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(s) Example(s) Notes

Honors Physics / Unit 04 / UBFPM

– 1 – from Modeling Workshop Project © 2006

Interlude: Vector Practice!Always use a protractor, ruler, and pencil when working with graphical vector constructions!

1. An object moving in a circular path with a constant speed of 2.0 m/s changes direction by 30º in 3.0 s.

a. What is the magnitude of the change in velocity over the 3.0 s interval? Remember that velocity is a vector. Draw a vector diagram and perform the graphical construction to find

€

Δv .

b. What is the magnitude of the average acceleration over the 3.0 s interval?

Honors Physics / Unit 04 / UBFPM

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2. 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

3. 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

Honors Physics / Unit 04 / UBFPM

– 3 – 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?

Objective of the experiment:

Whiteboarding and Post-Lab Notes

Honors Physics / Unit 04 / UBFPM

from Modeling Workshop Project © 2006 ! – 4 –

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

4. The 80 kg box travels to the bottom of the 2 m long ramp with a constant speed of 1.4 m/s.

a. What models apply to this situation and why? (See the italics + bold + underline? That part is way important!)

b. Draw at least four diagrams/graphs to illustrate the situation. Choose the diagrams and graphs that you find most useful. Draw these BIG enough to be useful and annotate them like crazy. The diagrams are key.

c. Using the models you have chosen, solve for any unknown quantities. Use more than one method to find the same answers whenever possible. Show your work and use units. Always start with a variable expression (that is, your first line should always start with symbols only, and you should plug in things that you know in a later step).

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Honors Physics / Unit 04 / UBFPM

– 5 – from Modeling Workshop Project © 2006

5. The 80 kg box starts from rest and travels to the bottom of the 2 m long ramp, but its motion is opposed by a 150 N frictional force.Follow the same general procedure as in the previous problem: tell which models apply, draw and annotate diagrams, solve for unknowns.

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Honors Physics / Unit 04 / UBFPM

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6. The 80 kg box starts from rest and travels to the bottom of the 2 m long, frictionless ramp. Again, follow the same general procedure.

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Honors Physics / Unit 04 / UBFPM

– 7 – from Modeling Workshop Project © 2006

7. In the preceding three problems, was the contact normal force that the incline exerted greater than, less than or equal to the weight of the box? Explain.

8. In problem 6, suppose the incline were tilted at a greater angle. How would your answers change? Use a side-by-side comparison of two vector addition diagrams as part of your explanation.

9. In problem 4, determine how unbalanced the forces on the cart are (the magnitude of the net force) at the instant its velocity is 1.4 m/s.

In problem 6, determine t how unbalanced the forces on the cart are (the magnitude of the net force) at the instant its velocity is 1.4 m/s.

Are your answers to the above two questions the same or different? If they are the same, explain how this is so even though there is no friction in problem 6. If they are different, explain how this is so even though the cart has the same instantaneous velocity of 1.4 m/s.

10. Suppose the cart in problem 6 were given a shove so that it started sliding up the ramp with a velocity of 3.0 m/s. How far up the ramp would it go?

Honors Physics / Unit 04 / UBFPM

from Modeling Workshop Project © 2006 ! – 8 –

Practice 2: 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 solve for as many unknown quantities as you can.

11. 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).

Honors Physics / Unit 04 / UBFPM

– 9 – from Modeling Workshop Project © 2006

12. 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.

Honors Physics / Unit 04 / UBFPM

from Modeling Workshop Project © 2006 ! – 10 –

13. 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.

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Honors Physics / Unit 04 / UBFPM

– 11 – from Modeling Workshop Project © 2006

UBFPM Model Summary

Concept Map 1.0

Honors Physics / Unit 04 / UBFPM

from Modeling Workshop Project © 2006 ! – 12 –