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3. What is a force? A force is a push, pull or twist.
Click on the arrows to see the force!

4. Weight, mass and gravity

5. Weight and mass are not the same.
Mass is the amount of matter in an object.
This will have the same value anywhere in the Universe including space.
Weight is a force and it is caused by the pull of gravity.
In fact, weight is the pull of gravity acting on a mass.

6. Weight and mass Weight is a force so is measured in Newtons.
Like other forces it has both magnitude and direction.
Mass is not a force, it is measured in kilograms.
A 1 kg mass will weigh less on the moon than it does on Earth.
This is because the force of gravity is less on the moon because the moon is smaller than the Earth.
An astronaut could jump 20 feet into the air on the moon because gravity is less.
However, he still has the same body, and the same mass, he just weighs less, because he is on the moon and gravity is weaker.

7. Weight and mass So, a scientist should NEVER say “He weighs 50kgs”
but, should say
“He has a mass of 50kgs”,
or the scientist could say:
“the gravitational force acting on his mass is about 500 Newtons”.
This is the same as saying:
“his weight is about 500 Newtons”.

8. Balanced and unbalanced forces

9. Forces Balanced Forces 10 N 10 N
If you link two newton meters and pull equally hard from both ends, the forces recorded on both will be the same.
We say that the forces acting on the central hooks cancel each other out - they are equal in magnitude and opposite in direction.
Because the forces are balanced the hooks do not move.

10. Forces unbalanced Forces 11 N 10 N
movement
What happens if the pull on one end is harder than on the other?
The forces acting on the hooks are no longer balanced. Both hooks will start to move to the left, that is, their speed will change. This is called acceleration.
Unbalanced forces lead to a change in speed or direction.

11. Friction
Friction always tries to slow moving objects down. We say it opposes motion.
Friction is created whenever two touching objects or surfaces move past each other.
Friction also occurs when things move through air. This is called air resistance or drag.
There is a maximum value for the frictional force which depends on:
The force pushing the two surfaces together.
The state of the surfaces in contact.

12. Friction On the diagram label all sources of friction.
tyre and road
brake pad and rim
wheel bearing
pedal bearing
links in chain
Click for answers
Air resistance or “Drag”
One more? Probably the most important…

13. Frictional forces

14. Forces in water What is thrust? A forward push
The upwards force on a body in a liquid or a gas – due to displacement.
What is upthrust?
Upthrust
Friction
Thrust
Weight

15. Label the forces acting on the moving ship shown:
Upthrust or buoyancy
Air resistance
Thrust
Friction
Weight

16. A ship arrives at port and is loaded with cargo:
Increases
What happens to the weight of the ship?
What happens to the upthrust on the ship?
Which is the largest force, weight or upthrust?
Increases
Increases
Increases
Increases
Both equal