Chapter 7 SE Quiz

A force produces power P by doing work W in a time T. What power will be produced by a force that does six times as much work in half as much time?

12P (Since Power = Work/Time, if you perform six times as much work in half the time, we have P1 = 6W/(0.5 T) = 12W/T = 12P)

The ratio of the final kinetic energy to the initial kinetic energy of an object is one half. If the initial velocity of the object is 10 m/s, what is the final velocity?

7.1 m/s (Use the definition of kinetic energy to solve this problem: KE = 1/2 m v^2.)

Kinetic energy is proportional to speed.

False

The work required to stretch a spring changes linearly with the amount of stretch.

False

Work is a vector quantity.

False

A truck has four times the mass of a car and is moving with twice the speed of the car. If Kt and Kc refer to the kinetic energies of truck and car respectively, it is correct to say that

Kt = 16Kc. ( Since KE = 1/2 m v^2, we have a multiplicative factor of 4 due to the mass differential and an additional factor of 4 due to the speed of the truck (2x the speed of the car). This results in a 16x KE multiplier for the truck).

Can work be done on a system if there is no motion?

No, since work involves a non-zero displacement. (Work, by definition, requires a displacement. So some motion must be involved if work is performed on a system.)

If the net work done on an object is positive, then the object's kinetic energy

increases. (The total work done on an object equals the change in its kinetic energy. So if the total work is positive, the kinetic energy of the object must increase.)

If you push twice as hard against a stationary brick wall, the amount of work you do

remains constant at zero. (Since the wall has no displacement, no work is being performed even though significant force is being applied.)

Consider a plot of the displacement (x) vs. applied force (F) for an ideal elastic spring. The slope of the curve would be

the reciprocal of the spring constant. (For an ideal elastic spring, the force and displacement are related by the following equation: F = -kx, where k is the spring constant and x is the displacement of the spring from equilibrium. Solving for x we have: x = (-1/k)F. Therefore the slope of the x vs. F graph is proportional to the reciprocal of the spring constant.)