Newton S Second Law of Motion

Experiment 3b Newton’s Second Law

How does a cart change its motion when you push and pull on it? You might think that the harder you push on a cart, the faster it goes. Is the cart’s velocity related to the force you apply? Or does the force just change the velocity? Also, what does the mass of the cart have to do with how the motion changes? We know that it takes a much harder push to get a heavy cart moving than a lighter one.

You will use a motion detector and force probe to investigate these questions as shown below.

< 0.5 m < Again be sure that the force probe handle does not extend beyond the back of the cart, and tape the cable back along the handle to assure that the motion detector "sees" the cart. At least 1.5 m to floor

OBJECTIVES  Collect force and acceleration data for a cart as it moves along the ramp.  Determine the relationship between force, mass, and acceleration.

MATERIALS Power Macintosh or Windows PC Logger Pro Lab Pro low-friction dynamics cart Vernier Dual Force Sensor Hanging Mass Motion Detector String Ramp Pulley

PRELIMINARY QUESTIONS 1. An astronaut in space throws a 2 kg rock and a 4 kg rock each with the same amount of force. Do the rocks both leave his hands at the same speed or different speeds? Explain your reasoning to me! 2. Is the force on the cart (diagram above) a constant force or is it changing as the hanging mass gets closer to the ground. Explain your reasoning to me! 3. In the absence of friction and other forces, if you exert a force, F, on a mass, m, the mass will accelerate. If you exert two times the force on a mass of 2m, would you expect the resulting acceleration to be twice as large or the same? Explain!

Physics with Computers 9 - 1 Experiment 3b Fawcett Handy Lab

PROCEDURE 1. Go to Group Shares – Fawcettam - Handout – Newton’s 2nd Law-Pulley.cmbl. You should now have velocity and force graphs on your screen. 2. Connect a Dual Range Force Sensor to Channel 1 (CH1). Connect the motion detector to DIG/SONIC 1 Channel on the Lab-Pro unit. 3. Attach the Force Sensor to a dynamics cart. Find the mass of the cart with the Force Sensor attached. Record the mass in the data table. 4. Place the cart on the horizontal ramp. Make sure the cart is not moving and zero the force sensor by clicking 5. Set up the ramp, pulley, cart, string, motion detector and force sensor as shown in the diagram. It is important to choose the amount of the falling mass so that the cart doesn’t move so fast that you can’t observe the motion. Experiment with different hanging masses until you get the cart to move across the ramp in about 2-3 seconds after the mass is released. Note: You will have to change the scale on the x-axis to about 3 seconds. Record the hanging mass that you decided to use in the data table. 6. Click on the button and when the motion detector starts clicking let go of the cart. You will need to catch the cart before it hits the end of the ramp! Be sure that the cable from the force probe is not seen by the motion detector, and that it doesn’t drag the cart. Repeat until you get good graphs in which the cart is seen by the motion detector over its whole motion. Use the autoscale button on the tool bar to make your data fit better on the axes. 7. Click on the force graph. We will now use the computer to find the average force acting on the cart during its motion. First drag the cursor across the force graph over a period of time where the force remains relatively constant and non-zero. Then go to the ANALYZE menu and select STATISTICS. Record the average force given to you in the data table. 8. Click on the velocity graph. Drag across the part of the graph where the cart was accelerating at a constant non-zero rate. Click the linear fit button and determine the average acceleration of the cart from the slope. Record your answer for the average acceleration on in the data table. Print the v-t and F-t graphs for WT 1. Include the fit and stat boxes. 9. Repeat steps 6-8 for three more trials using different hanging weights. Determine the mean force and mean acceleration and record them in the data table. 10. Add 500g to the cart. Use your first hanging mass. Repeat steps 6-8 above.

9 - 2 Physics with Computers Newton’s Second Law

DATA TABLE

MASS OF CART AND SENSOR = ______GRAMS = ______KILOGRAMS

PHYSICAL QUANTITY WT 1 WT 2 WT 3 WT 4 TRIAL 1 +500G

HANGING MASS (GRAMS)

MEAN FORCE (N)

MEAN ACCELERATION (M/S2)

ANALYSIS 1. Plot a graph of mean force (y-axis) vs. mean acceleration (x-axis) for WT 1-4. Can you use (0,0) as a data point? Produce a linear fit and make sure that the equation is displayed on your graph. Print out the graph with correct axes labels, title and group members names. 2. If you double the hanging mass, what effect does this have on the motion of the cart? 3. What does the slope of the graph (above) represent? Is it the mass of the cart, the hanging mass or the sum of the two masses? Explain your ideas! 4. What happened to the cart’s motion when you increased the cart’s mass? Explain!

Physics with Computers 9 - 3 Experiment 3b Fawcett Handy Lab

Scoring Sheet

1. Student has answered the 3 preliminary questions fully ______(6)

2. Student has produced a fully labeled data table with their mean force and acceleration recorded for each hanging mass ______(5)

3. Student has printed out the v-t and F-t graphs for WT1 and included fit and stat boxes for the correct intervals. ______(10)

4. Student has produced a fully labeled graph of mean force (y-axis) vs. mean acceleration (x-axis) for WT 1-4. They have included a linear fit with curve fit equation. The graph has title and group members’ names. ______(10)

3. Student has explained what effect doubling the hanging mass will have on the motion of the cart. ______(3)

4. Student has explained what the slope of their graph represents. ______(3)

5. Student has explained what happened to the cart’s motion when the mass of the cart was increased. ______(3)

Total Score ______(40)

9 - 4 Physics with Computers