Name: ________________________________

Unbalanced Force ModelPhysics Diagrams

Here you can keep track of all of the diagrams that you have learned to draw. You can come back and update it after future models, if you’d like. This list might come in handy when you start tackling complex “goal-less” problems.

Name of Diagram Model Example Notes

Physics! / Unit IV / UBFPM

– 1 – from Modeling Workshop Project © 2006

Experiment: Pulling Carts with Springs

Sketch and label the experiment setup:

What could we measure? How could we measure it?

The Objective:

Take data in an organized, labelled way:

Physics! / Unit IV / UBFPM

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Graph your data to see if there is a relationship:

Be sure to:‣ Use pencil‣ Label your axes with symbols and units‣ Give the graph a title (“[vertical axis variable] vs. [horizontal axis variable]”)‣ Draw a best fit line (don’t connect the dots).‣ Find the slope using points on the line (not data points).‣ Write the equation of the line using the variables from your axes (don’t default to “y and x”); make sure the

slope and intercept have the correct units attached to the numbers.‣ Put units on numbers, but never on variables.

Physics! / Unit IV / UBFPM

– 3 – from Modeling Workshop Project © 2006

Worksheet 1: “Goal-less” Problems! (Super cool.)

1. A person applies a 10 N horizontal force to a 10 kg block, initially at rest on a frictionless, horizontal floor. The person pushes it for 20 m.

a. Which models (CVPM, BFPM, CAPM, UBFPM) apply to this situation and why?

b. Draw AND annotate at least four diagrams and/or graphs to illustrate the situation. Choose diagrams based on the models you talked about in part a.

c. Using the models you have chosen, make calculations and solve for anything you can find. If possible, solve for the same quantities using at least two different methods. Beautifully show your work and use units.

Physics! / Unit IV / UBFPM

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2. The person once again applies a 10 N horizontal force to a 10 kg block on a horizontal floor, only now the floor is rough, and there is a frictional force of 3.0 N between the block and the floor. The person pushes it for 20 m. Follow the same general procedure as in the previous problem: tell which models apply, draw/annotate diagrams, make calculations.

3. Once the block from problem 2 has traveled 20 meters, the person stops pushing it. The surface still has friction. Again, follow the same general procedure.

Physics! / Unit IV / UBFPM

– 5 – from Modeling Workshop Project © 2006

4. Determine the magnitude of the net force (in Newtons) on the block from problem 1 at the instant it is traveling with a speed of…a. 3.0 m/s.

b. 5.0 m/s.

c. 0 m/s.

5. Determine the magnitude of the unbalanced force on the block from problem 3 at the instant it is traveling with a speed of 3.0 m/s.

6. Compare your answers to problems 4a and 5. If your answers are the same, explain how this is so even though in problem 3 the person is no longer pushing. If your answers are different, explain how this is so even though the object has the same instantaneous velocity of 3 m/s. Don’t worry, we’re definitely whiteboarding these! :)

Physics! / Unit IV / UBFPM

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Worksheet 2: Some thinking about vectors

7. The free body diagrams below are incomplete. Draw and label one more force appropriate to each situation that will result in the forces being balanced.

Fg Fg

FnFf

Fg

Fn

Fg

Ft

8. For each situation, draw a vector addition diagram. Use the vector addition diagram to find the net force on the object (the net force is not a type of force itself; rather, it tells you how unbalanced the forces are).

F1

F2F3

F1

F2

F3F4

F1F2

F3F4

Physics! / Unit IV / UBFPM

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Worksheet 3: Multiple Model Problem Solving, cont.Solve the following problems with the same approach as you did for the “goal-less” problems on Worksheet 1. List the models that apply (and why!), draw and annotate a lot of diagrams (at least four, but probably more), and then use the models to make as many calculations as you can.

9. When an 85 kg man who is standing in an elevator pushes the button for a higher floor, he starts from rest and accelerates upwards at 2.0 m/s2 for 1.25 s. (HINT: Choose as your system just the man, not the elevator or cable or anything else.)

After accelerating for 1.25 s, the man continues to rise with a constant velocity for an additional 6.0 s.

After traveling at a constant velocity for 6.0 s, the man nears the desired floor and slows down at a rate of 3.0 m/s2. !

Physics! / Unit IV / UBFPM

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10. The man at pulls the 70 kg box with a 400 N force at an angle of 30º to the horizontal. The box travels 15 m at a constant velocity of 5 m/s.

!

Physics! / Unit IV / UBFPM

– 9 – from Modeling Workshop Project © 2006

11. A 70.0 kg box is pushed by a 400 N force at an angle of 30°  to the horizontal.   Starting from rest, the box travels 15.0 m in 2.78 seconds.

12. A 70 kg box is pulled by a 400 N force at an angle of 30° to the horizontal.Starting from rest, the box travels 15 m in 2.47 s.

!

30o

!

Physics! / Unit IV / UBFPM

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13. Three blocks are in contact with each other on a frictionless horizontal surface. A person applies a horizontal force of 18 N to the smallest block. Hint: there are multiple ways to define your system here. You could draw several FBDs for the same instant (snapshot).

Physics! / Unit IV / UBFPM

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UBFPM Model Summary

My First Concept Map (Aww…)

Physics! / Unit IV / UBFPM

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