C5-6. Mechanics II

Question 1

What is the definition of work done?

Answer 1

Work is done when an external force causes an object to move through a certain distance. It is measured in Nm.

Question 2

What is the equation for work done?

Answer 2

W=Fx, where W is measured in Joules, F in Newtons, and X being distance in metres.

Question 3

What can we say about the relationship between energy transferred and work done?

Answer 3

Work done = energy transferred. When an object is falling for example, its weight does work, transferring gravitational potential to kinetic energy.

Question 4

How might we derive an equation for work done by a force applied at an angle?

Answer 4

work done = Fx cos(angle)

Question 5

Define kinetic energy, and give an equation to calculate it.

Answer 5

Energy due to motion of an object with mass. E=1/2mv^2

Question 6

Define GPE, and give an equation to calculate it.

Answer 6

Energy of an object due to location in a gravitational field. E=mgh

Question 7

Define chemical energy.

Answer 7

Energy contained within chemical bonds of atoms.

Question 8

Define elastic potential energy, and give an equation to calculate it.

Answer 8

Energy stored in an object due to a reversible change in shape. E=1/2ke^2

Question 9

Define electrical potential energy.

Answer 9

Energy of electrical charges due to their position in an electric field.

Question 10

Define nuclear energy.

Answer 10

Energy within the nuclei of atoms released when subatomic particles are rearranged.

Question 11

Define radiant/electromagnetic energy.

Answer 11

Energy of any EM wave stored within the oscillating electric and magnetic fields.

Question 12

Define sound energy.

Answer 12

Energy of mechanical waves due to the movement of atoms.

Question 13

Define internal/thermal energy.

Answer 13

The sum of random potential and kinetic energies of atoms in a system.

Question 14

What is the principle of conservation of energy?

Answer 14

The total energy contained within a closed system remains constant; energy can neither be created nor destroyed, only transferred.

Question 15

How can we explain and derive the equation for gravitational potential energy?

Answer 15

E=W=Fs
W=(mg)s and S=h
Therefore E=mgh

Question 16

If work is done by a resistive force on a propulsive force as an object is moving, what can we say about its kinetic energy?

Answer 16

Work is done against the propulsive force causing the final kinetic energy to be lower than. the initial.

Question 17

When an object is falling, what can we say about the transfer of energy through it?

Answer 17

• Initially the total energy is in the gravitational potential store
• This is gradually transferred to the kinetic store
• Air resistance does work against the GPE causing a lower final kinetic energy

Question 18

Define power and give an equation for it.

Answer 18

Power is the rate of work done (the rate of energy transfer), measured in Js-1 or Watts. P=W/t

Question 19

How could we derive the power equation for an object in motion?

Answer 19

P=w/t and W=Fx
P=Fx/t therefore P=Fv

Question 20

What can we say about out the forces and work done on a car moving at a constant speed on a flat plane?

Answer 20

The work done by the forward force provided by the car’s engine is equally opposed by the work done due to frictional forces such as air resistance, rolling resistance etc.

Question 21

What equation do we use to calculate the efficiency of a system?

Answer 21

efficiency = useful output/total input, where we can use energy in J or power in W.