# What is force?

Learn.

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.

Master.

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?