Braking Distance SEPUP Physical Science #82
One very important force frequently used in a car is the force applied by the brakes. Sometimes drivers can avoid an accident by coming to a stop before the accident occurs. The braking distance is the distance the car travels after the driver applies the brakes, until the car comes to a full stop. Other things being equal, the vehicle with the shorter braking distance is safer.
When the brakes of a car are applied, the brake pads push against a metal section of the wheels. The increased friction acting against the wheels’ motion slows down the turning of the wheels.
Objective: How can friction lower the risk of getting into an accident?
Materials:
· 1 cart
· 1 ramp
· 2 tracks
· 1 piece of cloth
· 1 meterstick
Procedure:
1. Use the following table to record your data:
Speed of Cart (Notch) / Braking Distance (cm) / Average Braking Distance (cm)Trial 1 / Trial 2 / Trial 3
Fast (A)
Medium (B)
Slow (C)
2. Set up the equipment as shown below, making sure that the ends of the cloth are held firmly between the pieces.
3. Place the cart so that the rear axle is at the high notch, Notch A, on the ramp. As you learned in a previous activity, this release height results in the fastest cart speed.
4. Release the cart, and let it roll to a stop.
5. Measure the distance it rolled along the flat track. Record this distance as Trial 1 in the table.
6. Repeat Steps 3-5 twice, and record the distances as Trial 2 and Trial 3 in the table.
7. Repeat Steps 3-6 with the cart starting at the middle notch, Notch B, on the ramp. This release height results in a medium cart speed. Record the data in the chart.
8. Repeat Steps 3-6 with the cart starting at the low notch, Notch C, on the ramp. This release height results in the slowest cart speed. Record the data in the chart.
9. Find the average distance the cart traveled for each speed. Record this in the right-hand column of the table.
Analysis:
1. In this model of a stopping car:
a. What does the cloth represent?
b. List some strengths and weaknesses of this model.
2. What effect does speed have on braking distance? Explain, using evidence from this activity.
3. Does the evidence you gathered in this investigation support the statement below? Explain why or why not, using evidence from your investigation.
A car moving at 55 MPH needs less distance than an identical car moving at 45 MPH.
4. Reflection: Why do you think excessive speed is a factor in about 20% of fatal car accidents?
Extension 1 (Required for Phases 2 and 3): Compare your data with the results posted on the Issues and Physical Science page of the SEPUP web site. How is it similar or different from the data from other schools? If possible, post your results on this page.
Extension 2: (Required for Phase 3). How would the point or time at which the braking started affect the braking distance? If the braking started while the cart was still on the ramp, how would this affect the distance? Alter the setup to investigate this. Give evidence or data to support your prediction.