Transcript Ch4-5-review

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?
m1
F
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
a1
F
m2
60
a2 = 2a1
4/3 a1
3/2 a1
1/2 a1
3/4 a1
2/3 a1
47
0
F
m2 m1
a
3
0
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.
.
.
99
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?
0%
0%
0%
0%
0%
1. this slows your initial velocity which is
already upward
2. you don’t go up, you’re too heavy
3. you’re not really pulling down -- it
just seems that way
4. the rope actually pulls you up
5. you are pulling the ceiling down
47
0
0
60
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?
A
F
0%
0%
0%
B
m2
m1
m2
1. Case A
2. Case B
3. Same in both cases
m1
F
47
0
0
60
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%
1. T1 > T2 > T3
47
0%
2. T1 < T2 < T3
0%
3. T1 = T2 = T3
0%
4. all tensions are zero
0%
5. tensions are random
0
0
a
3m
T3
2m
60
T2
m
T1
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.
0%
0%
0%
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%
The component of the gravity force
parallel to the plane increased
60
Normal
47
Net Force
0
Weight
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?
1.
2.
3.
4.
5.
0%
0%
0%
0%
0%
47
0
0
1
2
3
4
5
60
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?
0%
1. 1
60
0%
2. 2
0%
3. 3
0%
4. 4
0%
5. 5
47
0
0
1
2
3
4
5
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 ball’s path,
what is Fc equal to?
60
0%
1. Fc =FT - mg
2.
3.
4.
5.
0%
0%
0%
0%
Fc =FT + FN - mg
Fc =FT + mg
Fc =FT
Fc =mg
top
v
47
0
R
0
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?
1.
2.
3.
4.
5.
0%
0%
0%
0%
0%
47
1
2
3
4
5
2d
4
d
m
0
5
Earth
1
0
60
2
3
2m
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
5 – 2/3 a since m1+m2=3/2m1
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 mv^2/R=mg, then tension=0 and momentarily in
freefall
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