Untitled Document

Newton's Laws in One Dimension
Modified 8/13/03
With proper referencing, educators are welcome to use this for instructive purposes. Any other permission of publication (written or electronic) is denied without express prior consent from Dr. Philip Casabella.

1. Set up the equipment such that a track is placed on a table with one end of the track lining up with one end of the table. Place a motion detector at the end of the track farthest from the edge of the table. Place the cart on the track near the detector. Connect a string to the cart and thread it through a pulley at the opposite end of the track nearest the table's edge. Connect a weight to the string such that it can be dropped off the end of a table.

The frictional force acting on the cart is very small (almost no friction) and can be ignored. The cart is pulled with a constant force (the applied force, due to the weight hang on the string). Take 2 or 3 minutes to sketch an INDIVIDUAL prediction of what the following four graphs will look like for the motion shown above (cart starts close to the motion detector and moves away speeding up at a constant rate). Sketch predictions of:

Ignore the stop at the end of the track. Label your graphs "moving away and speeding up". Applied and net force are the same in this case. Why? Take another 2 or 3 minutes to compare your predictions with others in your group. Justify to each other why you believe that the graphs that you drew in the step above are the correct graphs. Come to a conclusion as a group about what you expect the graphs to look like.

2. With the equipment set up as above, practice the motion. Then take and record the actual data on your activity sheets. Ignore the stop at the end of the track. Label your graphs as in the prediction step above. Make sure that you include only relevant data in your sketch of the graph. Do your prediction graphs and actual graphs agree? If not, one of them is wrong. Determine which one it is. If your actual data is not correct (because you got your hand in the way of the detector or got too close to the detector or something) you must retake your data. Experimental results will be graded for correctness. Remember, record only relevant data.

Wondering what the position versus time graph would look like for this motion? You can display that graph by doing the following: Point your cursor to the word acceleration on the y axis of the bottom graph and left click. You will get a check box option menu. Remove the check from acceleration and add a check next to distance.

3. Take the friction pad provided and mount it on the bottom of the cart. The frictional force acting on the cart is now increased. Set the cart up again as shown in the figure above. The cart is pulled with the same constant force (the applied force due to the weight) as in question above. Adjust the friction pad so that the cart still moves under the pull of the hanging weight, but also make sure that the pad is dragging on the track to introduce a frictional force. (Fold the felt to make three layers and staple them if needed.) The cart starts close to the motion detector and moves away speeding up at a constant rate. Take 2 or 3 minutes to sketch on your graphs from above, an individual prediction of the velocity and acceleration of the cart and predictions of applied and net force on the cart after it is released. Note that the applied and net force are different now. Why? Which determines the acceleration?

4. Practice the motion (cart starts close to the motion detector and moves away, speeding up as it goes). Then take and record the actual data on your activity sheets. Ignore the stop at the end of the track. Label your graphs as in the prediction step above. Make sure that you include only relevant data in your sketch of the graph. Do your prediction graphs and actual graphs agree? If not, one of them is wrong. Determine which one it is.

5. Remove the friction pad from the cart so that you can ignore friction again.

6. Now start the cart as far away from the motion detector as possible and give it a gentle push toward the motion detector and then let go. A constant force pulls it in the direction away from the motion detector. The cart moves toward the motion detector slowing down at a steady rate (constant acceleration), comes to rest momentarily and then moves away from the motion detector speeding up at a steady rate. Before taking any data, sketch your individual prediction of the velocity, acceleration and net force for this motion after the cart is released and before it is stopped. Label your predictions "motion toward the detector, slowing down at a constant rate". Compare your predictions with those in your group.

7. Practice the motion above. DO NOT let the cart get closer than ½ meter from the detector. Then take and record the actual data on your activity sheets. Ignore the push to start the cart and the stop at the end of the track. Label your graphs as in the prediction step above. Make sure that you include only relevant data in your sketch of the graph. Do your prediction graphs and actual graphs agree? If not, one of them is wrong. Determine which one it is.

8. State in words how the signs of velocity and acceleration are related for speeding up and slowing down. Is the acceleration always positive if the object is speeding up? Is the acceleration always negative if the object is slowing down? State in words how the sign of acceleration is related to the sign of force. If the acceleration of an object is constant, does the net force acting on the object have to be constant too? Is there ever a situation where the net force and acceleration are not in the same direction?