## Friday, June 3, 2011

1. It the absence of air resistance, a rock is dropped straight down from a bridge into the river below.

a) Immediately after being released, is the magnitude of the rock’s acceleration greater than g, less than g, or equal to g? Explain how you know.

b) Immediately before hitting the water, is the magnitude of the rock’s acceleration greater than g, less than g, or equal to g? Explain how you know.

2. For a projectile, which of the following quantities are constant during the flight: x, y, vx, vy, v, ax, ay? Which of the quantities are zero throughout the flight?

3. If an object is not moving, does that mean that there are no forces acting on it? Explain.

4. Internal injuries in vehicular accidents may be due to what is called the “third collision.” The first collision is the vehicle hitting the external object. The second collision is the person hitting something on the inside of the car, such as the dashboard or windshield. This may cause external lacerations. The third collision, possibly the most damaging to the body, is when organs, such as the heart or brain, hit the ribcage, skull, or other confines of the body, bruising the tissues on the leading edge and tearing the organ from its supporting structures on the trailing edge.

a) Why is there a third collision? In other words, why are the organs still moving after the second collision?

b) If the vehicle was traveling at 60 mph before the first collision, would the organs be traveling more than, equal to, or less than 60 mph just before the third collision?

5. Suppose you are an astronaut in deep space, far from any source of gravity. You have two objects that look identical, but one has a large mass and the other a small mass. How can you tell the difference between the two? Why would your proposed method for distinguishing between the two objects work?

6. A light block of mass m and a heavy block of mass M are attached to the ends of a rope. A student holds the heavier block and lets the lighter block hang below it. Then she lets go. Air resistance can be neglected.

a) What is the tension in the rope while the blocks are falling, before either hits the ground?

7. When you drive fast on a highway with muddy tires, you can hear the mud flying off the tires into your wheel wells. Why does the mud fly off?

8. A screwdriver with a very thick handle requires less force to operate than one with a very skinny handle. Explain why this is so.

9. Suppose you are hanging from a tree branch. If you move out the branch, farther away from the trunk, the branch will be more likely to break. Explain why this is so.

10. You probably know that it feels better to catch a baseball if you are wearing a padded glove. Explain why this is so, using the ideas of momentum and impulse.

11. As you release a ball, it galls, gaining speed and momentum. Is momentum conserved?

a) Answer this question from the perspective of choosing the ball alone as the system.

b) Answer this question from the perspective of choosing the ball and the earth together as the system.

12. To change a tire, you need to use a jack to raise one corner of your car. While doing so, you happen to notice that pushing the jack handle down 20 cm raises the car only 0.2 cm. Use energy concepts to explain why the handle must be moved so far to raise the car by such a small amount.

13. A roller coaster car rolls down a frictionless track, reaching speed of vf at the bottom.

a) If you want the car to go twice as fast at the bottom, by what factor must you increase the height of theCould you answer some physics concept short answer questions?for #3, even if an object is motionless there are still forces acting on it, such as gravitational force