Resultant force

When they use the words "resultant force" in an exam question, they want you to talk about the balance between the driving force and the counter force on an object. They then want you to link it with the equation for Newton's Second Law:

Resultant force = mass x acceleration

F = ma

A favourite example is an object falling through air.

• At first, the only force is gravity(weight) downwards. It is not yet going fast enough for there to be air resistance. Gravity is the winner and hence the resultant force is downwards. There is a resultant force so the object accelerates, going faster.
• A faster object has more air resistance.
• The weight of the object does not change. So the force downwards stays the same whilst the the counter force upwards keeps increasing.
• The resultant force gets smaller and smaller. The objects keeps accelerating but the speed goes up by less and less each time.
• Eventually air resistance upwards = weight downwards.
• There is ZERO resultant force.
• There is NO acceleration.
• The object stays at the same speed, TERMINAL VELOCITY.