Newton’s three laws of motion, inertia, force and acceleration, momentum, and why momentum is conserved in collisions and recoil.
Key ideas
First law: an object stays at rest or keeps moving steadily unless a net force acts. This tendency is inertia, and more mass means more inertia.
Second law: the net force equals the rate of change of momentum, F = ma.
Third law: every action has an equal and opposite reaction, acting on a different body.
Momentum p = mv has a direction. In a collision or explosion with no outside force, total momentum stays the same.
A gun recoils because the bullet and gun get equal and opposite momentum.
Increasing the time of impact (sand pit, catching a ball by pulling the hands back) reduces the force.
Remember
Second law
F = ma; 1 N = 1 kg m/s2
Momentum
p = mv (kg m/s)
Impulse
F × t = mv − mu
Conservation
m1u1 + m2u2 = m1v1 + m2v2
Watch out
Thinking action and reaction cancel (they act on different bodies).
Using grams instead of kilograms in F = ma or p = mv.
Ignoring the direction (sign) of velocity in momentum problems.
Worked example
A rifle of mass 5 kg fires a bullet of mass 35 g at 600 m/s. Find the recoil velocity of the rifle. (Give the answer to 2 decimal places where needed.)
Working: total momentum stays zero: 5 × v = 0.035 × 600