1. C Force Forces Types of forces Balanced and unbalanced forces
Investigating the relationship between force and acceleration
Force, acceleration and mass
Example: Pushing cars
Example: Force calculations
Example: Mass and weight
Gravity and falling

2. Forces A force is a push or a pull that one object exerts on another.
It makes an object change its shape, speed or direction of movement.
Force is measured in newtons (N) .
Forces can be added together, but to do so you must know the size and direction of the forces. The answer you get when you add the forces together is called the total, net or resultant force.
Eg if a car has a thrust force of 50 N, but also has a friction force of 20 N backwards then there is a net force of 50 N – 20 N = 30 N acting forwards.

3. Types of forces
Forces can be contact forces, that need to touch when they are acting. For example:
A club hitting a golf-ball.
The wind blowing the leaves on a tree.
A ball rolling along the ground and slowing down.

4. Or forces can be field forces (non-contact) where there is no touching
Or forces can be field forces (non-contact) where there is no touching. For example:
Magnetic forces
Electrostatic forces
Gravitational forces.

5. Forces
A force is a push or a pull. It makes an object change its shape, speed or direction of movement.
Force is measured in newtons (N).
If more than one force is acting on an object, the forces can be added together but you must take into account the direction. The sum of the forces is called the net, total or resultant force.
Forces can be contact forces where the force touches the object. Eg a bat hitting a ball, the wind blowing the leaves. Or forces can be field forces (non-contact) where there is no touching. Eg magnetic, electrostatic, gravitational forces.

6. Balanced and unbalanced forces
If the net force is zero then the forces are said to be balanced.
When the forces are balanced, the object will remain stationary or, if it is moving, it will continue to move with its speed and direction unchanged.
Examples of balanced forces:

7. 300 N
500 N
If the net (resultant or total) force is not zero then the forces are unbalanced. This results in the object changing its speed and/or direction – accelerating in the direction of the net force.

8. Balanced and unbalanced forces
If there is no net force (the net force is zero), then the forces on an object are balanced. The object will remain stationary or continue moving with its speed and direction unchanged.
Examples of objects with balanced forces on them are a cup on a table or a balloon flying at constant speed in a straight line.
If there is an unbalanced force acting on an object it will speed up, slow down, change its shape or change its direction.
1C 1 Balanced and unbalanced forces

9. Mass and weight
Mass is a measure of how much matter something contains. Mass is measured in kg. Your mass is the same wherever you are.
Weight is the force of gravity on you. It changes as gravity changes in different places (eg on the Moon).
On Earth, gravity (g) pulls with an average strength of 9.81 N kg–1, which we often approximate to 10 N kg–1 to make the maths easier.
On the Moon, g = 1.63 N kg–1.

10. An astronaut has a mass of 80 kg. On Earth her weight is given by:
On the Moon g = 1.7 N kg–1 and her weight is 136 N.
Her mass on the Moon is the same as it is on Earth: 80 kg.

11. Mass is the amount of matter in an object in kg.
Mass and weight
Mass is the amount of matter in an object in kg.
Weight is the force of gravity on an object in newtons (N).
On Earth, g is about 10 N kg–1. On the Moon g is about 1.7 N kg–1.
Someone with a mass of 80 kg has a weight of
80 kg × 10 N kg–1 = 800 N on Earth,
80 kg × 1.7 N kg–1 = 136 N on the Moon.
Their mass remains 80 kg no matter where they are.
1C 2 A stretchy problem

12. Investigating the relationship between force and acceleration
This experiment is in your Workbook.
We shall use a falling mass to apply a steady force on a trolley, and use a ticker-timer to measure its acceleration.
Use the following slides to help you to set up the equipment below.

13. Set up the ticker-timer with about 2 m of tape.
Remember to connect the timer to the AC terminals of the power supply.

14. Each of these masses is 50 g, while the mass-carrier is also 50 g.
Two masses plus the carrier have a combined mass of 150 g.

15. Attach one end of a piece of string about 2 m long to the front end of a trolley.

16. Attach a pulley to a clamp stand.
Tie the other end of the string to the mass carrier and hook it over the pulley.

17. Put the clamp stand with pulley on a stool, and adjust the height of the pulley so that the string is horizontal.

18. Push the clamp stand out from the stool until the mass is able to fall freely to the floor without hitting the stool.

19. Use a G-clamp to hold the stand to the stool in the correct position.
(If you don’t have a G-clamp, use a pile of books.)

20. Pull the string until the mass-carrier is right up to the pulley.

21. Use tape or a drawing pin to attach the ticker-tape to the back of the trolley.
Bring the trolley as close to the ticker-timer as possible.

22. Move the stool away from the bench until the string is tight.

23. Hold the trolley in place.

24. Switch on the power supply. You will hear the hammer hitting the tape
Switch on the power supply. You will hear the hammer hitting the tape. THEN release the trolley.

25. As the mass falls, it pulls the trolley, which pulls the tape behind it.

26. Now do the experiment for yourself.
1C 3 Force and acceleration

27. Results
The dots on the tape get further apart, indicating that the trolley was accelerating.

28. Mark off every 5th dot...
... cut up the tape...
... and join the strips together to make a speed-time graph.

29. Label your finished graph with the mass used.

30. Repeat the experiment using a mass of 300 g.
5 x 50 g weights
plus
1 x 50 g carrier

31. The greater mass has produced a greater acceleration – the dots are further apart and gradient of the graph is steeper.

32. The gradient of the second graph is exactly double that of the first.
When the force pulling the trolley doubles, its acceleration doubles.

33. Force, acceleration and mass
An unbalanced force acting on an object will cause it to accelerate.
We know now that the acceleration produced is proportional to the force.
We could carry on making ticker-tape graphs to show the relationship between acceleration and mass for a constant force.
Another way to think of the relationship between force, mass and acceleration is to imagine pushing cars.

34. Pushing cars
The greater the force on the car, the faster it will accelerate.

35. If the forces are equal but the mass is doubled, then the acceleration halves.

36. Two people pushing two cars has the same acceleration as two separate people each pushing one car.

37. We have seen that acceleration goes up as force goes up, and acceleration goes up as mass goes down.
Thus: or
Where F, force is measured in newtons (N)
m, mass, is measured in kilograms (kg)
And a, acceleration, is measured in metres per second squared (m s–2)

38. force = mass × acceleration
Force, acceleration and mass
The greater the force applied to an object, the greater its acceleration. If the net force is zero then the acceleration is zero.
A moderate force will cause a small mass to accelerate more than a large one.
These two relationships are combined in the formula
force = mass × acceleration
Where F, force is measured in newtons (N)
m, mass, is measured in kilograms (kg)
And a, acceleration, is measured in metres per second squared (m s–2)

39. Example: Force calculations
A car with a mass of 1800 kg can accelerate at m s–2. What is the net force on the car?
m = 1800 kg a = 1.85 m s–2
F = ?
The car is loaded up with 5 people plus luggage. It now has a total mass of 2210 kg. What acceleration can they expect to reach with this force?
m = 2210 kg F = 3330 N
a = ?
1C 4 Force, mass and acceleration

40. Example: Mass and weight
A 70 kg person stands on the floor. How much force pushes on the floor? What is the support force supplied by the floor? The acceleration due to gravity is 10 m s–2.
m = 70 kg, a = 10 m s−2, F = ?
[Therefore, weight = mass × gravity]
Hence, the support force is 700 N.

41. Gravity and falling Gravity is the force that makes objects fall.
Gravity is always pulling objects down, whether they are moving downwards, stationary, or going up.
All falling objects in the same gravitational field will accelerate at the same rate (about 10 m s–2 on Earth).
Here an astronaut on the Moon dropped a hammer and a feather.
Without air resistance, the hammer and the feather fall with the same acceleration and land together.
feather
See the drop online.

42. On Earth, air resistance acts as a friction force opposing the motion (it pushes in the opposite direction to your movement).
Air resistance or friction only happens when an object is moving.
Stopped at the top
Friction force
Going up
Gravity force only
Gravity force
Friction force
Gravity force
Falling down

43. As the speed of a falling object increases, its air resistance increases.
Eventually the friction force (air resistance) is equal to the force of gravity.
When the forces are balanced, the object stops accelerating. Physicists say that the object has reached terminal velocity.
A parachute has a large surface area, giving it a high air resistance. With the parachute open, the sky-diver slows down until the forces again balance at a much slower terminal velocity.

44. Gravity and falling
The force of gravity always acts downwards, even if an object is moving upwards.
Gravity makes all objects fall with the same gravitational acceleration, 10 m s-2 approx. (assuming no air resistance) on Earth.
Air resistance and any other friction opposes the motion and only acts when things are moving. Air resistance increases with speed.
If there is air resistance then the net force between gravity and air resistance gives the acceleration of the object.
1C 5 More force calculations C 6 Living in gravitational fields C 7 Burnt out satellite

45. Proceed to next chapter
End of chapter
Proceed to next chapter