Forces, Uniform Circular Motion, and Gravity Review

1
Forces, Uniform Circular Motion, and Gravity
Review

• October 7

2
A force F acts on mass m1 giving acceleration a1.
The same force acts on a different mass m2
giving acceleration a2 2a1. What is the
acceleration if the same force pushes on both
blocks?

1. 4/3 a1
2. 3/2 a1
3. 1/2 a1
4. 3/4 a1
5. 2/3 a1

3
Astronauts can not weigh themselves on a
conventional scale (no normal force is produced).
Discuss with your neighbor ways of measuring the
mass of an astronaut while in space.

1. .
2. .

4
When you climb up a rope, the first thing you do
is pull down on the rope. How do you manage to
go up the rope by doing that?

1. this slows your initial velocity which is already
   upward
2. you dont go up, youre too heavy
3. youre not really pulling down -- it just seems
   that way
4. the rope actually pulls you up
5. you are pulling the ceiling down

5
If you push with force F on either the heavy box
(m1) or the light box (m2), in which of the two
cases below is the contact force between the two
boxes larger?

1. Case A
2. Case B
3. Same in both cases

6
Three blocks of mass 3m, 2m, and m are connected
by strings and pulled with constant acceleration
a. What is the relationship between the tension
in each of the strings?

1. T1 gt T2 gt T3
2. T1 lt T2 lt T3
3. T1 T2 T3
4. all tensions are zero
5. tensions are random

7
A box sits on a flat board. You lift one end of
the board, making an angle with the floor. As
you increase the angle, the box will eventually
begin to slide down. Why?

1. The component of the gravity force parallel to
   the plane increased
2. coeff. of static friction decreased
3. normal force exerted by the board decreased
4. both 1 and 3
5. all of 1, 2 and 3

8
A ping pong ball is shot into a circular tube
which is lying flat (horizontal) on a tabletop.
When the ping pong ball leaves the track, which
path will it follow?

1. 1
2. 2
3. 3
4. 4
5. 5

9
A rider in a barrel of fun finds herself stuck
with her back to the wall. Which diagram
correctly shows the forces acting on her?

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• 5

10
You swing a ball at the end of string in a
vertical circle. Since the ball is in circular
motion there has to be a centripetal force. At
the top of the balls path, what is Fc equal to?

• Fc FT - mg
• Fc FT FN - mg
• Fc FT mg
• Fc FT
• Fc mg

11
A planet of mass m is a distance d from Earth.
Another planet of mass 2m is a distance 2d from
Earth. Which force vector best represents the
direction of the total gravitation force on Earth?

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• 4
• 5

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Answers

• 5 2/3 a since m1m23/2m1
  
• Apply known force and measure acceleration (this
  assumes inertial mass gravitational mass, which
  is now experimental fact)
• 4
• 1 Case A Fcontact must push a larger mass for
  same acceleration
• 1
• 4 1 and 3
• 2
• 1 mg, friction, normal
• 3 if velocity is just right so mv2/Rmg, then
  tension0 and momentarily in freefall
• 2 gravity is directly proportional to m and
  inversely proportional to R2, so bigger planet
  farther away in this case has ½ the gravitational
  force, so does not add an equal component to
  total force as the smaller, closer planet