3.3 Work, Energy and Power

Question 1

What is the work done defined as?

Answer 1

The product of the magnitude of the force and the distance moved by the object in the direction of the force.

Question 2

When a force is required to provide motion, there is a transfer of energy.

What does the magnitude of this energy transferred depend on?

Answer 2

The force required and the distance the object moves as a result of this force.

Question 3

I want to calculate work done on an object.

I have a force at an angle to the object’s direction of motion.

What do I do?

Answer 3

Use the component of the force parallel to the direction.

Question 4

Define 1 joule.

Answer 4

The work done when a force of 1N moves an object 1m in the direction of the force.

Question 5

What two units can work be measured in?

What are the SI base units?

Answer 5

Joule, Nm

SI: kgm2s-2

Question 6

Energy is the capacity for doing ____. It is a ______ quantity, measured in ______.

Answer 6

Energy is the capacity for doing WORK. It is a SCALAR quantity, measured in JOULES.

Question 7

What does the principal of conservation of energy state?

Answer 7

In a closed system, energy cannot be created or destroyed, but only transferred from one form to another.

Question 8

What is work done equivalent to?

Answer 8

Energy transfer.

Question 9

Kinetic energy is associated with an object as a result of what?

Answer 9

Its motion.

Question 10

What is kinetic energy measured in?

Answer 10

Joules.

Question 11

What is the SI base unit for kinetic energy?

Answer 11

kgm2s-2

Question 12

What is the formula for kinetic energy?

Answer 12

Ek = 1/2 MV^2

Kinetic = 1/2 x mass x velocity^2

Question 13

What is gravitational potential energy?

Answer 13

An object’s capacity to do work as a result of its position in a gravitational field.

Question 14

Gravity is an attractive force.

What does this mean for an object that gets higher in a gravitational field?

Answer 14

The object gains GPE, as energy must be supplied to move it against the gravitational attraction. GPE is therefore lost when the object falls back down and gets closer to the source of the gravitational field.

Question 15

What is the formula for GPE?

Answer 15

Mass x Gravity x Height

Question 16

How can we use the formulas for GPE and kinetic energy to prove the mass of an object has no bearing on its final speed?

Answer 16

Because Ek(max) = Ep(max)

mass X gravity X height = 1/2 x mass x velocity^2

2 x gravity x height = velocity^2

velocity = SQRT2 x Gravity x Height

Question 17

Define power.

Answer 17

The rate of work done.

Question 18

What is power measured in?

Answer 18

Watts.

Question 19

What is one watt defined as?

Answer 19

A rate of energy transfer of 1 joule per second.

Question 20

What is the SI base unit for power?

Answer 20

kgm^2s^-3

Question 21

[Part 1]

What can we derive from the equation for power?

Answer 21

The equation for the power required to move an object at a constant velocity against resistive forces. (eg driving a car)

Question 22

[Part 2]

Derive the equation.

Answer 22

P = W/t = Fx/t = F * x/t = Fv

Question 23

[Part 3]

How can we use this knowledge in the driving a car example?

Answer 23

Constant velocity = No acceleration = No net force.
No net force = Magnitude of driving force is same as resistive force.

We use the driving force when considering the power required by the car to maintain the constant velocity.

Question 24

The conservation of energy principle states that energy is constant in a closed loop.

What is the actual reality?

Answer 24

We have energy loss.

Question 25

What is energy loss?

Answer 25

Energy is transferred from the closed loop to a form that is not as useful, such as the thermal energy generated by a lightbulb.

Question 26

Is total energy conserved when energy is lost to the surroundings?

Answer 26

Yes.

Question 27

What is efficiency?

Answer 27

A measure of how much energy is conserved as useful energy?

Question 28

What would a greater efficiency mean?

Answer 28

Less energy is wasted.

Question 29

Give the formula for efficiency.

Answer 29

Efficiency [%] = (Useful Output / Total Input) x 100