Question 7 A steel spring is stretched 0.10 m when subjected to 200 N of tension

Question

Question 7

A steel spring is stretched 0.10 m when subjected to 200 N of tension.

How much energy is stored in this stretched spring?

1. 5.0 J 2. 10 J

3. 20 J 4. 40 J

Question 8

A shot-putter accelerates a 7.12 kg shot from rest to a speed of 12.6 m/s.

If this motion takes 1.13 s, what is the average power used by the athlete?

1. 500 W 2. 565 W

3. 780 W 4. 880 W

Question 9

A 3.0-kg object moves to the right at 4.0 m/s. It collides in a perfectly inelastic collision

with a 6.0 kg object moving to the left at 2.0 m/s.

What is the total kinetic energy after the collision?

1. 72 J

2. 36 J

3. 24 J

4. 0 J

Question 10

A 2.0-kg softball is pitched to you at 20 m/s. You hit the ball back along the same path,

and at the same speed. If the bat was in contact with the ball for 0.10 s, what is the

magnitude of the average force the bat exerted?

1. zero

2. 40 N

3. 400 N

4. 800 N

Explanation / Answer

Q 7

force applied on spring = Tension = T = 200 N

extension in spring = x = 0.1 m

for spring ...

kx = T

where k = spring constant

so.. k*0.1 = 200

k = 200 / 0.1 = 2000 N / m

so energy stored = 0.5 * k*x^2 = 0.5 * 2000 & 0.1^2 = 10 J ------- option 2

Question 8

initial speed = u = 0

final speed = v = 12.6 m/sec

time = t = 1.13 sec

so... intial energy = 0.5 * m*u^2 = 0

final energy = 0.5 * m * v^2 = 0.5 * 7.12 * 12.6^2 = 565.1856 Joules

so average power = ( final energy - intial ) / time = ( 565.1856 - 0 ) / 1.13 = 500.16425 Watt

option 1...

Question 9

In a perfectly inelastic collision, after collision the bodies moves after same speed as they stick together... Let this combined speed after collision be v ...

so.. before collison ... momentum = 3*4 - ( 6*2 ) = 0 ( I am assuming right to be positive and left to be negative )

after collision ... momentum = ( 3 + 6 ) * v

so conserving momentum ....

inital = final

so... 9 *v = 0

so final velocity v =0

so ... final kinetic nergy = 0.5 * (3+6) * v^2 = 0 J ---- option 4..

Question 10

Initial momentum of the ball = mass * velocty = 2 * 20 = 40

final momentum = mass * velocity = 2 * ( -20 ) = - 40

so change in momentum = -40 - 40 = -80 kg m/sec

so foce = change in momentum / time = -80 / 0.1 = -800 N

so option 4

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