# Momentum and Force Lab

 Materials: School Network and Macintosh Computer w/Interactive Physics Time Allotment: 3 Class Days

### Purpose:

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This lab must be done on a Macintosh computer connected to the school network or upon a computer in the Science Computer Lab. To prepare for the lab, do the following steps.

1. Open the Physics Explorer application (Two Bodies model) by double-clicking on its icon; it is found in the directory /Local Drive/Physics/Physics Explorer/Two Bodies.
2. Choose Open... from the File menu and navigate to the Physics 163/Unit 4 folder of the same directory.
3. Open the file titled Exploring Two Bodies by double-clicking on its name. A Physics Explorer simulation is opened.
4. Open the file titled Momentum and Force using the appropriate button in the bottom window. Check that the middle window appears as shown below.

### Make a Prediction:

If you've ever driven a car, you know that the faster you're going, the more room you need to slow down. As a rule of thumb, safety experts recommend that you leave a minimum of one car length between you and the car ahead of you for every 10 miles per hour that you are traveling. If your car is heavily loaded, it may take even a longer distance to stop. A question arises: How does a car's speed and mass affect its stopping distance? Tricky question: Which stops more quickly - a lighter car traveling at high speed, or a more massive car traveling more slowly?

In this lab you will study the braking behavior of model vehicles with different mass and velocity, and find out what affects the braking time and the braking distance.

### 1.Practice Runs

a. Click on the "Momentum and Force" button in the Selecting Exploring Activity window.

Imagine you are looking at the remote controls for two model vehicles, a car (the patterned object) and a truck (the solid object). The "Go" button sets the vehicles in motion. The "Brake" button has the effect of gradually bringing them to a halt, like taking your foot off the accelerator and stepping on the brake pedal. The braking force is assumed to be the same for the two vehicles, independent of vehicle weight.

The Velocity Graph lets you watch how the velocities of the vehicles change during the course of the simulation. You also have controls for changing the mass of the vehicles and their initial speed.

To adjust the brakes click on the "Adjust Brakes" button. In the Braking force window, change the breaking force to 250 Newtons by clicking on the "Plus" or "Minus" buttons repeatedly. (The buttons change the force in increments of 25 Newtons.)

b. With the HAND tool, click on the "GO" button in the Control window (the middle window). Both vehicles have a velocity of 40 m/s in the x-direction and a mass of 50 kg.

• After the vehicles reach the marker, click the "Brake" button once.

• What do you notice about the stopping time and distance of each vehicle?

• Why is this so?

### 2. Braking Time

a. Increase the velocity of the car to 80 meters per second, and reduce the mass to 25 kg.

• What is the momentum of the car under these conditions? And that of the truck?

• Predict what you will see in the Model window. Draw a diagram (in the space below) of the relative positions of the two vehicles when the brakes are pressed, when they come to a stop, and include the trace lines in between (i.e., an "oil drop diagram").
• Also predict roughly what you will see on the Velocity-time graph. Draw the truck as a solid line and the car as a dotted line.

Diagram:

Graph:

• Will the two vehicles come to a stop at the same time, or at different times? _______________ Explain.

b. Now run the simulation. Record the actual results here:

Diagram:

Graph:

• Are the results what you had predicted? ________ How are they different?

c. Using data from both car and truck, run the simulation three times to complete the first three lines of the following table:

(kg)

(m/s)

(kg*m/s)

(N)

### Time

(sec)

40

2000

250

80

2000

300

2000

400

• From the values you found, find a relation between momentum, braking force, and stopping time? Make statements of the effect "as the mass is doubled, the braking force is _________."

• To check your above statement, run the simulation twice more (for your own values of mass, velocity, and braking force and fill in the empty rows in the table. Does your relation still hold? ___________ Explain what you did to test the relationship and how your results either confirm or deny your conclusion to the question above.

• From your results in the table above, which quantities appear to govern stopping time?

• Which stops more quickly in time - a slow, heavy truck or a fast, light car? _____________________ Explain your answer.

### 3. Braking Distance

a. Now that you know how the two vehicles behave in terms of braking time, we will look at what affects their braking distance.

• Set the truck and car to have the following values:

DIAGRAM

• Set the braking force to 500 N. Run the simulation and measure the braking distance of each vehicle with the RULER tool. Enter these values in the first two rows of the table.

### Distance (m)

50

20

50

40

50

60

50

80

50

• What do you think will be the braking distance for the car if it travels at 60m/s and 80 m/s?

• Run the simulation to check your answers and fill in these rows in the table. How close were you?

b. Look carefully at the values of speed and braking distance in your table. Can you see a relationship between them? What happens if the speed doubles? If it quadruples (speed increased by four times)?

c. Can you write a general expression that relates braking distance to velocity? (Hint: one of the terms is squared.)

• From your expression, predict the braking distance for a speed of 100 m/s.

• Run the simulation to verify your prediction. How close were you?

d. Which do you think will have a greater affect on braking distance; doubling the mass or doubling the speed?

### Distance (m)

50

40

50

80

100

40

• Set the mass and velocity for the three combinations shown in the table above. Run the simulation, take distance measurements, and record your data in the table above.
• Which quantity, mass or velocity, has a greater affect on braking distance?

### Conclusion:

Describe the variables which effect the stopping time and the stopping distance of a skidding automobile. In your description, use phrases such as "inverse", "directly", "first power", second power". etc. Be complete and clear in your description. Do a bang-up job.

This lab created by Tom Henderson and last updated on 11/26/97.

Special thanks to lab assitant Dan Reid for assistance with the typing.