# Energy and Momentum

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29 Νοε 2013 (πριν από 4 χρόνια και 5 μήνες)

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Energy

and Momentum

1.

You have two springs that are identical except that spring 1 is stiffer than spring 2 ( k
1

>k
2
). On which
spring is more work done (a) if they are stretched using the same force, (b) if they are stretched the
same
distance?

2.

Can the normal force on an object ever do work? Explain.

3.

A coil spring of mass m rests upright on a table. If you compress the spring by pressing down with your
hand and then release it, can the spring leave the table? Explain, us
ing the law of conservation of
energy.

4.

When a “superball” is dropped, can it rebound to a height greater than its original height? Explain.

5.

A car traveling along a level road at speed v slams on the brakes and skids to a stop. If the force of
frictio
n on the car is half the car’s weight, how far does the car slide?

6.

With what minimum speed must you toss a 100 g ball straight up to hit the 10 m high roof of a
gymnasium if you release the ball 1.5 m above the ground? With what speed does the ball hit t
he
ground? Use energy.

7.

A block sliding along a horizontal frictionless surface with speed v collides with a spring and
compresses it by 2.0 cm. What will be the compression if the same block collides with the spring at a
speed of 2v?

8.

A pendulum is form
ed from a small ball of mass m on a string of length L. A peg is height h = L/3
above the pendulum’s lowest point
.

From what minimum angle θ must the pendulum be released in
order for the ball to go over the top of the peg without the string going slack?

9.

You’ve decided to go bungee jumping. You stand on a bridge that is 100
.

m above a river and attach a
30
.

m long
bungee cord to your harness. A bungee cord, for practical purposes, is just a long spring, and
this cord has a spring constant of 40
.

N/m. Assume that your mass is 80
.

kg. After a long hesitation,
you dive off the bridge. How far are you above the wate
r when the cord reaches its maximum
elongation?

10.

A
s a ball falls toward Earth, the momentum of the ball increases. How would you reconcile this
observation with the law of conservation of momentum
?

11.

A

4.5 kg steel ball strikes a massive wall with a speed
of 10.0 m/s at an angle of 60
°

with the surface. It
bounces off with the same speed and angle. If the ball is in contact with the wall for 0.20 s, what is the
average force exerted on the ball by the wall
?

12.

A 10.0 kg block is released from point A
, 2.0 m above

on a track ABCD as shown below. The track is
frictionless except for the portion BC, of length 5.5 m. The block travels down the track and hits a
spring of force constant k = 2250 N/m and compresses it a distance of 0.30 m from its equilibr
ium
position before coming to rest momentarily. Determine the coefficient of kinetic friction between the
track portion BC and the block
.

13.

In a game of billiards, the player wishes to “sink” the target ball in the corner pocket. If the angle to the
corner pocket is 40.0
°
, at what angle
θ is the cue ball deflected? Assume that friction and rotational
motion (“English”) are unimportant, and assume the collision is elastic
.

14.

A 1500 kg car is rolling at 2.0 m/s. You would like to stop the car by firing a 10
.

kg blob of sticky clay
at it.
How fast should fire the clay?

15.

A 20.
g

ball of clay traveling east at 3.0 m/s collides with 30.
g

ball of clay traveling north at 2.0 m/s.
What are the speed and the direction of the resulting 50
.
g ball of clay?

16.

A 10 g projectile is traveling east at 2
.0 m/s when it suddenly explodes into three pieces. A 3.0 g
fragment is shot due west at 10 m/s while another 3.0 g fragment travels 40° north of east at 12 m/s.
What are the speed and direction of the third fragment?

17.

A 200
.

g steel ball hangs on a 1.0
m long string. The ball is pulled sideways so that the string is at a 45°
angle, then released. At the very bottom of its swing the ball strikes a 500
.

g steel block that is resting
on a frictionless table. To what angle does the ball rebound? Assume a
n elastic collision between the
block and ball.