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### Energy & Its Conservation Homework

1. A 60 kg shell is shot from a cannon to a height of 400 m. What is its potential energy at this height? How much will its potential energy decrease when the shell falls to a height of 150 m? Ans: 235,200 J; 147,000 J
2. A person has a mass of 45 kg and is moving with a velocity of 10 m/s. What is its kinetic energy? If the person’s velocity becomes 6 m/s, what is its change in kinetic energy? Ans: 2250 J; -1440 J
3. A child and a bicycle have a combined mass of 45 kg. The child rides the bike 1.8 km in 10 min at a constant speed. What is the kinetic energy? Ans: 202.5 J
4. A baseball that weighs 1.6 N leaves a bat with a speed of 40 ms. The ball struck the bat with a speed of 30 m/s. How much work did the bat do on the ball? (Remember: velocity has direction!) Ans: 200 J
5. A partially filled bag of cement with mass of 16 kg falls 40 m into a river from a bridge. What is its speed just before it hits the water? Ans: 28 m/s
6. During the hammer throw at a track meet, 784 J of work is done on an 8 kg hammer to throw it straight up. How high will it rise? Ans: 10 m
7. A 1.5 N force acts on a 0.20 kg cart so as to accelerate it along a frictionless track. If it is accelerated from rest, how fast is it going after 30 cm? Ans: 2.12 m/s
8. A 0.5 kg block is sliding along a tabletop with an initial velocity of 0.20 m/s. It slides to rest in a distance of 70 cm. Find the frictional force retarding its motion. Ans: 0.0143 N
9. In an electronics factory, small cabinets slide down a 30° incline a distance of 16 m along the incline to the next assembly stage. The cabinets have a mass of 10 kg each. Calculate the speed each cabinet would acquire if the incline were frictionless. What kinetic energy would a cabinet have? Ans: 12.52 m/s; 784 J
10. A 15 kg model airplane flies horizontally at point A that is 20 m above point B. It has a speed of 19.993 m/s at point A. It then flies horizontally at point B. If the loss of potential energy is reflected as an increase in kinetic energy, what is the speed of the model airplane at point B? Ans: 28.14 m/s