Chapter 7 questions
car 1 has four times the mass of car 2, but they both have the same KE. if the speed of car 2 is v, what is the speed of car 1
2v.
only the total forces acting on an object can do work. is this statement true or false.
False. Any force acting on an object can do work. The work done by different forces may add to produce agreater net work, or they may cancel to some extent. It follows that the net work done on an object can bethought of in the following two equivalent ways: (i) The sum of the works done by each individual force; or(ii) the work done by the net force.
give an example of frictional force doing negative work
Frictional forces do negative work whenever they act in a direction that opposes the motion. For example,friction does negative work when you push a box across the floor, or when you stop your car
the net work done on a certain object is 0. what can you say about the speed
If the net work done on an object is zero, it follows that its change in kinetic energy is also zero. Therefore,its speed remains the same.
a ski boat moves with a constant velocity. is the net force acting on the boat doing work?
The fact that the ski boat's velocity is constant means that its kinetic energy is also constant. Therefore, the net work done on the boat is zero. It follows that the net force acting on the boat does no work. (In fact, the net force acting on the boat is zero, since its velocity is constant.)
a package rests on the floor of an elevator that is rising with constant speed. the elevator exerts an upward normal force on the package, and hence does positive work. why doesnt the KE of the package increase?
gravity exerts an equal and opposite force on the package, which makes the net force on the package 0. which is no change in KE
Is it possible to do work on an object that remains at rest?
no - work requires that a force acts through a distance
engine 1 produces twice the power of engine 2. is it correct to conclude that engine 1 does twice as much work as engine 2?
no, power depends both on the amount of work done by the engine and the amount of time during which the work is performed.
engine 1 does twice the work on engine 2. is it correct to conclude that engine 1 does twice as much work as engine 2
no, we must also know how much time it take for engine 1 to do the work.
an object moves with a constant velocity. is it safe to conclude that no force acts on the object
no, what we can conclude is that the net force acting on the object is 0
a pitcher throws a ball at 90mi/h and the catcher stops it in her glove. is the work done on the ball by the pitcher + - 0. is the work done on the ball by the catcher +-0
the ball gains KE from the pitcher so the work is + bye call losses KE from the catcher so the work is -
give an example of frictional force doing positive work
the frictional force between your shoes and the ground does positive work
a block of mass m and speed v collides with a spring, compressing it. what is the compression of the spring if the force constant of the spring is increased by a factor of 4?
think 1/2k (delta x) ^2 if the force constant is increased by a factor of 4, the compression distance must be halved to produce the same amount of work. -- thus the compression of the spring is 1/2 delta x
a force that is always perpendicular to the velocity of a particle does no work on the particle. is this true or false
true, to do work on an object a force must have a nonzero component along its direction of motion
to get out of bed in the morning you have to do work, why?
you must do work against the force of gravity to raise your body out of bed
in one complete orbit ( by the space station @ 375km above the earth surface) is the work done by the earth on space positive, negative, or zero
zero, the gravitational force exerted by the earth is radial, but the displacement is tangential
