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Name: ________________________________

Balanced Force

Model A force is ____________________________________

Common Types of Forces

Type of Force Direction When is it present? Symbol

Newton’s 1st Law:

Newton’s 3rd Law:

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Notes: Interactions, System Schemas, FBDs

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Notes: Interactions, System Schemas, FBDs

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Practice Set 1: The Free Body Diagram (FBD) Draw a system schema, force diagram, and sketch of x-t, v-t, and a-t graph for each situation. First Attempt Corrections/Solutions

1. A box lies motionless on a floor.

<Note: The box is each diagram is 10 kg>

2. A box slides at a constant speed without friction.

3. A box slows due to friction.

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4. A box slides without friction down a ramp.

5. A box is at rest on a ramp.

6. A box is attached to a rope and is suspended from a ceiling.

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7. A box is attached to two ropes and suspended from a ceiling.

8. A box is attached to two ropes. One rope is fixed to a ceiling the other to a wall. The box is at rest.

9. A box is on a ramp and is attached to a rope that is fixed to a wall. The box is at rest.

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10. A box is attached to a rope and pulled along a floor with the rope parallel to the floor.

11. A box is attached to a rope and pulled along a floor with the rope at an angle to the floor.

12. A box is attached to a broom handle and pushed along a floor with the broom handle at an angle to the floor.

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13. A box is in free fall.

14. A box is falling at a constant velocity.

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Practice Set 2: Vector components and solving problems w/ FBD Determine the x and y components of each of the force vectors below. Show work.

1.

2.

3.

4.

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5.

A person pulls on a 50 kg desk with a 200N force acting at 30° angle above

the horizontal. The desk does not budge.

Draw a force diagram for the desk.

a. Write the equation that describes the forces that act in the

x-direction.

b. Write the equation that describes the forces which act in the

y-direction.

c. Determine the x and y components of the force of tension.

d. Determine the value of the frictional force. Do the same for the normal force.

6. Suppose in the diagram above, the person were pushing down at a 30° angle with 200 N of force.

The desk still does not move.

Draw a force diagram for the desk.

a. Write the equation that describes the forces that act in the

x-direction.

b. Write the equation that describes the forces that act in the

y-direction.

c. Determine the value of the frictional force. Do the same for the normal force.

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Practice Set 3: FBD Practice 1. A man pulls a 50 kg box at constant speed across the floor. He applies a 200 N force at an angle of

30°.

Quantitative Force diagram for the box:

How would the diagram change if the man pulled with a greater force than friction? Illustrate you’re

answer with another force diagram

2. Draw a quantitative force diagram for a 4 kg squirrel sitting still on a roof that makes a 18.5o with

the horizontal. Label the force vectors and use equality marks on the vectors.

How would the force of friction change if the squirrel jumped to your neighbor’s house with a roof angle

of 40o with the horizontal and stood motionless? Support your answer with another quantitative force

diagram.

3. Draw a quantitative force diagram for the 90 kg skier who slides with a constant velocity down a

slope that makes a 35o angle with the horizontal. Label the force vectors and use equality marks on

the vectors.

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5 kg

4. A 2000 kg elephant stands on a 20o ramp. Draw a force diagram to determine the components of the

elephant’s weight parallel and perpendicular to the ramp.

5. Determine the tension in each cable in case A and case B.

Case A Case B

6. Determine tension in each cable. (Hint: There is more than one way to define the system.)

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7. The 5.0 kg box hangs from two ropes. Determine all of the forces on the box.

8. A 950 kg car is driven up a hill at constant velocity of 7 m/s, where 1200 N of friction and drag

oppose its motion.

a. Draw a force diagram for the car.

b. What is the weight of the car?

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Practice Set 4: Interaction Problem Solving

1. The player in the photo exerts a 100 N horizontal force on a 25 kg blocking sled, pushing it across the grass with a constant speed of 2.0 m/s.

a. On the graph below, draw an FBD to a precise scale. Make sure you write down your scale! b. Also draw a force vector addition diagram (to the same scale). Be sure to annotate your diagrams.

c. How would the situation change if the player pushed with more than 100 N, while the frictional force between the grass and the sled remained the same? Illustrate your answer with another FBD and a motion map.

d. Describe, in terms of the amount of force the player would have to apply, what he would have to do to make the sled move with a constant velocity of 3.0 m/s. Assume that the frictional force between the grass and the sled remains the same under all circumstances. Illustrate your answer with diagrams and/or graphs as appropriate.

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e. If he pushes the sled as originally described with a velocity of 2.0 m/s, how far will it slide in 7.5

seconds? Draw at least two diagrams/graphs to illustrate this situation, then solve this problem

using at least two different methods (and getting the same answers). You will need your notes

and equations from Unit 1 and 2 (CVPM and UAPM) to help you with this one.

f. With the sled moving at a constant velocity of 2.0 m/s, the person reduces his force to 75 N. Describe what happens to the sled. Illustrate your answer with another FBD and motion map.

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2. The 80 kg box rests motionless on the 20º incline.

a. Fill out the chart below, determining all of the forces on the box (including their magnitudes).

(Qualitative) Sketch of FBD

System Schema

(Qualitative) Sketch of Vector Addition Diagram

REMINDERS: ◈ Does your system schema have a system boundary? ◈ In your FBD, did you represent the

system with a particle? ◈ Is it obvious when you intend two forces to be equal or when you intend one force to

be greater than another? ◈ Did you label your forces with an (on, by)? ◈ Did you add vectors head to tail?

b. In the space below, draw the vector addition diagram to a precise scale. Be sure to write down your scale! Be sure to use a ruler and a protractor! c. Is the contact normal force on the box by the ground [greater than, less than, or equal to] the gravitational force on the box by the Earth? Explain why your answer makes sense.

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3. A man pushes a 2.0 kg broom at a constant speed. The broom handle makes a 50o angle with the floor.

He pushes the broom with a 5.0 N force directed parallel to the handle. a. Fill out the chart below, determining all of the forces on the broom (including their magnitudes).

(Qualitative) Sketch of FBD

System Schema

(Qualitative) Sketch of Vector Addition Diagram

REMINDERS: ◈ Does your system schema have a system boundary? ◈ In your FBD, did you represent the

system with a particle? ◈ Is it obvious when you intend two forces to be equal or when you intend one force to

be greater than another? ◈ Did you label your forces with an (on, by)? ◈ Did you add vectors head to tail?

b. In the space below, draw the vector addition diagram to a precise scale. Be sure to write down your scale! Be sure to use a ruler and a protractor!

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c. Is the contact normal force that the floor exerts on the broom (greater than, less than, or equal to) the gravitational force that the earth exerts on the broom? Explain your thoughts. d. How would your answers change in 3a and 3b if the person exerted the same force of 5.0 N along the handle, but the handle were closer to parallel to the floor? Illustrate with another vector addition (or Force) diagram.

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Practice Set 5: Interactions and Newton's 3rd Law

For situations 1-5 compare the forces exerted by the blocks on each other. Friction is present in all

cases.

The choices are:

A) block A exerts a greater force on B than block B exerts on block A

B) block B exerts a greater force on A than block A exerts on block B

C) blocks A and B exert equal forces on each other

In the space below each case, construct separate qualitative force diagrams for blocks A and B.

1.

2.

3.

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4. 5.

6. How does the force the magnet exerts on the refrigerator compare to the force the refrigerator exerts

on the magnet? Draw and label a qualitative force diagram for the magnet and the refrigerator, using

equality marks on the vectors.

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7. How does the force the earth exerts on the person compare to the force the person exerts on the

earth? Draw and label force diagrams for the earth and the person, using equality marks on the vectors.

8. How does the force the earth exerts on the moon compare to the force the moon exerts on the earth?

Draw and label force diagrams for the earth and the moon, using equality marks on the vectors.

9. How does the force block A exerts on block B compare to the

force block B exerts on block A?

Draw and label a quantitative force diagram for each block, using

equality marks on the vectors. Identify and describe the equal and

opposite force pairs.

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10. Two people of the same mass on identical carts push off of each other to get their carts moving.

Treat the cart and person together as a system and assume the wheels make friction negligible. Draw

and label qualitative force diagrams for each cart & person system while pushing.

11. Why don’t we use cars that drive infinitely without energy input in a method seen above in the

picture. Draw qualitative force diagrams to support your answer.

12. You are stuck in the middle of a frozen lake (on top of the ice). The ice is soooooo slippery that you

can not generate friction to walk off of the lake. How can you get off of the lake? Draw qualitative

force diagrams to support your answer.

13. Using Newton’s 3rd Law, explain how walking or running works. Explain how a car moves.

14. Using Newton’s 3rd Law, explain the nature of swimming strokes.

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15. Explain why seat belts are necessary/effective

16. An SUV collides with a Mini-Cooper. Is the force exerted on the Mini by the SUV greater than, equal to, or

less than the force exerted on the SUV by the Mini?

17. Explain who wins in a tug-of-war game. Is it the group who can pull harder?

18. You decide to test your physics knowledge while going over a waterfall in a barrel. You take a baseball into

the barrel with you and let it go when you are falling vertically downward. What do you expect to see happen to

the ball?

19. If an object is acted upon by a single force, is it possible for the object to remain at rest or at constant

velocity?

20. Consider the gravitational interaction between the Sun and the Earth, does the Sun exert a greater than, less

than, or equal force on the Earth than the Earth exerts on the Sun? Why then, does Earth orbit the Sun and not the

other way around?

21. When you high five someone real hard, why do both of your hands hurt?

22. If gold were sold by weight, explain where you might want to have it weighed. Why?

23. In the motion picture It Happened One Night), Clark Gable is standing inside a stationary bus in

front of Claudetter Colbert, who is seated. The bus suddenly starts moving forward and Clark falls into

Claudette’s lap. Why did this happen?

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24. Do airbags reduce the amount of force on your face? If not, what do they do?

25. Explain how a rocket engine works.

26. A brick hits your toe (ouch…). Which will have the greater force of impact, the toe or the brick?

27. You are soooo cool that you have dice hanging on your rear view mirror. What happens to the

dice when you take off from rest? What about when you come to a stop? What force causes these

things to occur?

28. True or false: An object can have motion only if a net force acts on it.

29. Draw your spirit animal eating an ice cream cone of your favorite ice cream. Don’t worry, it’s not

lactose intolerant. It loves ice cream.