Except for the first two problems be sure to do an energy bar chart after making your sketch. Since this is pretty new for us here are a couple of potentially helpful links. Here is a Kahn video that will give a richer perspective on energy bar charts and a written perspective from another physics teacher.
1) In 1972 a large meteorite passed across the western US and was seen (in daylight) by many observers. It was estimated that the mass of the object was 4.106 kg and its speed was 15 km/s (typical of most meteorites). What was the kinetic energy of this object? Express this energy in Megatons of TNT (= 4.2.1015 J). The Hiroshima bomb had a yield of 13 kilotons of TNT.
[0.11Mton]
2) A box is placed on the bed of a flatbed truck. Consider the work done on this box by the frictional force due to the bed of the truck. In each of the following cases determine if the work done is positive, negative, or zero.
- i) If the box is dragged along the bed of the stationary truck?
- ii) If the box remains at rest on the bed of the truck as the truck moves down a straight road at constant speed.
- iii) If the box remains at rest on the bed of the truck as the truck moves down a straight road at increasing speed.
- iv) If the box slips backward relative to the truck as the truck moves down a straight road at increasing speed.
[-,0,+,+]
For the remaining problems be sure to do an energy bar chart after making your sketch. Be sure to check out the resources linked at the top of the page if you're not sure about the energy bar chart.
3) In 1955 an 80 kg paratrooper discovered during an exercise that he had a nonfunctional parachute. He landed in a snowbank and amazingly survived. His speed as he hit the snowbank was estimated at 50 m/s and subsequent measurements determined that he blasted approximately 1.0 m into the snowbank. Use energy methods and Newton's Laws to estimate the average force he felt as he was brought to rest
[1.0.105 N]
4) A 5 kg block, initially moving 4 m/s, slides 3.0 m down a ramp that is inclined 15 degrees above horizontal. The coefficient of friction is 0.2. After you have completed your energy bar chart and written the energy conservation equation consider which terms in the expression provide the answers to these questions.
- i) How much work is done on the block by the gravitational force as the block slides down the ramp?
- ii) How much work is done by the normal force as the block slides down the ramp?
- iii) How much work is done by the frictional force as the block slides down the ramp?
- iv) What is the total work done on the box by all forces acting on it?
- v) How fast is the block moving after 3.0 m?
[roughly +50 J, 0 J, -29 J, +11 J, 4.5 m/s]
5) An 80 kg bricklayer figures out a great way to get a free ride 10 m to the top of the building. He ties a rope to himself that goes up and over a (frictionless) pulley where it is tied to a partial pallet of bricks with a mass of 100 kg. The bricks are 10 m above the ground so he figures that when the bricks get to the ground he will arrive at the top of the building. What has he forgotten? How fast will he be going when he gets 10 m above the ground?
[7 m/s]
6)This is a silly logic problem but is a good test of your ability to write down what you think the problem means and keep track of symbols. A man racing his sister has half the kinetic energy of his sister who has half his mass. If he speeds up by 1.0 m/s he will have the same kinetic energy as his sister. What are the initial speeds of the man and his sister?
[2.4 m/s, 4.8 m/s]
7) For this problem you will need to extend your thinking a little. Power is the rate at which work is done which is most easily thought of as the work done devided by the time it takes to do the work. 1W = 1J/s. See if you can apply this idea in the following context. Your energy bar chart will be very helpful here. You are designing a freight elevator which must carry a total load of 1200 kg (including the elevator) 54 m straight up in 3.0 minutes. You choose a 950 kg counterweight to assist the elevator. How much work must the elevator motor do to raise the mass 54 m? What power rating should the motor have to accomplish this task in 3.0 min? How many hp (horsepower) is this? (1 hp =746 watts)
[1.0 hp]
8) In a spud gun the potatoe is pushed up the barrel by expanding hot gasses. The muzzle velocity of a typical 400 g spud is 30 m/s. The force on the spud is given by F(x)= K/(x+.08) where x is the distance along the barrel in meters. If the length of the barrel is 80 cm what is the constant K and what is the peak force on the spud? Because the force is not constant you will need to integrate but what?.
[750 J, 9400 N]