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Newton’s Third Law The Law of Interaction. Test Yourself. Hammer hits nail,____________ Hand punches wall, ___________ Book pushes down on table top,________ Bat hits ball, _____________ Car pushes road, _____________. Newton’s Common Sense.
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Test Yourself • Hammer hits nail,____________ • Hand punches wall, ___________ • Book pushes down on table top,________ • Bat hits ball, _____________ • Car pushes road, _____________
Newton’s Common Sense • Newton realized that nature was symmetrical. That is, things occurred in pairs. • He said that forces also occurred in pairs
F 1,2 = -F 2,1 • For every action there is an equal and opposite reaction
Earth and Rock • The earth pulls on a 1kg object with how much force? • Does the 1 kg object pull on the Earth? If so, how much?
A numerical example. • For every action there is an equal and opposite reaction • Lady bug • Mass = 3.5 g • Velocity = 4.8 m/s, south • F supplied by the car = 10 N Nissan Titan • Mass = 2289 Kg (5000 lb.) • Velocity = 24.6 m/s (55 mph), North • F supplied by the bug__________N
If the forces are equal and opposite, then why don’t they cancel each other out? • The reason they don’t cancel out is because they act on different objects. • For example: foot kicks ball. Ball kicks foot.
Gun Example • A gun with a mass of 1.35 kg fires shot with a mass of 45 g. If the recoil force is -16.2 N, what are the accelerations of the gun and the shot?
Solution(Assume the shot is moving to the right and this is the positive direction) • Shot • Fnet = ma • Fnet/m = a • +16.2 N/.045kg = a • a = 360 m/s2 • Gun • Fnet = ma • Fnet/m = a • -16.2 N/1.35 kg = a • a = - 12 m/s2 Would you want to shoot a huge (massive) bullet with a small (less massive) gun?
Newton’s 3rd Law in Zero Gravity • What would it be like?? (5 min)