Energy Conservation

joules second

Power is measured in ________, also known as watts.

Meter

________ is called a joule, abbreviated as 1 J.

opposite displacement

Work can be negative if the force is applied in the direction ________.

Energy

The ability to do work

Potential energy

Energy of position

h

the height of an object above a certain point, for vertical height above the zero of potential

A block of weight mg = 100 N slides a distance of 5.0 m down a 30-degree incline. How much work is done on the block by gravity?

(A) 500 J

(B) 430 J

(C) 100 J

(D) 50 J

(E) 250 J

E—The force of gravity is straight down and equal to 100 N. The displacement parallel to this force is the vertical displacement, 2.5 m. Work equals force times parallel displacement, 250 J

A block of weight mg = 100 N slides a distance of 5.0 m down a 30-degree incline.

If the block experiences a constant friction force of 10 N, how much work is done by the friction force?

(A) −43 J

(B) −25 J

(C) −500 J

(D) −100 J

(E) −50 J

E—The force of friction acts up the plane, and displacement is down the plane, so just multiply force times distance to get 50 J. The negative sign indicates that force is opposite displacement.

Two identical balls of mass m = 1.0 kg are moving toward each other.

What is the initial kinetic energy of the system consisting of the two balls?

(A) 0 joules

(B) 1 joules

(C) 12 joules

(D) 18 joules

(E) 36 joules

E—Kinetic energy is a scalar, so even though the balls move in opposite directions, the KEs cannot cancel. Instead, kinetic energy (1 kg)(6 m/s)2 attributable to different objects adds together algebraically, giving 36 J total.

A mass experiences a potential energy U that varies with distance x. The mass is released from position x = 0 with 10 J of kinetic energy. Which of the following describes the long-term motion of the mass?

(A) The mass eventually comes to rest at x = 0.

(B) The mass slows down with constant acceleration, stopping at x = 5 cm.

(C) The mass speeds up with constant acceleration.

(D) The mass oscillates, never getting farther than 5 cm from x = 0. (E) The mass oscillates, never getting farther than 10 cm from x = 0.

D—Because he starts with a KE of 10 J, he can, at most, have a potential energy of 10 J, which corresponds on the graph to a maximum displacement of 5 cm.