9709 · 4.4
Newton's laws of motion — FAQ
Frequently asked questions for 9709 Newton's laws of motion. Direct answers first, then deeper explanation — then practise with marking.
If an object is moving, doesn't that mean there must be a resultant force on it?
Not necessarily. According to Newton's First Law, an object moving at a constant velocity has zero resultant force acting on it. A resultant force is only required to cause acceleration (a change in velocity).
Why don't action-reaction forces from Newton's Third Law cancel each other out?
Because they act on different objects. To determine an object's motion, you only consider the forces acting on that object. Since the action and reaction forces act on separate bodies, they cannot cancel each other out in the force analysis of a single body.
In a pulley system, why can we assume the tension is the same on both sides of the string?
This is a modelling assumption. We assume the string is 'light' (has negligible mass) and the pulley is 'smooth' (has no friction). If the pulley had mass or friction, it would require a net turning force (a torque), meaning the tensions on either side would have to be different.
What's the difference between a 'smooth' and a 'rough' surface?
In mechanics problems, 'smooth' is code for 'frictionless'. A smooth surface exerts no frictional force. A 'rough' surface can exert a frictional force that opposes motion or attempted motion, up to a maximum value of .