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physics Physics For High Schools (KS4)

Solved problems for the chapter:

Power and Efficiency

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Unit conversion

1. Meaning of unit conversion

2. SI prefixes

3. Forum

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Detailed solution steps are available at:

https://en.openprof.com/wb/chapter:power_and_efficiency/5050/

Example 1.

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?

Example 2.

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.

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?

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.

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?

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?

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.

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?

10.

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?

11.

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?

12.

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?

13.

A crane takes to lift a load to the top of a building that is high. What is the useful power developed against gravity by the crane?

14.

A pump does of work to raise water to fill a tank. It takes to fill the tank. What is the power of the pump?

15.

An electric car is charged overnight. In , of energy is transferred. What is the power of the charger?

16.

A microwave oven is rated at . Which statement correctly describes the meaning of this value?

are transferred every second.
are transferred every second.
are transferred every second.
are transferred every second.

17.

Which two physical quantities must be used to calculate the power developed by a student running up a flight of steps?

force exerted and the vertical height of the steps only
force exerted and the time taken only
work done and the vertical height of the steps only
work done and the time taken only

18.

The engine of a motor vehicle develops a large power. Which statement is correct?

The driving force acting on the vehicle must be large.
The engine must have a very large volume.
The engine must transfer large amounts of energy each second.
The vehicle must be very fast.

19.

A wind turbine is efficient and has an output of of electrical power. What is the power input to the turbine?

20.

What is the unit of power?

joule
newton
volt
watt

21.

A boy takes to lift a book of mass from the top of a desk and place it on a shelf. The top of the desk is above the floor, and the shelf is above the floor. The gravitational field strength is .

Which power does the boy develop?

22.

What is meant by the power of an engine?

the energy that the engine transfers per unit time
the maximum force that the engine can exert
the maximum weight that the engine can lift
the total energy that the engine transfers

23.

A boy holds onto a bar and pulls himself up until his chin is level with the bar. He raises himself through in . The weight of the boy is . What is the average power he produces as he raises himself?

24.

The diagrams show four appliances and their power ratings. Which appliance transfers the most energy per second?

25.

A weightlifter picks up a stone ball and places it on a shelf. Each lift takes the same time. Which situation requires the greatest power?

26.

An engine produces of energy in . What is the power output of the engine?

27.

The power input to an electric motor is . The efficiency of the motor is . How much power is wasted?

28.

A crane is used to lift loads vertically. The output power of the crane to lift a car is . The crane then lifts a lorry, which has 3.0 times the weight of the car, through 0.25 of the distance in 0.50 of the time. What is the output power of the crane now?

29.

A car is moving along a straight horizontal road. The car has of kinetic energy. The car accelerates for until the kinetic energy of the car increases to . What is the minimum average power developed by the car engine for this acceleration?