The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

EXCERISES OF Home Work 5

E5. (a) Action: bat hits ball. Reaction: ball hits bat.

(b) While in flight there are two interactions, one with the Earth`s gravity and the other with the air. Action: Earth pulls down on ball (weight). Reaction: ball pulls up on Earth. And, action: air pushes ball, and reaction: ball pushes air.

E12. Yes, the baseball bat slows down when it hits a ball. This is because the ball provides a reaction force which acts on the bat to slow the bat down.

E14. No, acceleration is possible. When you push a car, you provide a non-zero net force on the car which causes the car to accelerate according to Newton's 2nd Law. Although there is a reaction force of the car -- this reaction force acts on you and does act on the car (hence you cannot include it within the net force which acts on the car).

E16. No, both cars with acquire the same speed. This is because, according to Newton's 3rd Law, the strong man provides the same force to each car. As the cars have equal mass, they will accelerate by the same amount and hence acquire the same speed.

E17. The forces on each cart are the same, but since the masses are different the accelerations will differ. The twice-as-massive cart will undergo only half the acceleration (recall a = F(net)/m) of the less massive cart and will gain only half the speed.

PROBLEMS OF Home Work 5

P1. F = ma = mDv/Dt = (0.003kg)(25m/s)/(0.05s) = 1.5N.

P2. The wall pushes back on you will a force of 30N. Therefore your acceleration will be a = F/m = 30N/60kg = 0.5m/s/s.