Energy & Matter

Fall 2009

Assignment for Chapter 6: Momentum

Start reading chapter 7 by Thursday, October 1.

Turn in the questions below on Tuesday, October 6.

1. What magnitude of impulse is required to stop a car weighing 12,000N if it is moving with a speed of 30m/s?

2. The same car would weigh about 2000N on the moon. What impulse would it take to stop that car, moving with a speed of 30m/s, on the moon?

3. A truck goes around a curve at a constant speed. Does its momentum change? Explain.

4. If I drop a ball, its speed increases as it falls. Does that violate conservation of momentum? Explain.

5. When you brake a car to stop at a stop sign, what force provides the impulse to stop the car?

6. I have a lump of clay and a rubber ball of the same mass, 0.1kg. I throw each at a wall and they hit the wall with a speed of 20m/s. The clay sticks to the wall but the ball bounces back with a speed of 15m/s. Which one experiences the larger impulse? (Calculate the impulses!)

7. Which would hurt worse, falling on a bare concrete floor or falling on the same floor covered by a thick carpet? Why?

8. One train car of mass 150,000kg is moving to the right with a speed of 4m/s. Another car of mass 100,000kg is moving to the left with a speed of 3m/s. They hit and stick together. What is their final velocity (magnitude & direction)?

Assignment for Chapter 7: Energy (This is an Important Chapter)

Start reading chapter 8 by Tuesday October 13. (It is a holiday, but start reading anyway.)

Turn in the questions below on Thursday, October 15?

1. I am pushing against a stationary wall that is 2m high with a force of 200N for 5s. How much work do I do on the wall?

2. A 0.5kg ball that is moving with a speed of 10m/s?

a) What is its kinetic energy?

b) How much work would I have to do to stop it?

3. A rocky asteroid 1km in diameter has a mass of about 2x10¹²kg and it is moving with a speed of 5000m/s. What is its kinetic energy? (An atomic bomb like the one dropped on Hiroshima releases about 10¹⁴J of energy. Obviously it would be unfortunate if such an asteroid hit the earth.)

4. A car has a kinetic energy of 60,000J. If I double its speed, what will its kinetic energy be?

5. I have a mass of 80kg. If I walk up a flight of stairs that is 3m high, how much does my gravitational potential energy change?

6. A 20,000N car is moving with a speed of 20m/s (about 45 mph). If the braking force between the car and the road is 15,000N, how far will the car go before it stops?

7. If I push (roll) a 100kg barrel up an incline 4m long with a force of 400N, how much work do I do on the barrel? The height of the incline is 1.5m.

8. How much would the barrel’s gravitational potential energy change as it goes from the bottom to the top of the incline?

9. How much would the barrel’s kinetic energy change as it goes from the bottom to the top? (Assume no energy is “lost” to friction.)

10. An electric motor uses 4800J of electrical energy do 4,000J of work. What is its efficiency? How does this compare to the efficiency of an internal combustion automobile engine which is roughly 25%?

11. Can an object have kinetic energy and potential energy at the same time? Explain why or why not.

12. I am on an exercise machine that tells me I am doing work on the machine at a rate of 200W. How much work would I do in 10min.?

Formulas for chapters 1-6

Change in Position = Final Position – Initial Position, 1N = 1kgm/s²

V_{A relative to C} = V_{A relative to B +} V_{B relative to C}