An apple is hanging on a branch of a tree. The resultant of the forces acting on the apple is equal to the weight of the apple.
A car is standing in front of a traffic light. The sum of all the forces acting on the car is zero.
A car moves evenly on a level road. The resultant of all the forces acting on the car is equal to the force of the engine.
We hang a 4 N weight on a spring balance which is attached to a retort stand. It stretches by 2 cm. In the end, the resultant of the forces acting on the weight is zero.
There is a package on a table. Because it is at rest, no force acts on it.

Example 2.

The table below describes events in which a force acts on one of the bodies described. Answer the questions on the columns for each event.

Event	On which body does the force act?	What do we call that force?	How do we know that a force acts on the body?
A car starts at a green light on a traffic light.
A brick breaks when it falls to the ground.
A child shapes a plasticine doll.
A snow avalanche starts on the hill and rushes into the valley with increasing speed.
A magnet attracts a needle on the ground.
We hold a spring in our hands and stretch it.
A child rotates on the hanging carousel seat.
We fill a balloon with air from the lungs.

Complete the table below with the appropriate values.

Subject	m	m	m	W
Traveller			20
Eraser	15
Student				520
Apple		17
Car			1350
Textbook				3.7

John pushes a cart with a force of 80 N, as shown in the picture below. How do we graphically represent this force?

Which of the following statements is/are correct?:

Force is known by its effect on a body.
The force unit, newton (N), is a fundamental unit of measurement.
We must always put an arrow above the force symbol.
The effect of the force on the body does not depend on the line of action.
The body can act with force on another body only by touch.
Force always acts at one point.

A metal ball weighing 2 kg falls to the ground.

What force acts on the ball?
Determine the magnitude, direction, and line of action of this force.
Draw the force to scale.

For the force in the following examples, write how it is distributed: point, planar or spatial.

Using the tip of a pencil, push an eraser across a table. What is the force of the pencil on the eraser?
The wind blows against the sail and pushes a sailboat. What is the force of the wind on the sail?
A string on which the weight is hung is attached to the ceiling. What is the force of the string on the ceiling?
Magnet attracts pieces of iron. What is the magnet's force on iron?
A crate lies on the ground. What is the force of the crate on the ground?
An apple falls from a tree. What is the force acting on the apple while falling?

Determine whether contact or field force acts in the cases described.

A locomotive pulls a wagon.
Engine force is transmitted to the wheels of a car.
A skier descended a gorge.
Our hair gets tangled during a storm.
The wind pushes a sailboat.
Magnet attracts iron.
A brick acts with force on the ground.

Let's take an iron block and bring a horseshoe magnet closer to it, as shown in the picture. The magnet acts on a square side with a force of 20 N.

Name the force.
Determine its direction and line of action.
Draw the force with a selected scale.

10.

A stone of mass 15 g is projected vertically upwards with a slingshot. After 5 s it returns to the ground again. Determine the magnitude and direction of the force acting on the stone:

when it moves vertically upwards,
when it stops for a moment at maximum height and
when it is falling to the ground.

Air resistance is neglected.

11.

Mark pulls a fishing boat with a rope towards the shore as shown in the picture. His traction is 450 N. Determine the scale if the force is drawn as a 3cm long line.

12.

A man walks on a tightrope and his weight is 500 N.

Draw his weight in a selected scale.
Where is the force with which his foot acts on the rope? What is this force?

13.

A table weighing 200 N has a triangular base as shown in the figure.

What is the magnitude and direction of the force with which the table acts on the ground?
Where is the line of action of this force?
Draw the force to scale.

14.

Two forces are on the same geometric axis and have their lines of action at the same point. The first force has a size of 120 N and the second 70 N. Draw them on a scale of 20 N to 1 cm if they are in:

the same direction,
opposite directions

15.

A student has a schoolbag in his hand as shown in the picture. The weight of the schoolbag is 20 N. Determine the size, direction, and centre of gravity of the forces acting on the schoolbag and draw them to scale.

16.

The diagram below depicts a force of 6 N pushing a crate on the ground. John drew the force on the crate as shown in the picture below. Is his drawing correct?

17.

The figure shows a force of 240 N. Determine in what scale it is drawn.

18.

In the figure below, a force of 3 N is drawn on a scale of 1 N 1 cm. The line of action is at point A. Draw the opposite force with the line of action at point B.

19.

Force in the described phenomena causes deformation of the body - that is, a change in shape. From the description, determine whether the deformation was flexible.

Jane stretches the spring.
At tension, the elastic is torn.
Due to the weight of the snow, the branches bend.
Ice broke the trees.
Housewife kneads bread dough.
The wind bends the spruce.

20.

Are the following statements correct?

If we act on a body with a certain force, the body also acts on us with an equal and opposite force.
Bodies interact only with direct contact.
A body acts on another body with a force F, and the second body on the first with the opposite same force. Since the sum of both forces is zero, the body is at rest or moving evenly and in a straight line.
The law of interaction also applies when a body acts with force on another body and accelerates it.
The earth acts on the Moon with the same force as the Moon on the Earth.

21.

A trolley on the tracks is pulled horizontally with a force of 600 N. The applied force acts at an angle with respect to the direction of the tracks. What component of the force acts in the direction of motion of the trolley?

22.

Determine whether the description of Newton's first or third laws applies in the given cases.

A car drives uniformly on a level road. It is acted upon by the force of the engine and, conversely, by the same force of air resistance.
The force of water resistance inhibits the navigation of the ship, and the ship acts with force on the water to cause waves. Both forces are equal and opposite.
Push a trolley on a flat surface with the force of your hands so that it moves uniformly.
A locomotive acts with force F on the train carriages and accelerates them. The wagons operate with the same opposite force on the locomotive.
A body hangs on a string. The sum of the force on the string and the weight of the body is zero, so the body is at rest.
A body hangs on a string. With its weight it acts on the string vertically downwards, and the string with the equal and opposite force on the body.

23.

Jack pushes a 30 kg box with a force of 300 N at an angle downwards as shown in the figure. Graphically determine the magnitude of the force component in the direction of motion of the box and the force component directed vertically downwards. With what force does the box act on the ground?

24.

A ball weighing 8 N is suspended on a string. It is displaced by an angle (to point A) and allowed to swing.

Determine graphically, the forces in the string and in the direction of motion at point A.
What are the forces in the string and in the direction of motion when the ball moves through point B?

25.

The drawn bow acts on an arrow with a force of 300 N when it is fired. With what force are the limbs of the bow tensioned if the angle between the arrow and each of the limbs is equal to . Solve the problem mathematically with the help of the knowledge of geometry - equilateral triangles.

26.

A log is placed on a slope with an inclination of , as shown in the figure. The weight of the log is 5 N. With how much force does the log act perpendicular to the slope and with how much force does it act parallel to the slope?

27.

John skis down a slope with an inclination of . His mass is 50 kg. How much force does he apply on the slope while still on the slope?

What are the two forces of John on the slope?
What are the two forces when he skis on the plain surface?

28.

A man strikes a log with an axe with a force of 200 N. With what force does the axe act on both sides of the log? The blade of the axe in the cross section makes an angle of as .

29.

The maximum force that a rope can withstand for it not to break is 6 kN. How much mass can be hung on the rope if it can be loaded with a maximum of 40% of the maximum force.

30.

A boat, tied to a rope, is towed on the water at a speed of 2 m/s. The traction force is 600 N. With what force does the boat resist the flow of the water?

31.

A ball rolls uniformly and in a straight line on a flat surface at a speed of 2 m/s - Figure A. At some point, two equal but oppositely directed forces of magnitude 3 N each begin to act on the ball - Figure B. How will the ball move after the forces start to act?

32.

Identify which of the listed motions below is described by Newton's first law. Justify the answer.

Brock meandering evenly along the snowy slope.
Raindrops falling from the sky at a speed of 8 m/s.
Children on the carousel rotating at a constant speed.
A passenger plane flying on a straight track and at a constant speed over the Atlantic.
A stone falling to the ground from a height of 10 m.

33.

A skier of weight 600 N steps on soft snow. The moment he steps on the snow, the force of the ground is 400 N.

What is the resultant force and in which direction does it act?
What force needs to change and by how much for the skier to rest?

34.

Joe pulls a sled at an angle to the horizontal surface. The frictional force between the sled and the snow is 42 N. With what force does Joe pull the sled to move uniformly?

35.

Four perpendicular forces of magnitudes 80 N, 40 N, 60 N, and 20 N act on a ball. Find the fifth force that can act with the four other forces on the ball so that the ball moves uniformly and in a straight line.

36.

Tom pulls a log with a force of 30 N at an angle of 45 ° with respect to the horizontal. The frictional force between the log and the ground is 21 N, the weight of the log is 40 N, and the ground acts with a force of 19 N on the log. Draw all the forces acting on the log on a scale and determine whether Newton's first law applies to the motion of the log.

37.

A car is driven on a flat road. The force of the engine is constantly opposed by the force of air resistance, which depends on the speed of the vehicle. Consider four cases:

The engine force is equal to the resistance force. The magnitude of each is 0 N.
The engine force is 800 N and the resistance force is 200 N.
The engine force is equal to the resistance force. The magnitude of each is 400 N.
The engine force is 200 N and the resistance force is 400 N.

Which of the cases does Newton's first law describe?

38.

A force of acts on a body. The body accelerates uniformly with an acceleration of . Calculate its mass.

39.

A body has a mass of . A force of acts on it. At what acceleration does it move?

40.

Two opposing forces and act on a trolley with a mass of . In which direction is the trolley moving and what is the acceleration?

41.

A locomotive with a mass of has a wagon with a mass of attached to it. The combination accelerates with an acceleration of . With what force does the locomotive engine work and with what force does the locomotive pull the wagon?