Interactive Physics Worksheet #5 — “Collision”
Overview
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 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 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.
Fig 1:The startup screen from “Collision”
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 adjustable using the sliders at top right. An adjustable force 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 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?
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.)
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 check your predictions.)
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 your predictions!)
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