Physics questions, please help?

[Nyx Lee]

1. A ball of mass 50g moving horizontally at a speed of 40cm/s strikes a suspended plate of mass 1000g and rebounds from it with a speed of 25cm/s. Find the speed with which the plate commences to move. (Answer: 3.25cm/s)

2. Suppose your hockey stick exerts an average force of 150N south on the ball as you hit it.
a) What is the average force the ball exerts on the hockey stick?
b) Does it follow that the net force on the ball is 0?

 

[Dark Destination]

1. A ball of mass 50g moving horizontally at a speed of 40cm/s strikes a suspended plate of mass 1000g and rebounds from it with a speed of 25cm/s. Find the speed with which the plate commences to move. (Answer: 3.25cm/s)

2. Suppose your hockey stick exerts an average force of 150N south on the ball as you hit it.
a) What is the average force the ball exerts on the hockey stick?
b) Does it follow that the net force on the ball is 0?

Q1. Mass of ball = 0.05 kg.

Change in velocity = 65 m/s.

So F=ma.

F = 0.05 x 65 = 3.25 N.

A 3.25 N force is exerted by the ball on the plate. It has a mass of 1 kg, so it's acceleration will be 3.25 m/s^2.

 

[A*(a*)]

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[yasoyasin]

1: use the equation for momentum. P=v*m.
P=constant momentum
v=velocity in m/s
m=mass in kg
so:
momentum ball before hitting plate: 0.4*0.05=0.02 (towards plate)
momentum of ball after hitting plate: 0.25*0.05=0.0125 (not towards plate)
momentum transferred to plate: 0.02+0.0125=0.0325
0.0325=1,000 * v so v =0.0325 m/s so 3.25 cm per second

2a: third law of Newton: so that is also 150N
2b: no, the force on the ball is still 150N and will accelerate because of this. The total force on all the parts conbined is indeed 0N.