Download Newton`s Third Law of Motion, Impulse and Momentum

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Investigation 3
Newton’s Third Law of Motion, Impulse and Momentum
Newton’s Third Law of Motion: “When one object exerts a force on a second, the second object exerts an
equal and oppositely directed force on the first.;” or “To every action there is an equal and opposite
reaction.”
Answer the following questions are clearly and concisely as you can.
1.
A physics student holds a ball in her hand at rest.
a.
b.
The two forces that are acting on the ball are:
1)
the gravitational force (gravity) – the earth pulling downward on the ball (noncontact)
2)
her hand exerts a force on the ball upwards (contact)
Name the reaction forces to the two forces above and indicate the directions of the forces.
1)
2)
2.
The physics student now releases the ball (lets the ball drop). Consider the ball as it falls toward the
ground without air resistance. The force that is acting on the ball as it is falling is the gravitational
force (gravity) pulling downward. Name the reaction force and indicate its direction.
3.
A physics student who weighs 800 Newtons (about 170 lbs) stands on a bathroom
scale in an elevator that is not moving (at rest).
a.
b.
The two forces that are acting on him are;
1)
the gravitational force (gravity) pulling downward with a force of 800
Newtons
2)
the bathroom scale pushing upward on him with a force of 800 Newtons
Name the reaction forces and indicate their strengths (in Newtons) and their directions.
1)
2)
calculator
4.
A physics textbook is at rest on a table. A calculator is
placed on top of the book. Name the three forces that are
acting on the book. Name the corresponding reaction forces.
book
table
action force
reaction force
1) table pushing upward on the book
1) book pushing downward on the table
2)
3)
5.
A physics student sitting in a stationary Lamborghini (car), is holding onto the steering wheel, and is
strapped in with her seat belt. She knows that in the vertical direction, 1) the gravitational force
pulls her downward and 2) the seat pushes her upward and that the net vertical force is zero, thus
resulting in no upward or downward acceleration. In the horizontal direction, 3) the back of the seat
exerts force on her and pushes her forward, and 4) the steering wheel exerts a force on her and pushes
her backward.
Name the reactions forces to each of the four forces described above, and indicate their directions.
reaction force
1)
2)
3)
4)
direction
Impulse and Momentum
The Impulse-Momentum relationship says: Impulse = Force x time of impact = change in momentum
1.
2.
An unfortunate bug splatters on the windshield of a car traveling at 60 mi/hr on the freeway.
a.
Compare the force of the car on the bug to the force of the bug on the car. Which one is greater?
b.
The time of impact is the same for both the bug and the car. Compare the impulse on the bug to
the impulse on the car. Which one is greater?
c.
Compare the change in momentum of the bug to the change in momentum of the car. Which
one is greater?
d.
Does the bug or the car undergo the greater acceleration? Explain briefly.
A block with a mass of 8 kg slides on a horizontal, frictionless surface at 4 m/s. It collides with and
sticks to a 2 kg block that is at rest.
a.
What is the value of the momentum of the 8 kg block before the collision occurs?
b.
What is the value of the momentum of the 2 kg block before the collision occurs?
c.
What is the value of the total momentum of the system before the collision occurs?
d.
What should the value of the total momentum of the system be after the collision occurs?
e.
How fast are the blocks traveling after the collision occurs?
Impulse and Momentum
3.
A 50 kg bungee jumper jumps off a bridge. She is in free fall for 3 sec. At 3 sec the bungee cord
begins to stretch, slows her down, and brings her to a stop 2 sec later (a total of 5 seconds). Take the
acceleration due to gravity to be 10 m/s2.
Fill in the blanks in the following table.
type of
motion
time (sec)
velocity
(m/s)
0
0
momentum
(kg m/s)
1 sec
2 sec
1
free
fall
2
3
3 sec
slowed by
bungee
v=0
4
5
0
5 sec
By how much did her momentum change from time t = 3 sec to time t = 5 sec?
____________________ kg m/s
What is the value of the impulse on her from 3 sec to 5 sec??
____________________ Newton sec
What is the average force that acted on her by the bungee cord from 3 sec to 5 sec?
____________________ Newtons
Would there be a problem if the bungee cord stopped her in 1/10 sec instead of 2 sec? Explain.
Momentum – Recoil
4.
A steel ball (A = 4 kg) and a wooden ball (B = 1 kg) are at rest and separated by a
compressed spring.
a.
What is the value of the total momentum of the system before the spring is released?
The spring is now released causing the balls to recoil in opposite directions. In this case,
b.
what should be the value of the total momentum of the system?
c.
Does the spring exert a greater force on ball A or ball B?
d.
Which ball has a higher recoil speed? Why? Explain.
A
B
spring
5.
If you were to jump straight upward from the earth’s surface, will the earth recoil? Explain.
Two-dimensional Elastic Collision
6.
The diagram shows the top view of the corner of a pool table with the cue ball and the eight ball.
Carefully draw the position of the cue ball when it makes contact with the eight ball so that it causes
the eight ball rolls into the corner pocket. Show also the path of the cue ball takes after the collision.
cue ball
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