Phy 131 Classical Physics I
Homework 4
Instructor: MacKenzie Lenz
Due 9/29/21 @ 11:59 pm
1. Problem 1: Free Body Diagrams
Draw a free body diagram for each of the situations below:
(a) A cat is sitting on a window sill.
Solution:
A good practice when drawing Free Body Diagrams (FBD) is to identify/specify your system and
then name all your forces withing that system acting on the object.
(b) An ice hockey puck glides across frictionless ice.
Solution:
Frictionless means that there is no friction force. Therefore, even though the puck is moving, its
net force should be zero because it has a constant speed. The net force does not always tell you
the direction of motion of an object.
(c) Your physics textbook is sliding across the table.
Solution:
Now we have friction in the system so we can see a non-zero net force. This is kinetic friction
because the book is moving.
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(d) A rocket is being launched straight up. Air resistance is not negligible.
Solution:
Here we have two forces in the same direction. You still want to draw them from a point at your
origin but can put them a little to the side to distinguish the size of the arrows. We should note
that for the rocket to move upward we need to have a F~thrust that is greater than the sum of the
arrows for F~drag and F~g .
(e) Your friend went for a loop-the-loop ride at the amusement park. Her car is upside down at the
top of the loop.
Solution:
Being upside down means that the normal force from the car and the force of gravity point in the
same direction.
(f) You’ve jumped down from a platform. Your feet are touching the ground and your knees are
flexing as you stop.
Solution:
This landing from our jump means we are slowing down from the speed of our drop so I know the
force is not zero and should point upward to decrease initial velocity.
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2. Problem 2: Newtons 1st Law
If a car stops suddenly, you feel “thrown forward.” We’d like to understand what happens to the
passengers as a car stops. Imagine yourself sitting on a very slippery bench inside a car. This bench
has no friction, no seat back, and there’s nothing for you to hold onto.
(a) Draw a picture and identify all of the forces acting on you as the car travels at a perfectly steady
speed on level ground.
Solution:
Our person is not moving and has an initial velocity that is equal to that of the car so the only
G
forces acting on them are the Force of gravity from the earth, FEP
, and the normal force of them
N
sitting on the bench, FBP .
(b) Draw your free-body diagram. Is there a net force on you? If so, in which direction?
Solution:
Below is the free body diagram for this situation with my two forces.
(c) Repeat parts a and b with the car slowing down.
Solution:
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Since there is no friction and not back to my chair, I know that the forces on the person are the
same as they were before since nothing attached to the car is acting on the person. So we still
G
have the Force of gravity from the earth, FEP
, and the normal force of them sitting on the bench,
N
FBP .
For the same reasoning I know that the free body diagram will be identical to the one in part b
since nothing changed in terms of interacting with the person.
(d) Describe what happens to you as the car slows down.
Solution:
Since nothing from the car acts on me I don’t slow down as the car does. This means that the
bench I am sitting on in the car would move out from underneath me.
(e) Use Newton’s laws to explain why you seem to be “thrown forward” as the car stops. Is there
really a force pushing you forward?
Solution:
Newton’s first law states that objects in motion stay in motion unless acted on by a force. This
is why I feel thrown as the car slows down because I have an initial velocity that no force acts to
slow so I keep moving while the car below me slows down.
(f) Suppose now that the bench is not slippery. As the car slows down, you stay on the bench and
don’t slide o↵. What force is responsible for your ‘deceleration’ ? In which direction does this
force point? Include a free-body diagram as part of your answer.
Solution:
The force due to friction between me and the seat would be the force that keeps me on the bench
in the car. It would oppose my motion so if the car is moving right originally, then my motion is
to the right and the friction would point to the left. This could be in the same direction as the
negative acceleration of the car causing me to slow down at the same rate as the car.
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