NEWTON’S THIRD LAWTHE 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

• 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)