# Week Nine – Investigating Forces Part 2

In Class Activity Plan

So it turns out that the plan from week 8 was a little more than a single week could hold, so we’re still doing some of that stuff.

20 min – Practice with Force Diagrams (Word, Pdf)

PURPOSE: Practice making, using, and interpreting force diagrams in conjunction with system schema.

Notes:

• Have them draw two different system schema for each of the similar situations, particularly for the ball leaving the hand problem
• This is a reasonable worksheet to give as a homework as well

10 min – Board Meeting

PURPOSE: Build consensus on the practice of making, using and interpreting force diagrams in conjunction with system schema.

Video Examples: (Discussion 1, Discussion 2)

• System schemas should have an arrow for every interaction

­So the case where the ball is no longer in the hand, there is no interaction between the ball and hand and thus there can’t be any force

­Watch for students trying to put 2 labels on a single arrow!

• System schema and Force Diagrams should be consistent with each other

­Fnet­ is the direction of the acceleration

­Directions are important

20 min – Whiteboard – Scales in an Elevator (Word, Pdf)

PURPOSE: Gain experience reasoning conceptually with force diagrams.

Video Examples: (Boarding 1, Boarding 2, Boarding 3)

Note: Probably want to whiteboard this, watch for same things as above. Emphasis on the direction of the acceleration matching the direction of the Fnet

20 min – Board Meeting

PURPOSE: Gain experience reasoning conceptually with force diagrams.

Note: Probably want to whiteboard this, watch for same things as above. Emphasis on the direction of the acceleration matching the direction of the Fnet

15 min – Whiteboard – Newton’s Second Law Variations on a Theme problem #1 (Word, Pdf)

PURPOSE: Practice modeling phenomena with new law, N2, and new representational tools.

Notes: They will try very hard to just answer the question without making a model

10 min – Board Meeting

PURPOSE: Build consensus about modeling phenomena with new law, N2, and new representational tools.

15 min – Whiteboard – Newton’s Second Law Variations on a Theme problems #2 & 3 (Word, Pdf)

PURPOSE: Practice modeling phenomena with new law, N2, and new representational tools.

Notes: Split up #2 & #3, have half of the class do one and the other half do the other.

Individual Problem notes:

• #1 – #3: All the same situation

Why bother? – Different quantities are given so we can find certain other quantities

­It’s always the same system schema and force diagram

10 min – Board Meeting

PURPOSE: Build consensus about modeling phenomena with new law, N2, and new representational tools.

Video Examples: (Discussion 3)

25 min – Whiteboard – Newton’s Second Law Variations on a Theme problems #4 or 5 (Word, Pdf)

PURPOSE: Practice modeling phenomena with new law, N2, and new representational tools.

Notes: Split up #4 & #5, have half of the class do one and the other half do the other.

20 min – Board Meeting

PURPOSE: Build consensus about modeling phenomena with new law, N2, and new representational tools.

Video Examples: (Discussion 4, Discussion 5)

Notes on each problem

#4 They don’t like the force from the rope on the child, and will try to put the force of the child on the rope in their force diagrams

#5: Using energy to get the velocities is a lot simpler. There are two parts to this problem (the way up and the way down)

Homework: Box on Box Problem (Word, Pdf)
Note: You will come back to this homework at the end of the upcoming lab. So it’s important that it is assigned before finishing the Newton’s 3rd Law Lab.

60 min – Newton’s 3rd Law Lab: Investigating Forces Part Two (Word, Pdf)

PURPOSE: Introduce Newton’s third law to models

Technical Notes:

• Have the students open the file for Newton’s Third Law in Logger Pro

­Logger Pro à Experiments à Physics with Vernier à 11 Newtons Third Law.cmbl

­(This automatically has one of the force sensors with direction reversed so that you get positive values for one and negative for the other)

• Remember to calibrate the force sensors upon plugging them into the LabPro

­Go to: Experiment à Calibrate à Calibrate Now à

(1) Enter 0N into the box when nothing is on the hook of the force sensor à Press KEEP

(2) Enter 9.8N into the box with a 1kg mass hanging from the force sensor à Press KEEP

• The force probes on the carts need to have the wide stoppers on them so that when they collide together, they actually push evenly on the stopper.
• The sampling rate should be increased to at least 500 samples/sec

­Go to Experiment à Data Collection… àSampling Rate

20 min – Whiteboard

PURPOSE: Describe results of Newton’s 3rd Law Lab

Video Examples: (Boarding 4, Boarding 5)

• What did you learn?
• What rules can you make?
• What would you say to your friend about the SUV argument?

45 min – Board Meeting

PURPOSE: Synthesize results of Newton’s 3rd Law Lab; introduce N3 to as law that models must obey.

Video Examples: (Discussion 6)

Goals:

• System schema for each different set-up

­The force sensors measure the contact interaction in the schema

­Each sensor reads one force, but it is the same interaction (2 forces for each interaction)

• Newton’s 3rd Law: “For each interaction, there are two forces one on each object, the forces are equal in magnitude and opposite in direction.”
• Rethink your box on box problem, carefully label each force, what is the same?
• Define Newton 3rd Law pairs as: the two forces that describe the same interaction

25 min – Whiteboard  – Identifying Newton’s 3rd Law Pairs (Word, Pdf)

PURPOSE: Integrate N3 into use of force diagram representational tool.

20 min – Board Meeting

PURPOSE: Build consensus about integration of N3 into use of force diagram representational tool

60 min – Whiteboard – Practice with doing Force Problems

PURPOSE: Build consensus about modeling phenomena with Newton’s  laws, new representational tools, integrating force with other approaches to modeling phenomena.