# High School Physics : Understand Power

## Example Questions

### Example Question #1 : Power

sports car accelerates from rest to  in .  What is the average power delivered by the engine?

Explanation:

Power is equal to the work done divided by how much time to complete that work.

Work is equal to the change in kinetic energy of an object.

We can solve for the work by solving for the change in kinetic energy.

Since the initial velocity is , this can be dropped from the equation.

needs to be converted to

We can now plug this into the power equation.

The average power is  Watts.

### Example Question #1 : Power

Of the following, which is not a unit of power?

Watt

newton meter /second

watt/second

joule/second

watt/second

Explanation:

The unit for power is the Watt.  A watt is a measure of the Joules per second that an object uses.  Joules can also be written as Newton Meters.  Therefore a watt could also be considered a Newton Meter/Second.  The incorrect answer is the watt/second since watt is the base unit of power on its own.

### Example Question #1 : Power

To accelerate your car at a constant acceleration, the car’s engine must

Develop ever decreasing power

Maintain a constant power output

Maintain a constant turning speed

Develop ever increasing power

Maintain a constant power output

Explanation:

Power is equal to the Work put into the system per unit time.

Work is equal to the force acting on the object multiplied by the displacement through which it acts.

Therefore power is directly related to the force applied.

Force is also directly related to the acceleration of an object.   A constant force will create a constant acceleration.

Since power is directly related to the force applied, and the force must be constant to maintain a constant acceleration, the power must also therefore be constant.

### Example Question #861 : High School Physics

A bicyclist coasts down a  hill at a steady speed of .  Assuming a total mass of  (bike plus rider) what must be the cyclists’s power output to climb the same hill at the same speed?

Explanation:

First we need to analyze the motion of the rider on the way down the incline.  During this time, the bicyclist has a constant velocity, which means that all forces acting on him must be balanced.  He has a portion of the force of gravity pulling him down the hill and the force of friction opposing his motion.

We can use components to find the force of gravity in the x direction.

Since the net force is equal to  we know that the force of gravity in the  direction is equal to the force of friction.

Now let us consider how the bicyclist will travel when he goes up the hill.  This time, the forces acting on the bicyclist are the force of gravity and friction resisting his motion and his applied force going up the hill.

The force of friction and the force of gravity are the same as when the cyclist was coasting down the hill.

The bicyclist doesn  of work to get back up the hill.  We can now use this in our power equation to determine the amount of power used.

Power is equal to the work divided by the time to complete the work.

Work is equal to the force times the displacement through which the object moved.

We can substitute this into our power equation to get

Velocity is equal to the distance over the time.

Therefore power could be written as

### Example Question #5 : Power

How long will it take an  motor to lift a  piano to a window  above?

Explanation:

Power is equal to the work divided by the time it takes to complete the work.

Work is equal to the force times the displacement.

In this case the motor is lifting the piano.  Therefore the force that it is exerting is equal to the force of gravity pulling the piano down.

We can now calculate the work done on the piano.

We can now plug this value into the power equation and solve for the time.

It will take  seconds to the lift the piano.

### Example Question #861 : High School Physics

The quantity  is

The work done on the object by the force

The kinetic energy of the object

The power supplied to object by the force

The potential energy of the object

The power supplied to object by the force

Explanation:

Power is equal to the work divided by the time to complete the work.

Work is equal to the force times the displacement through which the object moved.

We can substitute this into our power equation to get

Velocity is equal to the distance over the time.

Therefore power could be written as

### Example Question #7 : Power

Some electric power companies use water to store energy.  Water is pumped from a low reservoir to a high reservoir. To store the energy produced in  hour by a electric power plant, how many cubic meters of water will have to be pumped from the lower to the upper reservoir?

Assume the upper reservoir is an average of  above the lower. Water has a mass of  for every .

Explanation:

First, we need to calculate the amount of energy produced by the power plant.

We know that this energy is stored in the form of gravitational potential energy.

If there is  for every  of water, we can divide this number by  to determine the number of  of water is pumped from the lower to the upper reservoir.

### Example Question #1 : Power

How many 75 W light bulbs connected to 120V can be used without blowing a 15A fuse?

24

32

12

18

20

24

Explanation:

Known

Power = Current x Voltage

There are  in the circuit.  We can then divide this number by  to determine how many lightbulbs can go into the circuit.

lightbulbs

### Example Question #2 : Power

A person accidentally leaves a car with the lights on. If each of the two headlights uses 40 W and each of the two tail lights 6 W, for a total of 92 W, how long will a fresh 12V battery last if it is rated at 75Ah? Assume the full 12V appears across each bulb.

7.6hr

14.7 hr

9.7hr

6.3hr

9.7hr

Explanation:

First, we need to determine how much current is used by the system.

If the battery is rated at 75Ah we can divide this number by the current needed by these devices to determine how long the battery would run.

### Example Question #10 : Power

You buy a 75W light bulb in Europe, where electricity is delivered at 240V.  If you use the bulb in the United States at 120V (assume its resistance does not change) how bright will it be relative to 75W 120V bulbs?

The bulb is brightest in the United States

The bulb has the same brightness in both locations

The bulb is brightest in Europe