PHYS 2010             Sample Exam for CH's  6, 7, 8

3]     The concept of "impulse" is often used when dealing with the impact forces involved in
collisions. Two darts are thrown with the same speed at a standing block of wood. Although
they are identical in mass, one dart has a pointed tip (it sticks in the wood) while the other
has a rubber tip (it bounces back from the wood). Explain which dart will be more successful
in providing the force necessary to knock over the block of wood using the concept of

Answer: Need to show impulse provided by rubber dart will be twice that of pointed tip dart!

4]     Ball 1 (mass = 2.0 kg) travels to the right with a speed of 12 m/s. It has a head-on, inelastic
collision with Ball 2 (mass = 6.0 kg) which is traveling to the left with a speed of 4.0 m/s.
The balls do not sticktogether ; they recoil from each other. Ball 1 recoils with a speed of 8.0 m/s.
What was the impulse imparted to Ball 2 ?

Answer: 40 N x sec (opposite initial vel. direction) *[will need to find final vel. of ball 2 = 2.7 m/s]

3]     A)   A 10.0 kg object that is moving to the right with a constant velocity collides head-on with
a 15.0 kg object that is initially at rest. The two objects stick together and move off to the right
with a velocity of 8.00 cm/s. What was the initial velocity (don’t forget direction!) of the
10.0 kg object?

Answer: 20.0 cm/s to the right

B)   Now the stuck together masses moving to the right as one object have an elastic head-on
collision with a 25.0 kg mass that is at rest. Give the final velocities of both objects (the stuck
together masses and the 25 kg mass) after the collision.

Answer:    stuck together, v = 0 cm/s               25 kg mass, v = 8.00 cm/s to the right

4]     A baseball of mass 0.14 kg has an initial velocity of 38 m/s as it approaches the bat. The bat
applies a force very much larger than the weight of the ball and the ball departs from the bat
with a final velocity of 58 m/s in a direction exactly opposite to its initial velocity. If the contact
time between the bat and the ball is 4.0 x 10-3 s, what is the average force exerted on the ball
by the bat?

1]     A 0.150 kg object tied to a string is whirled in a horizontal circle at a constant angular speed
of 3.50 revolutions/sec. The tension in the string is 75.0 N. What is the length of the string?

2]     A wheel of diameter 0.80 m is initially at rest. It is then rotated with a constant angular
acceleration of 5.0 rad/s2 for exactly 10 revolutions of the wheel. Determine both the angular
speed of the wheel and the tangential speed for a point on the edge of the wheel at the end
of the 10 revolutions.

1]     A 150 gm object tied to a string is whirled in a horizontal circle such that the tension in the
string is 75.0 N when the angular speed is 3.50 revolutions/sec. How many meters, s, does
the object travel in 5.00 seconds? [ NOTE s is the circumferential distance traveled around
the circle.]

3]     A uniform beam 3.00 meters long has a weight of 245 N. It is supported on its left end by a cable.
A 500 N weight is located on the beam a distance of 1.00 meter from the left end. You supply a
force to the right end of the beam so that the beam is in equilibrium in the horizontal position (see
sketch). What are the tension in the cable, T, and the upward force, F, you must apply to the beam?
{HINT : Apply the equilibrium condition for two different axis’ of rotation !!}

ANSWER: T = 456 N , F = 289 N

5)   It is correct to say that impulse is equal to

A) momentum

B)  the change in momentum

C) the force multiplied by the distance the force acts

D) all the above   (A, B, and C)

E)  none of the above (A, B, or C)

6)  A bullet is fired from a gun.  The speed of the bullet will be about the same as the speed of the recoiling gun

A) as long as momentum is conserved.

B) because velocity will be conserved.

C) because both velocity and momentum are conserved.

D) only if the mass of the bullet is about the same as the mass of the gun.

E) none of the above are correct

7)   Compared with falling on a wooden floor, a wine glass may not break when it falls on a carpeted floor because

A) the impulse imparted to it will be smaller.

B) the time during which the stopping force is applied will be longer.

C) both A) and B) are true.

D) neither A) nor B) are true.

8)  Which one of the following statements is NOT true for a rotating CD undergoing angular acceleration?

A) The angular speed is the same at any point on the disk
B) The linear speed is the same at any point on the disk
C) The angular accerlation is perpendicular to the centripetal acceleration
D) It may have zero angular velocity
E) The angular accerlation is the same for any point on the disk

9)   A torque can be exerted on a body with a fixed axis of rotation

a. only by a centripetal force.
b. only by a force directed radially outwards.
c. only by a tangential force.
d. only by a force with a component directed radially outwards.
e. by any force perpendicular to but not pointing directly toward or away from the axis of rotation

10) Five identical cylinders are each acted on by forces of equal magnitude.
Which force exerts the biggest torque with respect to an origin located at the center of the cylinder?

5)  B
6)  D
7)  B
8)  B
9)  E

10)  A