Practice 1: Forces and Motion – Whiteboard Activity

Take care in reading every word in these questions. Make sure you know exactly when we are taking our snapshots in each part of the problem. When making a whiteboard, arrange your work so that all parts of one question can fit on one board (neatly).

A cardboard box with a rubber bottom contains a cinder block at rest on a rough, concrete, horizontal floor.

Draw a system schema for this situation.

Since the shape of the [box + cinder block]—your “system”—is unimportant, shrink it to a point (this is where we are treating the box like a particle) and then show and clearly label each force on the system. Make it obvious from your diagram which forces you intend to be equal and which you intend to be greater than others.

A person shoves the box horizontally so that it begins to move. Your answers to this part should concern the time while the person is still touching the box and shoving. (i) Draw a velocity-vs-time graph for the box, clearly marking the time when the box is at rest and the time when the person is still touching the box and shoving it. This should be qualitatively accurate (no numbers, but correct shape). (ii) Also draw a system schema for that same time period. (iii) Also draw a free body diagram for the box during that same time period. Label!

The shove ends when the box leaves contact with the person’s hands. (i) Draw a qualitatively correct velocity-vs-time graph for the box, clearly marking the time when the box is at rest, the time when the person is still touching the box and shoving it, and the time after the box loses contact with the person’s hands. Make it obvious which lines are horizontal, which have greater slopes, smaller slopes, or negative slopes. (ii) Also draw a system schema for the box when it leaves contact with the person’s hands. (iii) Also draw an FBD.

The rubber is now removed from the bottom of the box so that the cardboard surface rests directly on the same floor. It is then given a horizontal shove by a person. In the space below, modify your velocity vs. time graph as well as your system schemas and FBDs from problem 1 to accurately describe this new situation. Your diagrams do not have to be quantitatively accurate, but make it obvious which forces you intend to be equal and which you

intend to be greater or less than others, so that comparisons can be made among forces in this problem as well as between forces in this problem and in problem 1. Make any differences in your graphs and diagrams obvious.

Draw a velocity-vs-time graph for the box, clearly marking the three time periods.

Draw a system schema and an FBD for this situation while the box is at rest on the horizontal floor.

Draw a system schema and an FBD for this situation while the person is still touching the box and shoving.

Draw a system schema and an FBD for this situation during the time after the box loses contact with the person's hands.

The box is now placed on a very smooth and polished floor. In the space below, modify your velocity vs. time graph as well as your system schemas and FBDs from problem 2 to accurately describe this new situation. Your diagrams do not have to be quantitatively accurate, but make it obvious which forces you intend to be equal and which you intend to be greater or less than others, so that comparisons can be made among forces in this problem as well as between forces in this problem and problems 1 and 2. Make any difference in your diagrams obvious.

Draw a velocity-vs-time graph for the box, clearly marking the three time periods.

Draw a system schema and an FBD for this situation while the box is at rest on the horizontal floor.

Draw a system schema and an FBD for this situation while the person is still touching the box and shoving.

Draw a system schema and an FBD for this situation during the time after the box loses contact with the person's hands.

Suppose that we could somehow succeed in making the floor completely frictionless. Again, make new diagrams/graphs to represent this new variation in the situation. Make any differences obvious.

Draw a velocity-vs-time graph for the box, clearly marking the three time periods.

Draw a system schema and an FBD for this situation while the box is at rest on the horizontal floor.

Draw a system schema and an FBD for this situation while the person is still touching the box and shoving.

Draw a system schema and an FBD for this situation during the time after the box loses contact with the person's hands.

Return to the case of the cardboard box resting on the concrete floor, with friction. A person pushes it hard enough to start it into motion and then continues pushing so that it maintains a constant velocity. In the space below, modify your velocity vs. time graph as well as your system schemas and FBDs from problem 2 to accurately describe this new situation.

Draw a velocity-vs-time graph for the box, clearly marking the three time periods (at rest, velocity is changing, moving with a constant velocity).

Draw a system schema and an FBD for this situation while the box is at rest on the horizontal floor.

Draw a system schema and an FBD for this situation while the box is changing velocity.

Draw a system schema and an FBD for this situation while the box moves with a constant velocity.

After pushing the box with a constant velocity for a while, you reduce your force to half the value needed to maintain a constant velocity. Make a new (continued) velocity-vs-time graph to show what happens to the box while you continue to push with this force.