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Energy/Toy Car: Acceleration/Conservation of Energy

Estimated Time of Duration

1.5 Hours

Objectives

To familiarize the students with the concepts of force, mass and acceleration
To practice the students in data collection and calculations

Materials

Matchbox Cars Track
Stopwatches
Pencil
Paper
Calculator

Room Preparation

Assemble the race track and stetch it out along a length of the room. One end of the track must be raised in order to get the cars to run, so you need a table or chair to get a constant height reference for each run. Once it is flat on the ground, mark off intervals of the track with tape. If the class is divisible by three, but not by four, mark off two intervals. Otherwise, mark off three intervals.

Procedures and Activities

Lesson

Give background on F=ma. Give sports car/sub-compact car example to explain what acceleration is, and to show changes in F with constant m, varying a. Give bowling ball/ping-pong ball example to show changes in force with constant a, varying m. The sports car/sub-compact car example Two cars, a sports car and a sub-compact step on the gas as fast as they can from a stop. The sports car squeals its way up to 55mph, while the sub-compact slowly makes its way up to the same speed. The difference between the two cars is not how fast they went, but how quickly they got to that speed. Therefore, the faster car accelerated faster than the sub-compact car.

Give background on Kinetic and Potential Energy.

Experiment

Measure the distance between the marked-off intervals in the track.
Measure the height at which the cars are to be released.
Measure the mass of each car.
Split class into groups of three if there are two intervals, otherwise into groups of four.
Assign one person to time each of the intervals, and one person to let go of the car at the top of the hill.
Release the car at the top of the hill.
Have timers measure the time that the car is in their interval, and record the time.
If the track was initially pretty steep, make the track more shallow (or vice versa) and conduct a run.
Prop up the other end of the track and conduct another run with someone measuring how high up the track the car goes.

Post-Experiment Calculations

Refer to the Microsoft Excel document "Car Acceleration Wrksht."
Calculate the car's weight.
Calculate the car's potential energy.
Predict the car's velocity at the bottom of the hill.
Calculate the average speed of the car for each interval.
Calculate the change in average speed for two intervals.
Calculate the time across the two intervals.
Calculate the average acceleration for the two intervals.
Calculate the Kinetic Energy for the first interval (KE = 1/2 mV2).
Calculate the loss of energy due to friction (PE - KE).
Estimate the final Potential Energy (Initial PE - loss on way down first hill - loss on way up next hill).
Calculate the final height of the car based on final PE estimation.
Calculate the velocity of the first interval for the changed slope of the track.

Post-Experiment Discussion

Compare the calculated velocity for the first interval to the predicted velocity.
Give background on friction/drag. Friction acts in the direction opposite the movement. It is proportional to the weight of the object, and the surface it is moving over (e.g. greased glass vs sand paper). It results in a loss of energy due to heat (rub hands together hard and fast).
Compare the velocities for the different slopes of the track. Talk about conservation of energy and why the final velocity should theoretically be independent of the slope, but with friction, the steeper slope will result in the faster velocity.
Talk about the loss of energy in the "estimation of final height" calculations.