We are once again using a simulation: PhET Forces and Motion e. You will additionally need a stopwatch of any variety
(phone/web stopwatches work fine).
Part 1: Average Acceleration
1. Select the “Motion” tab of the simulation.
2. Check the boxes for “Values,” “Masses,” and “Speed.”
3. Simultaneously press the” +50N” force button three times and start your stopwatch.
The “+50N” button isn’t well labelled in the sim, here’s a picture indicating the button:
Applied Force
O Newtons
-500
0
500
4. After approximately 5s press the “-50N” button three times and stop your stopwatch.
. The “-50N” button isn’t well labelled either, but it is the button to the left of “O Newtons” in the above picture.
. Question 1: What was your final time on the stopwatch? What was the final speed of the crate on the
skateboard?
. Question 2: What is the average acceleration?
. Use the final and initial speeds as well as time to compute the average acceleration.
. Question 3: Compute the expected acceleration via Newton’s second law. Compare this to your answer from
Q2.
• Reminder, an excellent way to compare two individual numbers is % difference.
o Question 4: Which of Newton’s laws do we observe after the force was set to zero in step 4? Explain.
Part 2: Adding Friction
1. Select the “Friction” tab at the bottom of the sim.
2. Check the boxes next to “Values,” “Masses,” and “Speed” once again.
3. Simultaneously press the”+50N” force button three times and start your stopwatch.
4. Stop your stopwatch and press the “-50N” button another three times when the crate reaches the same speed as
:S
. Question 5: What were the speed and time when you stopped the stopwatch?
. Question 6: Did this process take more or less time that was needed to achieve the same speed in part 1? What
does that imply about the acceleration?
. Question 7: Compute the net acceleration and force.
• Use the definition of acceleration, and Newton’s second law.
. Question 8: Use the net force computed in Q7 and the applied force of 150N to compute the force due to
friction. Additionally, find the coefficient of friction.
• To find the coefficient of friction (ju) it will be helpful to know that Ffric = uN where N is the normal force
(and in this case it is equal to weight).
. Question 9: What happens to the box after we stopped pushing it? Explain (using Newton’s laws).
Part 3: The Mystery Box
1. Put the crate away, and replace it with the mystery box.
2. Follow the exact same process that we used in Part 2 to set up the summation of horizontal forces.
3. Use the definition of friction Ffric = uN, along with the normal force (in this case: N = mg) to compute the mass
of the mystery box.
This question does use a bit of algebra. Let me know if you need any assistance.
. Question 10: Assuming the coefficient of friction is the same as it was in part 2, compute the mass of the
mystery box.
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