Work, Energy and Power

What is meant by work done?

A force moving an object a distance in the direction of the force

What are the units of Work Done?

Nm or Joules

What is the energy transfer of lifting an object off the ground, and how is work done?

The object has gained gravitational potential energy

energy is transferred when work is done.

This is because applying a force on the object will make it move through a certain distance.

What is meant by energy?

Energy is defined as the capacity to do work

How do you calculate work done?

Work done= Fcosθ(Force) X x(Distance)

What equation can be used when the loss in gravitational potential energy= gain in kinetic energy to get velocity?

Derive through the equations of KE and GPE

Loss in GPE= mgh

Gain in KE= ½ mv²

mgh= ½ mv² →mass cancels out

gh= ½ v²

v= √2gh

Derive an expression relating power, to constant speed, v?

Power= W(energy transferred) / t (time taken)

Power= Force X distance / Time taken

→ Distance/ time = velocity

Power = Force X Velocity

P=Fv

How is efficiency calculated?

Useful output energy/ Total input energy X 100%

What does it mean if something has high efficiency?

The total energy of a closed system remains constant, and most the energy is useful and will not be dissipated into the surroundings

Define Power in words?

The rate of work done or energy transferred per unit time

What is the equation for power relating energy transferred and time taken?

Power (W/ Watts) = W(energy transferred) / t (time taken)

What are 2 alternative units for powers?

• Joules per second Js^-1

• Newton meters per second Nms^-1

Give an example where a constant force has to be applied on an object to maintain constant speed and explain how?

A car

The rate of work done by the forward force provided by the car engine is equal to the rate of work done against the frictional forces acting on the car

What is gravitational potential energy?

The capacity of doing work as a result for an object’s position in a uniform gravitational field

• GPE is gained when an object is higher

• GPE is lost when an object goes lower

How is GPE calculated? include units and terms

E (GPE) =

M(Mass/Kg) x g(gravitational field strength) x h(height/m)

Why does the equation of GPE only work for objects in a uniform field?

As there is no acceleration

Give an example of where KE and GPE are exchanged?

A pen dropping

An object falling from rest will lose GPE, and from the principle of conservation of energy, the object will gain an equal amount of KE

The equation v= √2gh will only work if what assumption is made?

There are no frictional losses

What is Kinetic Energy?

Kinetic energy is the energy with an object as a result of its motion

How is KE calculated? Define terms and Units

E (Kinetic energy)=

½ X M(Mass/Kg) X v² (Velocity/ms^-1)

For objects travelling at the same speed, how is KE related to mass?

KE is directly proportional to the mass

What are the 2 general classifications of energy systems with mass?

• GPE

• KE

State the principle of the conservation of energy?

The total energy of a closed system remains at constant energy, and energy cannot be created nor destroyed, but it can only be transferred from one to another

What does the term “potential” mean?

Hidden or stored

Give 2 examples of Potential energy?

• Elastic Potential energy

• Gravitational Potential Energy

Is Energy a Scalar or Vector Quantity?

Scalar

What is the SI Unit of Energy?

Joules/ J