work and power and energy
A metal ball is held stationary at a height h0 above the floor and then dropped downward. Which graph best shows the relationship between the Total energy E of the metal ball and its height h with respect to the floor?
B
7. A 15 kg cart is moving with a velocity of 7.50 m/s down a level hallway. A constant force of 10 N acts on the cart, and its velocity becomes 3.20 m/s.
a. How much work is done on the cart?
b. How far did the cart move while the force acted?
A: -345 J
B: 34.5 J
9. A 2kg ball rolls off a 0.8m high table with an initial speed of 7 m/s. Ignore air resistance. As it first leaves the table, what is its kinetic energy?
49 J
16 J
11 J
7 J
A: 49 J
11. As an object freely falls from a building, what happens to the energy of the object as it falls?
A. Its potential energy and kinetic energy increase.
B. Its potential energy and kinetic energy decrease.
C. Its potential energy increases and its kinetic energy decreases.
D. Its potential energy decreases and its kinetic energy increases.
answer:D
13. A worker lifts a 1 kg box from the floor, moves it 5 meters, then places it on a shelf 1 meter from the floor. Four students in physics class attempt to explain how much network was done on the box.
Student 1: The potential energy of the box on the floor was zero and now it is 10 J because PE = mgh. Work is the change in energy. Therefore, 10 J of work was done on the box.
Student 2: The box weighed 10 N because weight = mg. A force of 10 N was exerted for 1 m while lifting the box. Force times distance equals work, so, 10 J of work was done on the box.
Student 3: You are forgetting the box was moved 5 m while it was being held. That must have been work too. So, more than 10 J of work was done on the box.
● Student 4: No! No! No! Work is the change in kinetic energy. The box started out at rest and ended up on the shelf at rest, so there was no change in kinetic energy. Don’t overlook the fact that gravity was doing negative work on the box while the person moving the box was doing positive work on the box. Therefore, no work was done on the box!
Which student’s explanation is correct?
A. Student 1
B. Student 3
C. Student 4
D. Both Student 1 and Student 2
answer: A