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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
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?