PH 201 Basic Physics I Fall 1995
EXAM III – SYSTEMS OF PARTICLES
All work on this exam must be on your own. You may use a calculator
and a 3" x 5" card of equations. Clearly indicate your final answer for
each question.
Section 1: Multiple Choice. No partial credit will be
given for this section. Circle only one answer for each question.

The center of mass of a system of particles remains at the same place if:

it is initially at rest and the external forces sum to zero

it is initially at rest and the internal forces sum to zero

the sum of the external forces is less than the maximum force of static
friction

no friction acts internally

none of the above

Two objects, P and Q, have the same momentum. Q can have more kinetic energy
than P if it:

weighs more than P

is moving faster than P

weighs the same as P

is moving slower than P

is moving at the same speed as P

Why does a rocket accelerate when it is fired horizontally?

exhaust gases push against outside air

exhaust gases, which the rocket pushes backwards, push the rocket forward

outside air exerts more force on the back of the rocket than on the front

exhaust speed is greater than the rocket speed

atmospheric pressure at the back of the rocket is reduced

Sphere X, of mass 2 kg, is moving to the right at 10 m/s. Sphere Y, of
mass 4 kg, is moving to the left at 10 m/s. The two spheres collide headon.
The ratio of the magnitude of the impulse exerted by X on Y to that exerted
by Y on X is:

¼

½

1/(Ö 2)

1

need to know whether the collision is elastic or inelastic

An elastic collision is one in which:

a momentum is not conserved but kinetic energy is conserved

total mass is not conserved but momentum is conserved

kinetic energy and momentum are both conserved

momentum is conserved but kinetic energy is not conserved

the total impulse is equal to the change in kinetic energy

If the angular velocity vector of a spinning body points out of the page
then, when viewed from above the page, the body is spinning:

clockwise about an axis that is perpendicular to the page

counterclockwise about an axis that is perpendicular to the page

about an axis that is paralleled to the page

about an axis that is changing orientation

about an axis that is getting longer

For a wheel spinning with constant angular acceleration on an axis through
its center, the ratio of the speed of a point on the rim to the speed of
a point halfway between the center and the rim is:

1

2

½

4

¼

A wheel rolls along a horizontal road as shown. The velocity of the center
of the wheel is represented by ® . Point
P is painted on the rim of the wheel. The instantaneous velocity of point
P is:

to the right

to the left

up

zero

Two uniform cylinders have different masses and different rotational inertias.
They simultaneously start from rest at the top of an inclined plane and
roll without slipping down the plane. The cylinder that gets to the bottom
first is:

the one with the larger mass

the one with the smaller mass

the one with the larger rotational inertia

the one with the smaller rotational inertia

neither (they arrive together)

When a woman on a frictionless rotating turntable extends her arms out
horizontally, her angular momentum

must increase

must decrease

must remain the same

may increase or decrease depending on her initial angular velocity
Section 2: Problems and questions. Choose four of the following five
problems.
Do all sections of each problem you choose. If you attempt more than
four problems, please indicate which four you wish to have graded. Partial
credit will be given for this section.
SHOW ALL WORK and JUSTIFY all answers. Be sure your answers include
UNITS where appropriate.

A man is marooned at rest on level frictionless ice. In desperation, he
hurls his shoe to the right at 45 ft/s. If the man weighs 180 lbs. and
the shoe weighs 1.0 lb.

What is the man's speed and direction?

When the man and his shoe are 3 ft apart, where is the center of mass of
the manshoe system?

A 3gram bullet is fired horizontally into a 10kg block of wood suspended
by a rope from the ceiling. The block swings in an arc, rising 3 mm above
its lowest position.

How fast were the block and bullet moving (together) just after the bullet
entered the block?

What was the velocity of the bullet before striking the block?

Is the collision elastic?

Ten seconds after the switch of an electric fan is turned on, the fan rotates
at 300 rev/min.

What is the average angular acceleration of the fan.^{}

Through how many revolutions do the blades turn in these ten seconds?

A 8.0 cm radius disk with a rotational inertia of 0.12 kg·
m^{2} is free to rotate on a horizontal axis. A string is fastened
to the surface of the disk and a 10kg mass hangs from the other end. The
mass is raised by using a crank to apply a 9.0N·
m torque to the disk.

What is the total torque acting on the disk?

What is the angular acceleration of the disk?

What is the linear acceleration of the mass?
brief answers