i)

Power is the rate of doing work

OR

Power is the rate at which work is done/energy is used

OR

Power is energy used up/work done per unit time.

OR

Power = $\frac{\mathrm{Work\; done}}{\mathrm{Time}}$

ii)

Power = $\frac{\mathrm{Work\; done}}{\mathrm{Time}}$

Work done = Force x Distance
Force = Mass x Acceleration due to gravity

Mass of the packing case = 50 kg
Acceleration due to gravity = 10 m^-2

Force = 50 kg x 10 m^-2 = 500 N

Distance = 10 m

Work done = 500 N x 10 m = 5000 J

OR

Work done = potential energy = Mass x Acceleration due to gravity x Height (mgh)

Work done = 50 x 10 x 10 = 5000 J

Time = 30 s

Power = $\frac{\mathrm{5000\; J}}{\mathrm{30\; s}}$ = 166.67 W/166.7 W

i)

Simple machine is a device that can be used to make work easier or faster.

OR

Simple machine is a device on which a force/an effort is applied at one point to overcome another force/load at another point.

OR

Simple machine is a device which enables a small force (effort) to overcome a large force (load).

ii)

Examples of a simple machine

1. Lever (Examples are a crowbar, claw hammer, a pair of pliers)
2. Wheel and axle (Examples are the steering wheel of a car, bicycle pedal)
3. Pulley (Examples are a pulley used in raising a load)
4. The inclined plane (Examples are ramps, staircases, hilly roads)
5. The wedge (Examples are knife, axe, plough)
6. Screw (Examples are A screw bolt)

(i)

Power is the time rate of doing work

OR

Power is the rate of energy dissipation/rate of using energy

OR

Power = $\frac{\mathrm{Work\; done\; (W)}}{\mathrm{Time\; (T)}}$

OR

Power = $\frac{\mathrm{Energy\; transformed\; (E)}}{\mathrm{Time\; (T)}}$

(ii)

The S.I unit of power is watt (W)

(i)

Work input = Effort x Effort distance

Work input = 25 N x 4 m = 100 J

(ii)

Work output = Load x Load distance

Work output = 20 N x 2 m = 40 J

(iii)

Efficiency of the machine = $\frac{\mathrm{Work\; out}}{\mathrm{Work\; input}}$ x 100%

Efficiency of the machine = $\frac{\mathrm{40\; J}}{\mathrm{100\; J}}$ x 100% = 40%

OR

Efficiency of the machine = $\frac{\mathrm{Mechanical\; Advantage\; (M.A)}}{\mathrm{Velocity\; Ratio\; (V.R)}}$ x 100%

Mechanical Advantage = $\frac{\mathrm{Load}}{\mathrm{Effort}}$ = $\frac{20}{25}$ = $\frac{4}{5}$

Velocity Ratio = $\frac{\mathrm{Effort\; Distance}}{\mathrm{Load\; Distance}}$ = $\frac{4}{2}$

Efficiency of the machine = $\frac{\frac{4}{5}}{\frac{4}{2}}$ x 100%

Efficiency of the machine = $\frac{4}{5}$ ÷ $\frac{4}{2}$ x 100%

Reciprocate the fraction after the ÷ and change the ÷ to multiplication (x)

Note: reciprocate means the numerator becomes the denominator and the denominator becomes the numerator.

Efficiency of the machine = $\frac{4}{5}$ x $\frac{2}{4}$ x 100%

Efficiency of the machine = $\frac{\mathrm{4\; x\; 2}}{\mathrm{5\; x\; 4}}$x 100%

Note: the 4s cancel each other.

Efficiency of the machine = $\frac{2}{5}$x 100%

Note: 5 divides itself 1 time and 100, 20 times.

Efficiency of the machine = 2 x 20% = 40%

(i)

Second class lever

(ii)

I → Effort
II → Load
III → Pivot/Fulcrum

(iii)

Examples of second class levers

1. Wheelbarrow
2. Bottle opener
3. Nutcracker
4. Staplers
5. Nail clippers
6. Doors or gates

i)

Why an inclined plane is classified as a machine

1. It enables work to be done easier/faster
2. A heavier load could be overcome with a smaller effort by pushing it up the plane than by lifting it directly upwards

ii)

Reasons why the output energy of a machine is always less than the input energy

1. Part of the input energy is used to overcome friction
2. Part of the input energy is used to overcome inertia
3. Part of the input energy is used to overcome gravitational force

i)

Efficiency of a machine = $\frac{\mathrm{Output\; energy}}{\mathrm{Input\; energy}}$ x 100%

Efficiency of a machine = $\frac{\mathrm{5.0\; J}}{\mathrm{7.5\; J}}$ x 100%

7.5 = $\frac{75}{10}$

Efficiency of a machine = 5 ÷ $\frac{75}{10}$ x 100%

Reciprocate the fraction at the right of the ÷ and change ÷ to multiplication (x)

Note: reciprocate means the numerator becomes the denominator and the denominator becomes the numerator.

Efficiency of a machine = 5 x $\frac{10}{75}$ x 100%

Note:
1. 5 divides itself 1 and 75, 15 times
2. 5 divides 15, 3 times and 10, 2 times
3. 2 x 100 = 200

Efficiency of a machine = $\frac{200}{3}$% = 66.67%

ii)

Why the efficiency of a machine is always less than 1 or 100%

The efficiency of a machine is always less than 1 or 100% because part of the energy input is used to overcome friction and gravitational forces

OR

The efficiency of a machine is always less than 1 or 100% because part of the energy input is wasted in the machine

i)

A machine is a device that helps us to do work easier/faster.

OR

A machine is a device that enables a small force (effort) to overcome a large force (load).

ii)

Efficiency of a machine = $\frac{\mathrm{Work\; output}}{\mathrm{Work\; input}}$

OR

Efficiency of a machine = $\frac{\mathrm{Output\; energy}}{\mathrm{Input\; enery}}$

OR

Efficiency of a machine = $\frac{\mathrm{Mechanical\; Advantage\; (M.A)}}{\mathrm{Velocity\; Ratio\; (V.R)}}$

i)

Work is said to be done when an application of a force moves an object through a distance in the direction of the force

OR

Work is said to be done when a force moves a body through a distance in the direction of the force.

ii)

The unit of work is joules

i)

A machine is a device that makes work easier or faster

OR

A machine is a device which enables a small force (effort) to overcome a large force (load).

ii)

(i)

Work is said to be done when the point of application of a force moves an object through a distance in the direction of the force.

OR

Work is said to be done when a force moves a body through a distance in the direction of the force.

OR

Work is the product of force and the distance moved in the direction of the force.

OR

Work = Force x distance moved in the direction of the force

(ii)

Power is the rate of doing work.

OR

Power is the rate at which energy is dissipated

OR

Power = $\frac{\mathrm{Work\; done}}{\mathrm{Time\; taken}}$

(i)

A lever is a simple machine which consists of a rigid bar/rod free to turn about a fixed turning point called the fulcrum/pivot.

(ii)

Examples of second class levers

1. Wheel barrow
2. Nut cracker
3. Bread knife
4. Bottle opener
5. Paper cutter
6. Staplers
7. Doors or gates
8. Nail clippers

(i)

Mechanical advantage of a machine is the ratio of the load to the effort needed to raise the load

OR

Mechanical advantage (M.A) = $\frac{\mathrm{Load\; (L)}}{\mathrm{Effort\; (E)}}$

(ii)

Velocity ratio of a machine is the ratio of the distance moved by the effort to the distance moved by the load

OR

Velocity ratio (V.R) = $\frac{\mathrm{Distance\; moved\; by\; effort}}{\mathrm{Distance\; moved\; by\; load}}$

α)

β)

Effort distance + Load distance = Length of bar

Length of bar = 100 cm
Effort distance = 80 cm

80 cm + Load distance = 100 cm
Load distance = 100 cm - 80 cm
Load distance = 20 cm

(i)

Devices that could be used to carry a heavy body onto a truck

1. Inclined plane
2. Pulley
3. Crane
4. Lifting magnets/Electromagnet
5. Forklift

(ii)

Simple levers used in everyday life

1. Wheel barrow
2. Can opener
3. A pair of scissors
4. Nutcrackers
5. Forceps
6. Sugar tong
7. Bottle opener
8. Crowbar
9. Claw hammer
10. A pair of pliers
11. Knife
12. Paper cutter
13. A pair of pincers
14. See saw

(iii)

Note: any of the above diagrams represents labelled lever.

(i)

Work

Work is said to be done when a force moves a body through a distance in the direction of the force

(ii)

Work = Force x Distance

Force = 20 N
Distance = 3.2 m

Work done = 3.2 m x 20 N = 64 J

i)

ii)

Factors that limit work output

1. Friction/wear and tear
2. Age of machine/period of usage
3. Weight of machine parts