CHAPTER 7 11/04/2010
4.- Which requires more work-lifting a 50-kg sac a vertical distance of 2 m or lifting a 25-kg sack avertical distance of 4 m?
The 50-kg sac weighs 500 N, and the force required to lift it will be 500 N. And the force required to lift the 25-kg sack will be 250 N.
The work done lifting the 50 kg sac willbe: W=F X= 500 N x 2 m = 1000 joules
The work done lifting the 25 kg sac will be: W=F X= 250 N x 4 m = 1000 joules
Equal amount of work are done in both cases.
They require the same amount ofwork because the formula for work is force times distance.
5.- If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for eachcompare? How about for the case in which the lighter sack is moved its distance in half the time?
Power for each is the same because the same work is done in the same time. Twice the power is required todo the same work on the lighter sack in half the time.
8.- Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, how do their gains ofpotential energies compare?
10.- A moving car has kinetic energy. If it speeds up until it is going 4 miles as fast, how much kinetic energy does it have in comparison?
11.- Compared with some originalspeed, how much work must the brakes of a car supply to stop a car that is moving 4 times as fast? How will the stopping distance compare?
12.- If you push a crate horizontally with 100 N across a10-m factory floor and friction between the crate and the floor is a steady 70 N, how much kinetic energy is gained by the crate?
15.- An apple hanging from a limb has potential energy because of itsheight. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?
Plug and chug:
4.- Calculate the watts of power expended when a force of 2 N moves a book 2...