3. What are forces?

4. Push and pull

5. Types of forces
All forces involve interactions between objects. There are several different types.
Gravity and magnetism are forces that can act over distances.
Friction and upthrust are forces that involve direct contact between objects.
Teacher notes
Forces during take off include gravity, thrust and air resistance (friction).
Photo credit: NASA Marshall Space Flight Center (NASA-MSFC)
All types of forces can occur whether objects are still or moving.
What forces are acting during this rocket launch?

6. Forces affecting objects

7. Forces changing an object’s shape
Teacher Notes
The gradient of the graph increases when the 6 Newton weight is added because the spring has reached its elastic limit. 7

8. Balanced forces
There are two wind machines applying forces to the ice skater. The forces acting on the skater are equal in magnitude and opposite in direction.
The forces are balanced, so they cancel each other out. The skater does not move.

9. Unbalanced forces
What if only one wind machine is blowing on the skater?
The forces acting on her are no longer balanced so she will start to move to the left. Her speed will change – this is called acceleration.
Unbalanced forces lead to a change in speed or direction.

10. Measuring forces

12. Force pairs Teacher Notes
Encourage pupils to predict the outcome of each simulation.

13. Balanced and unbalanced forces
Imagine a car travelling at a constant speed of 50 km/h.
The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed.
500 N
500 N

14. Balanced and unbalanced forces
A crosswind acting on the car produces a sideways force.
500 N
500 N
100 N
cross wind
The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. The car will veer sideways.
If the car turns right so that the wind is now behind the car, what will happen to the speed?

15. Resultant force
The sum effect of more than one force is called the resultant force.
The resultant force is calculated by working out the difference between opposing forces in each direction.
What is the resultant force on this truck?
A resultant force of 100 N is accelerating the truck.
400 N
500 N

16. Balanced and unbalanced forces – summary
If the forces on an object are balanced:
and the object is stationary, it will remain stationary
and the object is moving, it will continue to move at the same speed and in a straight line.
In other words, the object will continue to do what it is already doing without any change.
If the forces are unbalanced, two things can happen:
The speed can change. This is called acceleration.
The direction of motion can change.

17. Resultant forces – question 1
1. What is the resultant force on the satellite?
5 N 5 N
Resultant force = 20 N – 10 N = 10 N down
The satellite will accelerate downwards.
20 N

18. Resultant forces – question 2
2. What is the resultant force on the bird?
The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.
Resultant force = 5 N – 5 N = 0 N
The vertical forces are not balanced, the bird will accelerate in a downwards direction.
5 N
5 N
Resultant force = 5 N – 0 N = 5 N down
5 N

19. Resultant forces – question 3
3. What is the resultant force on the yacht?
10 N
10 N
13 N
20 N
10 N
The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction.
The horizontal forces are not balanced, so the yacht will accelerate to the right.
Resultant force = (20 N +10 N) – 13 N
= 17 N right

21. What is gravity?
When the netball is thrown, why does it fall back down?
There is a gravitational force pulling it towards the Earth.
Gravity is a force that occurs between all objects.
Gravity always acts to pull objects towards each other.
The Earth and the ball are pulling each other together.
However, the ball moves much more than the Earth because it has a much smaller mass.

22. Sir Isaac Newton
Sir Isaac Newton is a very famous physicist who lived in England 1643–1727.
Newton wrote down his ideas in the Philosophiae Naturalis Principia Mathematica; a very important book about forces and gravity.
Some accounts suggest that one of Newton’s greatest discoveries occurred when an apple fell on his head and it made him think about the reason it fell…

23. Gravity and Newton
Newton realized that the motion of falling objects and objects orbiting in space must be caused by the same force – Gravity!
He wrote in the Philosophiae Naturalis, “It is an attractive force that makes apples fall from trees and the planets orbit the Sun.”
Other scientists had already noted the effects of gravity but Newton was the first to calculate the force of gravity on objects.

24. What is gravity?
Gravity is an attractive force that acts between all objects that have mass. The size of the force depends on the masses of the objects.
Satellite’s gravity
Earth’s gravity
All objects produce a gravitational force. This is very large for huge masses such as planets.
When you jump, the gravitational force of the Earth pulls you down. Your gravitational force also pulls the Earth towards you, but you don’t notice it because the Earth is too heavy to be visibly affected by your gravity.

25. Gravity and distance
The force of gravity between two objects depends on their masses and the distance between them.
Spacecraft produce a very large force, called thrust, to overcome the force of gravity.
As a spacecraft gets further away from Earth, the force of gravity gets smaller.
Photo credit: © 2008 Jupiterimages Corporation
Why do spacecraft lose their large fuel tanks and booster rockets once they have left the Earth’s surface?

26. Gravity during a rocket launch

27. What are mass and weight?
Mass is the amount of matter in an object and is measured in kilograms.
Mass is not a force.
An object, such as this satellite, has the same mass at any point in the Universe.
Weight is a force and is caused by the pull of gravity acting on a mass.
Photo credit: © 2007 Jupiterimages Corporation
Weight is measured in newtons and has both magnitude and direction. An object’s weight changes depending on where it is in the Universe.

28. Investigating mass and weight

29. Mass and weight on the Moon
The force of gravity on the Moon is only one-sixth of that on Earth because the Moon has a much smaller mass.
Any object on the Moon weighs one-sixth of the amount it would weigh on Earth.
Astronauts can jump up 20 feet on the Moon due to there being such a low gravitational force.
Photo credit: © 2008 Jupiterimages Corporation
However, the astronaut still has the same mass – they just weigh less because gravity is weaker.

30. Mass and weight on different planets
Worksheet 1 accompanies this slide.
It provides further practice at calculating weights in different gravitational fields.

31. Weight and mass activity

32. Missing words about gravity