Mechanics Week 11
Week 11 - Finishing Friction & Investigating Momentum
In Class Activity Plan (Word, Pdf)
Week 11: Finishing Friction & Investigating Momentum
Homework: Full Model with Friction (Word, Pdf)
Note: You probably want to start this problem in class. They’ve got a lot of tools/representations to work with at this point, and probably could use some help coming up with them all. The goal in this problem is to represent the motion using every tool/representation we have thus far (e.g. energy pie charts, force diagrams, kinematic graphs)
30 min Whiteboard - Pushing Boxes Static Friction (Word, Pdf)
PURPOSE: Practice modeling situation with static friction.
15 min Board Meeting
PURPOSE: Build consensus on use of static friction in models.
15 min Whiteboard - Box Held on Wall (Word, Pdf)
PURPOSE: Practice modeling situation with static friction.
Video Example:
Video Example: (Boarding)
10 min Board Meeting
PURPOSE: Build consensus on use of static friction in models.
30 min Whiteboard - Box on a Wall with Angle (Word, Pdf)
PURPOSE: Practice modeling situation with static friction.
Video Examples:
Video Examples: (Boarding1, Boarding2)
15 min Board Meeting
PURPOSE: Build consensus on use of static friction in models.
Video Example:
Video Example: (Discussion)
75 min Investigating Collisions Lab (Word, Pdf)
PURPOSE: Investigate momentum conservation
Video Examples:
Video Examples: (Investigating1, Investigating2, Investigating3)
Notes:
This lab should be quick because they’ve already done a similar lab with Newton’s Third Law
Note that there are two motion detectors in this lab set up on either end of the track. This is a case where using the bigger tracks can be useful.
They will default to doing similar labs as to what they did with Newton’s Third law, but you should encourage including the following experiments:
A thorough study of mass. Vary the proportions of mass on the carts.
A thorough study of velocity. Directions, speeds, etc.
Magnets bouncing off magnets
Velcro sticking to Velcro
Cars at rest together, then pop the spring and fly apart
Seed:
It looks like car 1 is transferring something to car 2, what is it?
They will suggest energy, have them check it (Velcro to Velcro is a great example)
Think about the ratios of masses and speeds
Lead them to mv = mv
Momentum vectors as a tool
Sometimes kinetic energy is conserved, can we find out the conditions? (Elastic and inelastic collisions)
15 min Whiteboard - Investigating Collisions Lab
PURPOSE: Report results of investigations of momentum conservation
Video Examples:
Video Examples: (Boarding1, Boarding2)
What did you learn?
What rules can you make?
What questions do you still have?
60 min Board Meeting
PURPOSE: Introduce momentum conservation as a law that governs all models; introduce impulse and relate to Newton’s 3rd law.
Video Example:
Video Example: (Discussion)
Goals:
Something is being conserved, what is it?
Momentum is the same before the collision and after the collision: p = mv
New tool – momentum vectors
Difference between elastic/inelastic collisions
Impulse (instructor-led, can be done in the center of the discussion circle):
We’ve already done this lab in the Newton’s Third Law lab – what did the force graph look like during the collision?
Notice that the curves are curvy and the force car 1 reads is opposite of the force car 2 reads
What is the area under each of these curves? The same! – What’s the same in these collisions? So the area must tell us something about momentum
So the change in momentum for each of the carts must be the same. What are the units of the area (N·t = kg m/s)?
Define Impulse: J = FΔt = Δp
Notice the vectors – having a negative impulse means the change in momentum is in the negative direction