work energy and power grade 12

An applied force F accelerates an object of mass m on a horizontal frictionless surface, from velocity v to velocity 2v.

The net work done on the object is equal to:

If air resistance is negligible, the total mechanical energy of a free-falling body…

• decreases
• remains constant
• decreases

• becomes zero

The engine of a car does work, W, to increase the velocity of the car from 0 to v.

The work done by the engine to increase the velocity from v to 2v is…

• 3W
• 5W
• 4W
• 2W

Power is defined as the rate …

• speed at which work is done
• as the rate at which work is done or energy is expended

• amount of work done
• distance traveled per unit time

A car moves up a hill at constant speed. Which one of the following represents the work

done by the weight of the car as it moves up the hill?

• ΔE_k

• -ΔE_k

• ΔE_p

• -ΔE_p

Answer explanation

the weight of the car pulls the car down, working in the direction opposite to motion, i.e. negative work is done by the weight of the car. Since it moves up the hill at constant speed, there is no change in the kinetic energy of the car. It is the potential energy that changes. The car’s motor must do positive work to overcome the negative work of the car’s weight

An object moves in a straight line on a rough horizontal surface. If the net work done on the object is zero, then…

• the object has zero kinetic energy.

  
  

• the object moves at constant speed.

• the object moves at constant acceleration.

• there is no frictional force acting on the object.

Answer explanation

W_net = ΔE_k, so if the net work is zero, there cannot be a change in kinetic energy. Thus, the object moves with constant speed

A diver dives from a diving board into a swimming pool.

Which one of the following graphs represents his gravitational potential energy versus the time from the moment the diver leaves the diving board until just before he enters the water? Ignore the effects of friction.

• 
• 
• 
• 

Which one of the following is incorrect? Wnc is…

• equal to ΔEk.

• equal to the change in mechanical energy of an object.

• the total work done by a sum of non-conservative forces.

• a sum of works.

Answer explanation

Wnc= ΔEk +ΔEp – which is equal to the change in mechanical energy of an object

An object is pulled along a straight horizontal road to the right without being lifted.

The force diagram shows all the forces acting on the object.

Which one of the above forces does positive work on the object?

• W

• N

• F

• f

Answer explanation

the force acting in the direction of the motion does positive work.

The kinetic energy of object X is E.

Object Y has double the mass and moves with twice the velocity of X.

The kinetic energy of Y is …

• 4E

• 8E

Answer explanation

Ek (of Y) = ½ (2m)(2v)2 = ½ ·2·4 mv2 = 8E.