# Physics Worksheet Two

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```					                                  Interactive Physics Worksheet #5 — “Collision”

Overview                                                                                    Time Card
In “Collision” we examine the interaction forces between two objects, one
of which gets pushed by an adjustable force and the other of which gets       Name:
pushed by contact with the first object. Once again we limit our attention
to one-dimensional motion. We will examine the motion of the two                Date         Time In         Time Out
objects before and after they come into contact (i.e., “collide”) and see
what we can discover about the nature of interaction forces.
Please “punch in” and “punch out” of the computer lab using the time card
at right and write your answers to each question on the worksheet itself in
the space provided.

Getting Started
Locate and open the Interactive Physics module—“Collision”—in the
Physics 121 folder. When the window opens you will see a scene like that
shown in Figure 1.
The two objects appear as a
blue square (on the left) and a
red square (on the right), The
masses of the two objects are
to the right is also applied to
the blue object. The
accelerations of both objects
are graphed as functions of
time and their instantaneous
speeds are indicated at the
bottom of the screen.
Make sure the forces and
masses are set to the values
shown in Figure 1 and run the
simulation until both objects
have been pushed to the right
side of the screen. Then stop
the simulation. Notice that
there is a brief period after the
collision during which the
accelerations of the two objects    Fig 1:   The startup screen from “Collision”
are not constant due to the fact
that they bounce against each other a few times before settling down. Drag the frame counter at the bottom of the window to
a time before the collision.
Q1: What was the acceleration of the blue object before the collision? Does this value make sense given the mass and
the applied force?

AJM:10/2/95
Now drag the frame counter to a time well after the collision.
Q2: What were the accelerations of the two objects after the collision? Does this value make sense given the masses
and the applied force?

Q3: How would you answer someone who says , “The accelerations after the collision should be different because the
masses are different and both objects are subject to the same force”?

Notice that, after the collision, two red force vectors labeled “FN” (for “normal force) are shown. These indicate the
magnitudes of the forces of contact between the two objects.
Q4: Which, if either, appears to be the greater contact force—that on the red object or that on the blue object? Does
this make sense? (Why or why not?)

Now reset the simulation and adjust the masses to 5 kg each. (Leave the force at 30 N.) Again run the simulation until both
objects are pushed to the right edge of the screen. Again use the tape player controls to look at frames before and after the
collision.
Q5: Now what are the accelerations before and after the collision and do they make sense given the magnitude of the
force and the object masses? Explain.

Q6: How much net force would be required to give either one of the masses the acceleration that you observe them both
to have after the collision?

Q7: In the case of the red mass, the only force is the force of contact with the blue mass. So what must the contact force
(of the blue mass on the red mass) be? (Give the direction and magnitude.)

Q8: In the case of the blue mass, the net force is the vector sum of the applied force and the force of contact with the red
mass. So, given the applied force and the required net force, what must the contact force (of the red mass on the blue
mass) be? (Give the direction and magnitude.)

AJM:10/2/95
Reset the simulation and adjust the masses to 1 kg (blue) and 9 kg (red). Run until well past the collision. Note the
acceleration of the two objects after the collision.
Q9: What force of the blue mass on the red mass is required to give the red mass this acceleration?

Q10: What force of the red mass on the blue mass is required to give the blue mass this acceleration? (Remember that
the applied force is also acting on the blue mass.)

Q11: Do the relative lengths of the three displayed force vectors agree with your predictions? (If not, go back and

Finally adjust the masses to 9 kg (blue) and 1 kg (red) and run the simulation until well past the collision. Again, note the
post-collision acceleration.
Q12: What force of the blue mass on the red mass is required to give the red mass this acceleration?

Q13: What force of the red mass on the blue mass is required to give the blue mass this acceleration? (Remember that
the applied force is also acting on the blue mass.)

Q14 Do the relative lengths of the three displayed force vectors agree with your predictions? (If not, go back and check

Play with the simulation a little more and see if you can discover anything else. For instance, you can get the blue object
moving and then remove the force to see what happens to the two objects in the collision when there are no external forces
applied.
Q15: What would you say is the most important thing you have learned from this module?

Q16: Any suggestions?

AJM:10/2/95

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