Friction Lab: The Number the Line Gives You
The cart slides down the track on its own and tells you its acceleration. Out of six runs and one straight line come two numbers: one you already know, which is how you check yourselves, and one you do not, which is what you came for. You work in a group the site draws for you, and each of you is given a job.
Before you are put in a groupfour questions
Answer these on your own. They are not marked and they are not part of the fifty points. When you have answered all four the page shows you every answer worked through, and the lab opens after that.
1. As the cart slides down the raised track, which force acts along the surface and points back up the slope?
Friction lies along the surface and always opposes the sliding, so a cart sliding down has friction pointing up the slope. The weight points straight down and the normal force is perpendicular to the track, so neither of them lies along the surface.
2. On the raised track, how does the normal force on the cart compare with the cart’s weight?
Only the part of the weight that presses into the track has to be balanced, and that part shrinks as the track is raised. Raise it further and the normal force falls, which drags the friction force down with it.
3. You raise the end of the track higher and release the cart again. What happens to the acceleration?
Steeper means more of the weight acts along the slope and less of it presses into the track. The pull down the slope grows and the friction shrinks, and both of those push the acceleration up.
4. The raised end sits 12.0 cm above the low end, and the two supports are 60.0 cm apart measured along the track. What is the angle, to the nearest tenth of a degree?
The sine of the angle is 12.0 over 60.0, which is 0.200, and the inverse sine of 0.200 is 11.5 degrees. Measure the 60.0 cm along the track and not along the table, or the angle comes out too small.
Not open yet
Type your class code above. The lab opens when Mr. Tuna opens it, and your group and your job appear here as soon as he draws them.
What each group needs
- one dynamics track
- one Smart Cart with the friction accessory fitted, pad down
- a balance
- a meter stick
- something steady to raise one end of the track with
The three jobs, one each. The site gives you yours.
- Track and angle
- Sets the angle, measures the height and the length, and keeps the track still. Writes Part 1.
- Cart and data
- Runs the software, releases the cart, and reads the acceleration off it. Writes Part 2.
- Recorder and checker
- Writes every reading down as it is called and repeats it back, then checks the arithmetic. Writes Part 3.
1. Before you touch anything10 points
Fit the friction accessory to the cart with the pad facing down. Weigh the cart with the pad on it and write that mass at the top of your page.
(a)Draw every force on the cart while it slides down the raised track. Then write one sentence naming the force that is holding it back and saying what the track has to do with how big that force is.4 pts
(b)Take Newton’s second law along the surface and perpendicular to it, and show that the cart’s acceleration is $a = g(\sin\theta – \mu\cos\theta)$. Write the two equations you started from and the line that puts them together.6 pts
2. Let it slide down on its own20 points
Raise one end of the track. Take six angles, spread out, and keep the cart away from the end of the track.
- Measure the height of the raised end above the low end, and the distance along the track between the two places it is supported. The angle is the inverse sine of the first divided by the second.
- Start at an angle where the cart slides on its own with no push at all.
- Release the cart from rest and read the acceleration off the cart while it slides. Use the steady stretch, away from the release and away from the end.
- Do each angle twice. If the two runs disagree, do a third and say so.
(a)Put the angle and the acceleration in, and work out the last two columns yourself. Now find the pair of axes that gives a straight line, and say in a sentence which pair worked.14 pts
(b)Say what went wrong with the first pair of axes you tried. Then say what a line that curves upward would have been telling you about the physics, not about your arithmetic.6 pts
3. What the line tells you10 points
The straight line you found has a slope and an intercept, and both of them are physics.
Your slope should be a number you already know. Say what it is and how close you came to it. Then use the intercept to work out the coefficient of friction, and write the line of algebra that gets you there.10 pts
4. Predict, test, and agree on what you found10 points
This part belongs to all of you. Write it together, and every one of you has to approve it before you leave.
(a)Use your own coefficient to work out the angle at which the cart should just begin to slide from rest. Set the track to that angle and try it. Say what happened. If the cart needed a steeper angle than you worked out, say what that tells you about the two kinds of friction.5 pts
(b)Write your group’s conclusion. Give your coefficient of friction, say how much you trust it and why, and name one thing about the way your group did it that would push the value up or down. Say which way it would push.5 pts