Download Kreutter: Dynamics 11 Lesson 11: Newton`s Third Law: Quantitative

Kreutter: Dynamics 11
Lesson 11: Newton’s Third Law: Quantitative
11.1 Observe and Find a Pattern
The goal of this experiment is to determine a mathematical relationship between the force that
object A exerts on object B and the force that object B exerts on object A when the two objects are
interacting.
Available Equipment: Force probe sensors with hooks on the end and computers.
Note on equipment: You will be using a new sensor for this experiment; it is called a force probe. A
force probe is a sensor that sends a signal to the computer indicating the force exerted on its tip. The
software interface helps you plot force as a function of time for two different force probes.
a) Take one of the probes, connect it to the computer, and gently pull or push on it. Examine
the graph on the screen and make sure it makes sense to you.
b) Design and perform enough experiments to find a pattern in the readings of the two probes
when they record forces that two interacting objects exert on each other. For example,
place one force probe on the table and tap it gently with the second probe. The probes are
very delicate, and if you use them to tap each other, you must do it lightly. Even a light tap
will register on the graph.
c) Write a paragraph summarizing the experiments and your findings. Be sure to include the
following:
1) Describe the setup for each experiment and sketch the graphs that you see on the
computer.
2) Find a pattern in the pairs of graphs representing the force-versus-time functions
recorded by each probe during an interaction.
3) Formulate a tentative rule relating the force that object A exerts on object B to the
force that the object B exerts on the object A. This is your hypothesis.
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
11.2 Test Your Hypothesis
Available Equipment: Force probe sensors with hooks on the end, a track, carts, objects of different
masses to put on the carts, cushions, elastic bands, and computers.
a) State the rule you are going to test which you devised in 11.1.
b) Brainstorm a list of possible experiments whose outcome can be predicted with the help of
the rule. Choose two experiments from your list. Check with Mrs. K about the validity of your
experiment.
c) Briefly describe your experiment. Include a labeled sketch.
d) Use the tentative rule (hypothesis) to make a prediction about the outcome of each
experiment.
e) Perform each experiment. Record the outcome.
f) Does the outcome support the prediction? Explain.
g) Based on the prediction and the outcome of the experiments, what is your judgment about
the rule?
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
h) Think of the assumptions that you used to make the predictions for the testing experiments.
How could these assumptions have affected your judgment?
i)
Jigsaw with other groups (Mrs. K will help with this—basically count to 4 in each lab group;
then the 1’s all get together, the 2’s, etc) and discuss your experiment. The goal of this jigsaw
is to see if other experiments performed by the other lab groups also supported your
hypothesis.
j)
Use the attached rubric to write your report. Notice that the rubric for this lab report has a
section about the incorporation of other groups’ results into your lab report. Therefore
during the jigsaw discussion it is important to discuss and understand every other group’s
experiment.
11.3 Reason
Use the rule that you devised and tested to decide who exerts a larger force for the following
situations:
1.
A mosquito on a car’s windshield or a car’s windshield when a mosquito smashes into it;
2. A reflex hammer (Taylor hammer) on your knee or your knee on the hammer when a doctor
taps your knee with it.
Reconcile your answers with your observations of these phenomena.
Newton’s Third Law of Motion: When two objects interact, object 1 exerts a force on object 2.
Object 2 in turn exerts an equal-magnitude, oppositely directed force on object 1:
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
r
F1 on 2   F2 on 1 . Each force above is caused by one object and is exerted on another object. Since
these two forces are exerted on two different objects, they cannot be added to find a net force.
11.4 Reason A horse is pulling a carriage. According to Newton’s third law the force exerted by the
horse on the carriage is the same in magnitude but points in the opposite direction of the force
exerted by the carriage on the horse. How are the horse and carriage able to move forward?
(a) The horse is stronger.
(b) The total force exerted on the carriage is the sum of the forces exerted by the horse and the
ground’s surface in the horizontal direction.
(c) The net force on the horse is the sum of the ground’s static friction force on its hooves and the
force exerted by the carriage.
(d) Both b and c are correct.
Homework
11.5 Reason
a) A book sits on the tabletop. What is the Newton’s Third Law pair for the force that Earth
exerts on the book?
b) If Earth exerts a 5 N force on the book, what is the force that the book exerts on Earth?
c) What is the acceleration of the book if Earth is the only object exerting a force on it?
d) Why does the book fall onto Earth but Earth does not fall onto the book? The mass of Earth is
6.00 x 1024 kg.
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
Use Earth’s mass to calculate Earth’s acceleration. What does this tell you about the motion of Earth?
e) The Sun’s mass is 2.00 x 1030 kg. It pulls on Earth, exerting a force of about 1020 N. What is the
force that Earth exerts on the Sun?
11.6 Reason
Two students sit on office chairs with wheels. Student A pushes student B away from him. Student B
does nothing. Does student B exert the force on A? Explain your reasoning.
11.7 Reason
a) You hit a stationary puck with a hockey stick. The stick exerts a 100 N horizontal force on the
puck. What is the force exerted by the puck on the stick? How do you know?
b) A truck rear ends a small sports car that is moving in the same direction as the truck. The
collision makes the truck slow down and the sports car is propelled forward. What object
exerts a larger force on the other object: the truck on the car or the car on the truck? Explain
how your answer is consistent with Newton’s third law and with the fact that the sports car
is damaged more than the truck.
11.8 Reason
Use Newton’s third law to predict what will happen if you try to open a door wearing rollerblades.
Draw a force diagram to help make the prediction.
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
11.9 Represent and Reason
Your friend says that if Newton’s third law is correct, no object would ever start moving. Here is his
argument: “You pull a sled exerting a 50 N force on it. According to Newton’s third law the sled
exerts the force of 50 N on you in the opposite direction. The total force is zero, thus the sled should
never start moving. But it does. Thus Newton’s third law is wrong.”
What is your opinion about this answer? How can you convince your friend of your opinion?
11.10 Reason
The Moon orbits Earth because Earth exerts a force on it. The Moon, therefore, has to exert a force
on Earth. What is the visible result of this force?
11.11 Regular Problem
A person of mass m is standing on the floor of an elevator that starts from the first floor and reaches
the 21st floor.
a) Make two kinematics models for the motion of the elevator. Describe them in words. What
is the same about the two models? What is different?
b) Now describe the same models with motions diagrams, with position, velocity and
acceleration versus time graphs, and with algebraic functions.
c) Choose one of the models and draw force diagrams for the person for three different parts
of the trip.
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
d) Write a mathematical expression that will help you determine the magnitude of the force
that the person exerts on the floor when the acceleration of the elevator is upward and
again when the acceleration is downward. What is a reasonable magnitude for the elevator’s
acceleration?
e) Who is pushing harder – the elevator’s floor on the person or the person on the floor? Earth
on the person or the floor on the person?
11.12 Reason
Your young sister jumps off a chair holding a 0.50-kg container of ice cream. What is the force
with which the container pushes on her hand when she is in the air? Explain.
(a) Almost 5 N .
(b) The same as she exerts on the container.
(c) 0 N .
(d) Both b and c are correct.
11.13 Reason A spaceship moves in outer space. What happens to its motion if there are no
external forces exerted on it? What happens to the motion if there is a constant force exerted on
it in the direction of its motion? If something exerts a force opposite its motion?
(a) It just keeps moving; it speeds up with constant acceleration; it slows down with constant
acceleration.
(b) It slows down; it moves with constant velocity; it slows down.
(c) It slows down; moves with constant velocity; it stops instantly.
11.14 Reason You stand on a bathroom scale in a moving elevator. What happens to the scale
reading if the cable holding the elevator breaks suddenly?
(a) The reading will increase. (b) The reading will not change.
(c) The reading will decrease a little.
(d) The reading will drop to 0 instantly.
11.15 Estimate Imagine that all people on Earth decided to jump simultaneously. What will happen
to Earth?
Reflect: What did you learn about Newton’s third law so far? How is it different from the
second law? Give 3 examples of Newton’s third law from everyday life.
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey
Kreutter: Dynamics 11
Adapted from PUM: Dynamics
©2010, Rutgers, The State University of New Jersey