Forces, Uniform Circular Motion, and Gravity Review

October 7

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?

0

0

47

60F a1

m1F a2 = 2a1

m2

F m2 m1 a3

20%

20%

20%

20%

20% 1. 4/3 a1

2. 3/2 a1

3. 1/2 a1

4. 3/4 a1

5. 2/3 a1

Astronauts can not weigh themselves on a Astronauts can not weigh themselves on a conventional scale (no normal force is produced). conventional scale (no normal force is produced). Discuss with your neighbor ways of measuring the Discuss with your neighbor ways of measuring the

mass of an astronaut while in space.mass of an astronaut while in space.

99

1. .

2. .

When you climb When you climb upup a rope, the first thing you do is a rope, the first thing you do is pull downpull down on the rope. How do you manage to go up on the rope. How do you manage to go up

the rope by doing that?the rope by doing that?

0

0

47

600%

0%

0%

0%

0% 1. this slows your initial velocity which is already upward

2. you don’t go up, you’re too heavy3. you’re not really pulling down -- it

just seems that way4. the rope actually pulls you up5. you are pulling the ceiling down

If you push with force F on either the heavy box (mIf you push with force F on either the heavy box (m11) )

or the light box (mor the light box (m22), in which of the two cases below ), in which of the two cases below

is the contact force between the two boxes larger?is the contact force between the two boxes larger?

0

0

47

60

FF mm22

mm11

AA

FF mm22

mm11

BB

0%

0%

0% 1. Case A

2. Case B

3. Same in both cases

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?

0

0

47

60

T3 T2 T13m 2m m

a

0%

0%

0%

0%

0% 1. T1 > T2 > T3

2. T1 < T2 < T3

3. T1 = T2 = T3

4. all tensions are zero

5. tensions are random

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?

0

0

47

60

Net Force

Normal

Weight

0%

0%

0%

0%

0% 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

0%

0%

0%

0%

0%

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?

60

0

0

47

1. 1

2. 2

3. 3

4. 4

5. 5

0%

0%

0%

0%

0%

A rider in a “barrel of fun” finds herself stuck with her A rider in a “barrel of fun” finds herself stuck with her back to the wall. Which diagram correctly shows the back to the wall. Which diagram correctly shows the

forces acting on her?forces acting on her?

0

0

47

1 2 3 4 5

1. 1

2. 2

3. 3

4. 4

5. 5

60

You swing a ball at the end of string in a vertical You swing a ball at the end of string in a vertical circle. Since the ball is in circular motion there has to circle. Since the ball is in circular motion there has to

be a be a centripetal force.centripetal force. At the top of the ball’s path, At the top of the ball’s path, what is what is FFcc equal to? equal to?

0

0

47

60

R

vtop

0%

0%

0%

0%

0% 1. Fc =FT - mg

2. Fc =FT + FN - mg

3. Fc =FT + mg

4. Fc =FT

5. Fc =mg

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?

0

0

47

60

0%

0%

0%

0%

0% 1. 1

2. 2

3. 3

4. 4

5. 5

1 23

4

5

2d

d

2m

m

Earth

Answers

1. 5 – 2/3 a since m1+m2=3/2m1

2. Apply known force and measure acceleration (this assumes inertial mass = gravitational mass, which is now experimental fact)

3. 4

4. 1 – Case A – Fcontact must push a larger mass for same acceleration

5. 1

6. 4 – 1 and 3

7. 2

8. 1 – mg, friction, normal

9. 3 – if velocity is just right so mv^2/R=mg, then tension=0 and momentarily in freefall

10. 2 – gravity is directly proportional to m and inversely proportional to R^2, 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