Friction Lab: What the Graph Shows You
The cart slides down the track on its own and tells you how fast it is picking up speed. You write down two numbers for each run, the site draws the graph, and then you read it. There is nothing to calculate anywhere in this lab. 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. The cart slides down the raised track. Which force is holding it back?
Friction is the rubbing between the pad and the track, and rubbing always works against the sliding. Gravity is what makes it go, not what holds it back.
2. You raise the end of the track higher and let the cart go again. What happens to the acceleration?
The steeper the track, the more gravity is pulling the cart along the slope rather than into it, so the cart picks up speed faster.
3. What is the friction pad on the bottom of the cart for?
A cart on its wheels barely rubs at all, so it never slows down and there is nothing to measure. The pad puts a rubbing surface under the cart, which is the whole point of this lab.
4. For every single run, which two numbers does your group have to write down?
Those two are what the graph is made of. If one of them goes missing for a run, that run cannot go on the graph at all, so say the numbers out loud and have somebody repeat them back.
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 meter stick or a ruler
- something steady to raise one end of the track with
The three jobs, one each. The site gives you yours.
- Track and angle
- Raises the track and measures how high the end is. Writes Part 1.
- Cart and data
- Runs the software, lets the cart go, and reads the acceleration. Writes Part 2.
- Recorder and checker
- Writes every number down as it is called and repeats it back. Writes Part 3.
1. Before you touch anything10 points
Fit the friction accessory to the cart with the pad facing down, and put the track flat on the table to start with.
Give the cart a gentle push along the flat track and watch it. Then take the pad off and push it again the same way. Write down what was different. Say which of the two carts you would rather have brakes on, and why.10 pts
2. Let it slide down on its own20 points
Now raise one end of the track. You are going to do this six times, a little higher each time, and write down two numbers every time.
- Raise the end a little and measure how high it is above the table, in centimeters. Write it down.
- Let the cart go from rest and read the acceleration off the cart while it slides. Write that down beside the height.
- Raise it a bit more and do it again. Six heights altogether.
- Do not let the cart run off the end of the track. One of you catches it.
(a)Put your six pairs of numbers in. The site draws the graph for you. Put the height across the bottom and the acceleration up the side.14 pts
(b)Look at your graph and describe it in words. Do the points climb in a straight line, or do they bend? Say what the graph tells you about what happens to the cart as the track gets steeper.6 pts
3. Read the line10 points
The site fits the best straight line through your points and prints its slope underneath. You do not have to work anything out. You have to say what it means.
Write down the slope the site gives you. Then say in a sentence what it means for your cart: for every extra centimeter you lift the end, what happens to the acceleration? Then say whether a cart with no pad on it would give a steeper line or a flatter one, and why you think so.10 pts
4. Find the smallest slope that works, and agree on what you found10 points
This part belongs to all of you. Do it together, and every one of you has to approve it before you leave.
(a)Lower the track slowly until the cart will not slide on its own any more. Write down the smallest height at which it still slides. Then give it the tiniest push at that height and say what happens.5 pts
(b)Write your group’s conclusion. Say what the friction pad does to the cart, what your graph showed, and one thing you would do differently if you did this again.5 pts