Dynamics: Newton's laws & non-uniform motion
Kinematics told us HOW things move; dynamics tells us WHY. In this AS lesson we meet the three laws that Isaac Newton gave us for the connection between force and motion. We state that mass is the property of a body that resists a change in motion, define linear momentum as p = mv, and then state Newton's second law in its full form — that the resultant force equals the rate of change of momentum, F = Δp ÷ Δt — before deriving F = ma as the special case for constant mass. We look at weight as the effect of a gravitational field, W = mg, and at bodies falling through air until they reach terminal velocity. Finally we practise the exam skills that hold it all together: drawing free-body force diagrams and resolving forces on an inclined plane. Momentum conservation is a separate lesson; here we build the force–motion foundation, step by careful step.
By the end you should be able to (NSSCAS Physics (AS) 1.4):
- State that mass is the property of a body that resists a change in its motion (inertia)
- State and apply Newton's first law of motion (the law of inertia)
- Define and use linear momentum as the product of mass and velocity, p = mv
- State Newton's second law as the resultant force being equal to the rate of change of momentum, F = Δp ÷ Δt
- Derive F = ma as the special case of Newton's second law for constant mass, and appreciate that a and the resultant force are always in the same direction
- State and apply Newton's third law of motion, identifying action–reaction pairs that act on different bodies
- Describe and use weight as the effect of a gravitational field on a mass, W = mg
- Describe and explain qualitatively the motion of a body falling through air, including terminal velocity, and recall that acceleration can be constant even when the motion is non-uniform
- Draw free-body force diagrams and resolve forces acting on an inclined plane to find the resultant force and acceleration