Download LAB-11-NewtonsLawsUSE.doc

Newton’s LAWS LAB
You have probably noticed an empty feeling in your stomach when an elevator starts its descent. That feeling is a
result of a decrease of pressure against your feet and a corresponding change in the tightness of the muscles in your
abdomen. You feel less pressure, because the floor of the elevator is going out from under you momentarily. Use
Newton’s Second Law to find out how you could measure this feeling in more concrete terms, and identify the
acceleration of the elevator. According to Newton’s Third Law for every action force there is an equal (in size) and
opposite (in direction) reaction force. Forces always come in pairs — known as "action-reaction force pairs."
Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects
and making two statements describing who is pushing on whom and in which direction. We’ll also, of course, use
Newton’s First Law to describe all aspects of motion.
Purpose: To ACTIVELY investigate Newton’s Laws of Motion. To think about the forces involved, the
directions in which they act, and the motion they may cause.
Read through the entire activity and formulate a simple statement of what you think you will
discover in this activity. Place this statement in your purpose section before you begin.
Data and Calculations: Newton’s 2 nd/3rd Law
In this part you will use the scale in the elevator to calculate the value of the acceleration as the elevator ascends and
descends. Do two trials – one person versus another
1. On the 1st floor – While at rest carefully find the mass of each group member outside the elevator and
convert mass to weight (N) and record your results. Enter the elevator and confirm that the scale records the
same "rest" value for each of your group members. Convert and record this value as the “stopped” or “rest”
Reaction Force (FR). It should be the same as your weight.
2. With one of your group members acting as subject and another acting as recorder, take the elevator UP
to the 2nd floor. Record the reading on the scale as the elevator accelerates upward, continues upward and
as it decelerates at the chosen floor. You should have a couple of seconds when the scale reading is stable.
Be certain to wait until it stabilizes and be certain that the person on the scale is still. Record values for
“starting up”, “continuing up” and “slowing down”.
3. Finally, record a “rest” value when the elevator has come to a complete stop.
4. Repeat the measurements as the elevator descends DOWN to the ground floor. Repeat the measurements
at least three times for each subject and use each team member as a subject.
5. Use your data to find the Net Force acting (Fnet) and the Acceleration of the elevator.
Weight (lbs)
from scale
Trial 1
Stopped (rest)
ELEVATOR
MOVING
UP
starting up
continuing up
slowing down
Stopped (rest)
Stopped (rest)
ELEVATOR
MOVING
DOWN
starting down
continuing
down
slowing up
Stopped (rest)
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Trial 2
Your Weight (N)
Trial 1
Trial 2
Reaction Force - FR
(N) (scale lbs to Newtons)
Trial 1
Trial 2
Fnet = FR - Weight
Trial 1
Trial 2
Acceleration
a = Fnet / m
Trial 1
Trial 2
Newtons 3rd/2nd Law
1. Obtain a scooter. Carefully SIT on the scooter near a wall. By touching only the wall, not the floor,
cause yourself to move away from the wall and “coast” across the floor. Use the physics terms that we
have been discussing for the past few weeks. Be thorough in your descriptions.
a. How far were you able to glide by pushing off from the wall? What is the relationship
between the direction of your push and the direction of motion?
b. In what direction - and on what object - do you apply a force?
c. How do you think, on the basis of amount and direction, the two forces compare?
d. Do you undergo a positive acceleration at any point? In what direction? Explain.
e. What is the source of the force that causes you to positively accelerate?
f. Is there a Net Force involved with you pushing on the wall? Explain.
g. For what distance does the positively accelerated motion last?
h. Neglecting all forms of friction, how far should you travel?
i. Is there any negative acceleration involved here? When does it occur and what causes it?
2. Find another group. Sit on your scooter and push off from a partner on another scooter.
How far were you able to glide by pushing off from another scooter rider? How does this distance compare
to you pushing off of the wall? Be sure you try pushing off partners with different masses and explain any
differences you observed from question 1.
3. Find an empty stretch of hallway and test whether or not accelerated motion occurs with constant force
(THINK ABOUT WHAT THIS MEANS!) applied. One person must sit on the scooter and one must push.
The pusher must try to keep the same force applied to the back of the scooter/rider. Record your
observations then switch places for a second and third trial.
4. Think about ALL OF the forces involved when you are running or walking on a horizontal surface and
answer the following questions.
a. How is walking horizontally on the ground similar to you pushing against the wall with the
scooter? How is it different?
b. Since you move forward and not backwards when you walk, there must be a force in the forward
direction that causes you to move. Identify where the forward force comes from, and compare its size
and direction to the rearward force exerted by your shoe with each step so that you accelerate.
c. Would it be possible to run or walk normally on an extremely slippery ice-skating rink when
wearing ordinary shoes? Discuss why or why not in terms of applied forces.
Questions:
1. Hypothetically speaking, if the elevator piston breaks and the safety brake fails, such that the elevator's
acceleration down is equal to "g", what should you expect the scales to read? Explain.
2. What happens to your weight when you begin your ascent? How long does the change last?
3. What happens to your weight when you begin your descent? How long does the change last?
4. Does a person's initial weight have anything to do with the amount of change recorded? Explain.USE
YOUR DATA IN YOUR EXPLANATION!
5. While driving, Lopez observed a bug striking the windshield of his car. Obviously, a case of Newton's
third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is
greater: the force on the bug or the force on the windshield?
6. How would a fully dressed person that was sitting completely still in the middle of a totally frictionless
– iced over – pond make it to the shore? Think about it. Be thorough.
7. A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from
the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun
backwards. The acceleration of the recoiling gun is ... (Explain your answer)
a.greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
Conclusion: NONE
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USE ALL THREE of Newton’s laws to completely explain your motion in the elevator,
on the scooter and while you run/walk.. Be Thorough. Be Clear. Be Concise.
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