What is force?


You may have been taught in earlier years that a force is a push, pull or twist. However, this description is very limited for year 11 and 12. We will think of force as a conceptual understanding that allows us to explain phenomena we see around us.

Newton described force as a mass undergoing acceleration. This was the second law of mechanics and can be written as F = ma – mass is always in kg and acceleration is always in m/s/s. This means we are all experiencing a force, since we are consistently undergoing a downward force due to gravity, g. When there is no normal force, ie the force of the ground pushing up on you, then we would accelerate downwards at 9.8 m/s/s.

The unit for force is Newtons (N) – this will be a vary useful thing to know in later lessons.

Forces can be add together and they can be subtracted. This is because Force has both a magnitude (how big it is) as well as a direction (left, right, up, down etc.).

When solving problems it can be useful to use a free body diagram to illustrate and understand what forces are acting where. Remember – two equal forces in opposite directions equal out to 0 N net force – this is why we are stationary when we are standing on the ground. The force from gravity pulling us down and the earth pushing us up equal out.


Question 1. [Not exam style question]

A car accelerates at 15 m/s/s and it weighs 1000 kg – what force is acting on it.

Question 2. [Not exam style question]

A child is thrown into the air and accelerates upwards at 4.6 m/s/s, they have a mass of 50 kg, what was the initial force acting on them.

Question 3. [ 1 mark]

A car is accelerating at 10 m/s/s and the engine is supplying a force of 300 N, how many Kg must the car way?

Question 4. [2 mark]

Two people are pushing each other one weighs 60 kg and is applying a force of 40 N to the right, the other weighs 80 kg and is applying a force in the opposite direction of 30 N. In what direction and with what acceleration will they be moving?

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