7.6 Conservation of Energy

You do 30 J of work to load a toy dart gun. However, the dart is 10 cm long and feels a frictional force of 10 N while going through the dart gun’s barrel. What is the kinetic energy of the fired dart?

1. 30 J
2. 29 J
3. 28 J
4. 27 J

A child has two red wagons, with the rear one tied to the front by a stretchy rope (a spring). If the child pulls on the front wagon, the ____ increases.

1. kinetic energy of the wagons
2. potential energy stored in the spring
3. both A and B
4. not enough information

Which of the following are closed systems?

1. Earth
2. a car
3. a frictionless pendulum
4. a mass on a spring in a vacuum

A 5.0-kg rock falls off of a 10 m cliff. If air resistance exerts an average force of 10 N, what is the kinetic energy when the rock hits the ground?

1. 400 J
2. 12.6 m/s
3. 100 J
4. 500 J

Before railroads were invented, goods often traveled along canals, with mules pulling barges from the bank. If a mule is exerting a 1200 N force for 10 km, and the rope connecting the mule to the barge is at a 20 degree angle from the direction of travel, how much work did the mule do on the barge?

1. 12 MJ
2. 11 MJ
3. 4.1 MJ
4. 6 MJ

Using values from the table, how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? (These electrons were not dangerous in themselves, but they did create dangerous x rays. Later model tube TVs had shielding that absorbed x rays before they escaped and exposed viewers.)

Using energy considerations and assuming negligible air resistance, show that a rock thrown from a bridge 20.0 m above water with an initial speed of 15.0 m/s strikes the water with a speed of 24.8 m/s independent of the direction thrown.

(a) Use of hydrogen fusion to supply energy is a dream that may be realized in the next century. Fusion would be a relatively clean and almost limitless supply of energy, as can be seen from the table. To illustrate this, calculate how many years the present energy needs of the world could be supplied by one millionth of the oceans’ hydrogen fusion energy. (b) How does this time compare with histo…