Monday, January 25, 2010

Unit 7 Review Problems: Force and Newton's Laws

The Unit 7 Test is tomorrow, Tuesday 1/26/10. In addition to the questions below, you will be expected to find the net force using a free-body diagram and to identify objects in static equillibrim.

1. Many automobile passengers have suffered neck injuries when struck by cars from behind. How does Newton's first law apply here? How do headrests help to guard against this type of injury?

2. Suppose you place a ball in the middle of a wagon, and then accelerate the wagon forward. Describe the motion of the ball relative to a) the ground and b) the wagon.

3. If an elephant were chasing you, its enormous mass would be most threatening. But if you sigzagged, its mass would be to your advantage. Why?

4. If you are sitting in a bus that is travelling slong a straight, level road at 100 km/h, you are travelling at 100 km/h too.
a. If you hold an apple over your head, how fast is it moving relative to the road? Relative to you?
b. If you drop the apple does it still have the same horizontal motion? Explain.

5. State Newton's second law in words and in the form of an equation.

6. How much support force (Normal force) does a table exert on a book that weighs 15 N when the book is placed on the table? What is the net force on the book in this case?

7. If a loaded truck can accelerate at 1 m/s^2 and loses its load so it is only 3/4 as massive, what acceleration can it attain for the same driving force?

8. A rocket fired from its launching pad not only picks up speed, but its acceleration increases significantly as firing continues. Why is this so? (Hint: About 90% of the mass of a newly fired rocket is fuel.)

9. Your weight is the result of the gravitational force of the earth on your body. What is the corresponding reaction force?

10. If you walk on a log that is floating in the water, the log moves backward. Why?

11. If you step off a ledge, you noticeably accelerate toward the earth because of the gravitational interaction between the earth and you. Does the earth accelerate toward you as well? Explain.

12. If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impast force greater? Which vehicle undergoes the greater change in motion? Explain.

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