 20 J. What is its kinetic energy just before it touches the ground? If it bounces off the ground several times before coming to rest, by how much does its internal energy increases?

86.

A spherical object has a potential energy of 60 J at altitude . What is its kinetic energy at altitude ?

87.

A ball of mass 0.4 kg falls from a height of 1.4 m. After the first bounce off the ground, it reaches a height of 0.8 m. By how much has its internal energy increased?

88.

A ball rolls down a 6 m long and 2 m high slope. It has a potential energy of 12 J at the top of the slope.

Describe the energy conversions during rolling.
What is its kinetic energy at the foot of the slope?

(Apply the law of conservation of energy).

89.

A 100 g arrow is shot vertically upwards with a bow and reaches a height of 40 m. What was the elastic energy of the bow at launch?

90.

A jumper weighing 60 kg and tied to an elastic rope, jumps from a bridge that is 40 m high above a river. The length of the rope is 30 m. When jumping, the rope is stretched for 10 m so that the jumper stops just above the river surface.

What is his potential energy before the jump?
What are his potential and kinetic energies when he falls through the length of the rope?
What is the elastic energy of the rope when the jumper is at its lowest point?

91.

A stone falls from a height, h. Just before it hits the ground, it reaches a final speed of . Calculate the height, h from which it fell.

92.

A body falls freely from a height of 6 m. Its potential energy changes with height, as shown in the graph below. Draw a graph of the kinetic energy against the height of the body.

93.

A body of mass 0.1 kg attached to a pendulum is displaced from its equilibrium position by 4 cm before it is released to freely oscillate. What is the kinetic energy of the mass as it passes through the equilibrium position?

94.

The tables below give energy values for different bodies. Calculate the missing energy in the tables:

First body:

Kinetic energy: Potential energy: Elastic energy: Total energy:
100 40 20 ?
Second body:

Kinetic energy: Potential energy: Elastic energy: Total energy:
0 ? 400 600
Third body:

Kinetic energy: Potential energy: Elastic energy: Total energy:
60 120 ? 240
Fourth body:

Kinetic energy: Potential energy: Elastic energy: Total energy:
? 60 40 160

95.

A wagon with a mass of 6 tonnes crashes into a spring at a speed of . The spring is compressed by 40 cm when the wagon stops. What is the elastic energy of the spring?

96.

A weight of 1 kg falls from a height of 40 m. Calculate the kinetic energy, potential energy, and total energy of the weight for every 10 m of fall.

(Acceleration due to gravity, )

97.

A body has a total energy of 360 J. At the moment observed, its potential energy is three times the elastic energy and its kinetic energy is twice the elastic energy. Calculate all three energies.

98.

A stone weighing 50 g is shot vertically upwards with a catapult. When the catapult is taut, it has an elastic energy of 18 J. At what height after launch is the kinetic energy of the stone twice the potential?

99.

Which of the following statements are correct:

As a body falls, its kinetic and potential energies remain unchanged.
A plasticine falls to the ground and stops without bouncing. The potential energy before the fall is equal to the internal energy when it stops on the ground.
A parachutist falls evenly towards the ground. The potential energy of the parachute is converted into the internal energy of the parachute and the surrounding air during the fall.
An athlete throws a spear with the force of his body. After 50 m of flight, the spear sinks into the ground. The law of conservation of energy applies from the moment the athlete picks up the spear.
A pendulum oscillates from one extreme position to another. The total energy of the pendulum is equal to its kinetic energy at its lowest, i.e., equilibrium position.
A car accelerating up a hill applies the law of conservation of energy.

100.

A plasticine ball weighing 0.1 kg is thrown against the wall at a speed of . The plasticine clings to the walls and soothes. What does the kinetic energy turn into? Calculate the change in internal energy of plasticine.

101.

A gymnast weighing 60 kg jumps on a trampoline and pushes him 3 m high. What is the elastic energy of the trampoline at the start?

102.

Which of the following objects have kinetic energy?

A student riding a bicycle.
An apple hanging on a branch.
A bus heading towards the station and stops.
An aircraft parked on the runway.
A bee flying through the air.

103.

For the movements listed below, determine whether the kinetic energy of the observed bodies increases or decreases.

A student rides his bike down the slope and does not brake.
An apple is torn off the branch and falls to the ground.
A bus is heading towards the station and stops.
An aircraft takes off on the runway on take-off.
A ball kicked vertically upwards moves towards the highest point.

104.

Calculate the kinetic energy of a 76 kg cyclist riding at a constant speed of .

105.

A 145 tonnes train runs at a speed of . Calculate its kinetic energy.

106.

A car weighs 1.4 tonnes. At what speed must he drive to have a kinetic energy of 280 kJ?

107.

A football player kicks a ball at a speed of . The moment the ball takes off, the kinetic energy of the ball is equal to 3 J. Calculate the mass of the ball.

108.

A father weighs 80 kg and runs at a speed of and the son, weighing 40 kg, runs at a speed of . Which of them has the highest kinetic energy?

109.

A skier weighs 90 kg and has a speed of . As she descends a slope, her speed increases by . By how many kJ does her kinetic energy increase?

110.

A car weighing 800 kg travels at a speed of . It then decelerates uniformly, reducing its speed by every second. Calculate the change in the car's kinetic energy after 8 s of deceleration.

111.

A rider weighs 80 kg together with a bike. While driving, he sees an obstacle in front of him, so he applies the brakes quickly. He brakes with a constant force of 200 N and stops after covering a distance of 8 m. Calculate the speed the cyclist had when he started to apply the brake.

112.

A 600 kg aircraft accelerates in a horizontal direction from a speed of 180 km/h to 216 km/h against air resistance of magnitude 1200 N and covers a distance of 500 m.

Calculate the kinetic energy of the aircraft at the speed of 180 km/h.
What is the change in the kinetic energy of the aircraft during acceleration?
Calculate the average thrust with which the engine acts on the airplane during acceleration.

113.

A boat, together with all equipment and passengers, weighs 350 kg. While the boat is sailing at a speed of , a diver weighing 90 kg jumps into the water and the boat continues to run at a constant speed. By how many kJ does the kinetic energy of the boat change when the diver jumps into the water?

114.

A cube, initially resting on a smooth ice base, is pushed by a boy with a force so that it begins to move with acceleration. When the boy performs 4 J of work, the cube has a velocity of . Calculate the mass of the cube.

115.

A plastic baby truck with its trailer weighs 1.6 kg and runs on batteries by driving steadily at a speed of 9.0 km/h. While driving, a child puts a toy weighing 800 g in the trailer. The speed of the truck does not change.

Calculate the kinetic energy of the empty trailer truck.
Compare the kinetic energy of the empty trailer truck and the kinetic energy of the trailer truck with the toy.
By how much does the kinetic energy of the truck change when the child put the toy in the trailer?
At what speed should the truck drive after having the toy in the trailer in order to have the same kinetic energy as at the beginning?

116.

Which of the following statements about cars are correct? Correct the wrong statements.

If two cars have the same mass and the same speed, they will have the same kinetic energy.
The car with a higher mass certainly has higher kinetic energy.
If a car drives straight uniformly, its kinetic energy cannot change in any way.
When the speed of a car increases while driving at a constant mass, its kinetic energy will also increase.
If two cars have the same kinetic energy and the first has a higher mass than the second, then the second has a lower speed than the first.
The following applies to two cars moving straight at the same speed: A car with twice the mass has twice the kinetic energy.
The following applies to two cars with the same mass moving straight: The car with twice the speed has twice the kinetic energy.

117.

An airplane weighs 600 kg. The engine of the airplane works with a power of 70 kW and performs 8.4 MJ of work for 6 km of uniform horizontal flight.

How long does the plane fly 6 km on such a flight?
What is the thrust force of the engine on the plane?
Calculate the speed of the plane.
What is the kinetic energy of the plane?
What is the force of the air resistance against the airplane during the flight?

118.

A Formula 1 race car together with the racer weighs 605 kg and drives at a speed of 306 km/h. Calculate the mass of a truck moving at 72 km/h that has the same kinetic energy as the race car.

119.

A student kicks a slipper weighing 80 g to slide on the floor perpendicular to a wall. The slipper is subjected to a constant frictional force of 0.24 N. When it is 1.5 m away from the wall, it has a speed of .

What is the kinetic energy of the slipper when it is 1.5 m away from the wall?
How much work is done by the frictional force over the 1.5 m distance?
At what speed does the slipper collide with the wall?

120.

How fast can a Cheetah, weighing 56 kg, run to have as much kinetic energy as an Elephant, weighing 5.6 tonnes and walking at 9 km/h?

121.

Apple fruit is torn off the branch of a tree and falls to the ground. The apple fruit weighs 175 g and the branch is 3.2 m high. Neglecting air resistance:

does the apple have kinetic energy during the fall?
does the kinetic energy of the apple change during the fall? Clarify the answer.
What is the kinetic energy of the apple just before it falls to the ground?

122.

A train is moving at a constant speed of with a power of . What is the traction force of the locomotive of the train?

Kinetic energy:	Potential energy:	Elastic energy:	Total energy:
100	40	20	?