Chapter 7 Flashcards
The SI units of energy are
kg*m^2/S^2
If mechanical work is done on a body, the body must
move
Initially a body moves in one direction and has kinetic energy K. Then it moves in the opposite direction with 3 times its initial speed. What is the kinetic energy now?
9K
K=1/2mv^2
Negative work means
The applied force is opposite to the displacement
A body moves with decreasing speed. Which statement is true?
The net work done on the body is negative, and the kinetic energy is decreasing.
A 5-kg object slides down a frictionless surface inclined at an angle of 30 degrees from the horizontal. The total distance moved by the object along the plane is 10 meters. The work done on the object by the normal force of the surface is
zero
A force F pushes a block along a horizontal surface against a force of friction Ff. If the block undergoes a displacement s at constant velocity, the work done by the resultant force on the block
is zero
A particle moves halfway around a circle of radius R. It is acted on by a radial force of magnitude F. The work done by the radial force is
zero
A person pushes against a spring, the opposite end of which is attached to a fixed wall. The spring compresses. Is the work done by the spring on the person positive, negative or zero?
Negative
A variable force is represented on an F-versus-x graph. Which of the following is the work done by this force?
LOOK UP
You release an object from rest at a high altitude. If air resistance is considered, which curve best represents the kinetic energy of the body as a function of the distance fallen?
Arched line from zero (not upside down)
A motor is lifting a mass of 35.0 kg at a constant speed of 6.00 m/s. If friction is neglected, the power developed by the motor to do this lifting is
2.1 X 10^3 W
If the average power output of a car engine is the same as a 100-W light bulb, how long would it take a 1200 kg car o go from 0 to 96 km/hr (60 mph)?
P= Delta KE/ t
t = Delta KE/ P
t = 1/2 (1200 kg)(26.7 m/s)^2/ 100W
4.3 x 10^3 seconds
Power P is required to lift a body a distance d at a constant speed v. What power is required to lift the body a distance of 2d at constant speed 3v?
P1/P2 = V1/V2 p2 = 3V/V x P P2 = 3P
Power P is required to do work W in time interval t. What power is required to do work 3W in time interval 5t?
3P/5
