Inertia Practice quiz

What you need to know or be able to do:

1. The definition of motion and the meaning of relative motion 2. State of motion 3. The definition of inertia 4. The relationship between mass and inertia 5. Definition of acceleration and equilibrium 6. Identify or give examples of objects that are accelerating or in equilibrium 7. The definition of force 8. Force as a vector—finding Fnet in basic (non-trig) problems 9. Balanced and unbalanced forces as they relate to acceleration and equilibrium 10. The law of inertia/Newton’s first law of motion 11. Three forces on Earth that make Newton’s first law hard to observe 12. Explain everyday events in terms of the law of inertia/Newton’s first law of motion

Sample problems/questions Answer the questions below on a separate piece of paper. Write complete thoughts in complete sentences. If appropriate, use a diagram to explain your answer.

1. A fellow student is sitting next to you in Physics class, dead asleep. It could be argued that the student is at rest. It could also be argued that the student is in motion. Both arguments are valid. Explain.

2. If you get a speeding ticket, you’re getting a ticket for your relative speed. What frame of

reference do the police use to determine your speed?

3. Drivers hate to be behind big trucks at quick green lights, because typically only one tractor trailer gets through the light before it turns red again. Are truck drivers just out to annoy us, or is there some other way to explain why they move so slowly once the light turns green?

4. You are watching a science fiction movie. In it, the spaceship captain orders the ship to

shut down the engines while the ship is at ludicrous speed. The crew follows the order, the ship stops, and the ship and everyone in it is fine. Give two reasons why this is bad physics. Explain.

5. Some might argue that if Newton’s first law of motion is accurate, then you shouldn’t

need to depress the gas pedal at all once you get your car going in a straight line at a constant speed. If you get up to 60 mph along a straight stretch of highway, you should be able to turn you car off and just let its inertia keep it in its state of motion. Experience tells us that this is not the case at all; you have to keep giving the car gas or else it will slow down and eventually stop. Was Newton wrong? Explain.

6. Is the Earth accelerating? How do you know?

7. Decades ago, scientists discovered that the universe is expanding. Several years ago, they were shocked to learn that large-scale objects in the universe (like clusters of galaxies) are moving away from each other at increasing speeds; that is, the expansion of the universe is accelerating. What does this tell us about the forces that must be acting on the large-scale universe? In what direction are these forces pointing?

8. Explain, in terms of inertia, how the game of volleyball would be different if a volley ball

had twice its normal mass.

9. Which two cars experience the most similar motion? Explain.

10. Explain the cartoon.

40 mph

0 mph

40 mph 3

2

1

11. State whether the objects below are accelerating or in equilibrium.

30 mph

20 mph

7 N 10 N

120 N

120

N

12. You are driving 50 mph straight up a hill. Which are greater: the forces pushing you forward or the forces pushing you backward? Explain.

13. A tractor trailer is following too closely behind you on the highway and trying to blind

you with its high beams. You’re tired, decaffeinated, cranky, behind in your school work, broke, and newly rejected from your first-choice college, to which your friend—who is an idiot—was accepted. In a moment of road rage, you slam on your brakes to teach the truck driver a lesson and get him off your bumper. Explain, in terms of inertia, why slamming on the brakes when a tractor trailer is behind you is foolish.

14. The quarterback throws a long pass downfield. After the ball leaves his hand, what force

is acting on the ball in the forward direction?

15. A car is at rest in a parking lot. Diagram what, if any, forces are acting on it.

16. During a long road trip, you strap some of your belongings to the roof of your car. A suitcase falls off the roof while you are traveling down the highway at 60 mph. Which way will the suitcase roll once it hits the pavement? Explain.

17. Though you’re traveling 60 mph, a fly inside your car manages to hover without being

affected by the speed. Why?

18. The Earth is circling the sun at an average speed of about 65,000 mph. a. That’s really fast. Why don’t we fall off? b. What would happen to us if the Earth suddenly stopped during its orbit? Explain.

19. Once a figure skater pushes off from the ice, what upward force allows the skater to

continue rising in the air?

20. You are riding a carousel, which is moving at a constant speed of 3 mph. Are you accelerating?

21. A track and field athlete jumps a hurdle. What horizontal force keeps her moving forward

after she’s left the ground?

22. A water skier is pulled by a rope attached to a speedboat. He moves in a straight line at a constant speed. Which is greater: The force pulling him forward or the forces pushing him backward?

23. When running the 100-m, you find that you always travel the second 50-m much faster

than the first 50-m. Why?

24. Explain, in terms of forces and acceleration, how it is possible to jump.

25. You go to the home improvement store and pick up six 50-lb bags of cement and put them in your trunk. Explain how and why your car’s response will be different when:

a. You pull out of the parking lot and speed up on the main road b. You brake when approaching a red light

26. In driver education classes, they warn students to keep a big following distance when

driving behind motorcycles. This is not only because a biker is far more vulnerable to injury. Based on what you know about inertia, why is it important to stay well behind a motorcycle when driving?

27. You and your friends pile into your car and head out to a movie. Since you eschew death,

disfigurement and dismemberment, you always wear your seatbelt. No one else in the car chooses to buckle up. Explain, in terms of inertia, how an unrestrained backseat rider could hurt or kill you if you were ever in a serious front-end collision.

28. Tractor trailer accidents can be catastrophic, not only because these vehicles are so

massive, but also because they can “jackknife”, as shown below. Explain, in terms of inertia, how this happens.

29. Many amusement parks have swings. Once the ride gets going, why do the riders swing outward, away from the center and their original positions?

30. Describe and give a clear example of the three forces we discussed that make it difficult to observe Newton’s first law.

31. Diagram 3 general ways that an object can accelerate and 2 general ways that an object

can be in equilibrium.