Remember:
1.
Use “LoggerPro” software to collect and analyze data
2.
To collect data, click the collect button on the data collection software, listen for the ticking of the motion detect and then let the cart go.
3.
Keep your hands and everything else clear of the motion detector. It will detect whatever is closest and that may not be the cart if you are not careful.
4.
The motion detector cannot pick up anything closer that ½ meter away.
5.
Make sure the track is level.
6.
"Sketch" means that you should carefully show the shape of the curve produced, but don't bother with exact numbers on the graph.
You open the file you will need for this activity by going File, Open,
, L03A2-1 (Motion & Force).mbl .
1.
Set up the equipment shown below.
Release from rest--keep hand out of way of motion detector
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 hanging on the string ). Take
2 or 3 minutes to sketch a 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 a) acceleration versus time b)velocity versus time c) applied force (from the string) versus time and d) net force vs time. 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?
2.
With the equipment set up as shown above, practice the motion. Make sure you hold the cord of the force probe so it does not pull on the cart. Zero the force probe by clicking the “Zero” button near the top with no force on the string. Then take data with LoggerPro and sketch the same four graphs as in step 1. Remember, show only relevant data (between letting go and stopping). Do your predictions and measurements agree within expected measurement error? If not, what was wrong?
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.
Now set up the cart and track as shown in the picture below. Practice the following motion: Give the cart a gentle push toward the motion detector and then let go. A constant force pulls it in the direction away from the motion detector. (Hold the force probe cord so it does not pull on the cart.)
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.
After watching the motion, but before taking any data with
LoggerPro, sketch your predictions for this case as you did in step 1.
COPYRIGHT

1999, 2000, 2001 Thornton, Sokoloff, Laws, Cummings; Rev. 2004 Bedrosian

4.
Take data with LoggerPro. DO NOT let the cart get closer than ½ meter from the detector .
Sketch the relevant data and compare with your predictions. Do your predictions and measurements agree within expected measurement error? If not, what was wrong?
5.
State in words the relationship between the velocity of the cart and the net force on the cart when the cart is speeding up. State the relationship when the cart is slowing down.
6.
Is there ever a situation where the net force on an object and its acceleration are not in the same direction? If your answer is yes, give an example and have your instructor or TA check it. If your answer is no, use a principle of physics we discussed in class to justify your answer.
An empty elevator car is supported by a cable with tension T. The weight of the elevator (W) is 10,000
N. The plot to the right shows T as a function of t
(time). Use g = 9.8 N/kg for the constant of gravity.
nd
T (N)
11,000
10,000
0 0.2
0.4
0.6
0.8
1.0
7.
Identify the forces on the elevator car. (Ignore friction and air resistance.)
8.
Choose a coordinate system.
9.
Draw a free-body diagram.
10.
Determine for each force whether it is in the positive or negative direction for your coordinates.
11.
Write Newton’s 2 nd Law for your free-body diagram. Make sure all forces are on the left of the equation and m a is on the right. Make sure the signs on your forces agree with 10 above. t (sec)
12.
Solve the equation from 11 for a, the acceleration of the elevator car. You will get an algebraic expression involving T, W, m. You will use this expression in 13.
13.
Draw a graph (not a sketch) of a versus t. Label the axes and make sure the shape of the curve(s) you draw and the values of acceleration are clear.
14.
The elevator is moving down at t = 0.2 seconds. Does that change any of your analysis in steps 7-13?
At that instant, is the elevator speeding up or slowing down? Explain your answer.
COPYRIGHT

1999, 2000, 2001 Thornton, Sokoloff, Laws, Cummings; Rev. 2004 Bedrosian