Simple Machine

In this chapter Simple Machine, we discussed about concepts of machines, work, energy, power, mechanical advantage, velocity ratio, efficiency in Grade 8 Science under the Unit Physics .

Simple Machine 

The devices which are simple in structure and makes our work easier and faster are called simple machines.

OR

Simple Machines are those devices in which effort is applied at a more convenient point in a more convinient direction.

Advantages of simple machines:

1. Simple machines multiply force i.e. a larger load is lifted by applying a smaller effort.
2. They transfer force from one point to another.
3. They increase the rate of doing work.
4. They change the direction of applied force.
5. They help to do work safely.

Mechanical advantage(MA):

MA of a machine is defined as the ratio of load overcome by the machine to the effort applied.

MA = (load overcome) / (effort applied) = L / E

It is the ratio of two similar physical quantities i.e. two forces. MA of a machine shows how many times the machine multiplies the applied force. For example; MA of a machine is 4 means that the machine can lift a load 4 times heavier than the effort applied. OR, the machine multiplies force 4 times. MA of a simple machine can be less than 1, or equal to one or more than 1.

MA < 1, means that load is less than the effort applied, and the machine cannot multiply force.

MA > 1, means that load is greater than the effort applied and the machine can multiply force.

MA = 1, means that effort needed is equal to the load and the machine cannot multiply the applied effort.

Generally, machines are made having MA greater than one. MA of a machine is affected by friction which exists in the machines as it is ratio of two forces. So, to increase the MA of a machine, friction in it should be decreased by oiling or greasing.

Velocity Ratio (VR):

VR = Ed / Ld = (Distance moved by effort) / (Distance moved by load)

Velocity ratio is the ratio of distance travelled by the effort to the distance travelled by the load of the machine. Friction is not involved in it as it is not impacted by two forces. 

Velocity of load = Ld / T

Velocity of effort = Ed / T

So, Velocity ratio = Velocity of effort / Velocity of load = Ed / Ld (as T= time is cancelled)

Efficiency(η):

It is the ratio of output of the machine to the input appplied to the machine. It is denoted by η. 

Efficiency (η)= output / input =( L* Ld) / (E*Ed) * 100% = MA / VR * 100%

where, L = load, Ld = load distance, E= effort, Ed= effort distance.

For an ideal machine, efficiency is 100%. In real world, machine cannot have 100% effieciency because of the presence of frictional force in machines.

Nuclear energy (atomic energy):

The energy stored un the nucleus of an atom is called nuclear energy. Nuclear energy is released out when a heavier nucleus splits into lighter nuclei or lighter nuclei fuse together to form a heavier nucleus.

The process in which a heavier nucleus splits into two or more than two lighter nuclei with the release of large amount of energy is called nuclear fission.

The process in which loghter nuclei fuse together to form a heavier nucleus with the release of large amount of energy is called nuclear fusion.

Examples of nuclear fusion; energy is produced in the sun by nuclear fusion process. In the hydrogen, nuclei fuse together to form a helium nucleus. During this process, large amount of energy is produced in the sun.

Examples of nuclear fision; Uranium: Barium + Krypton + Energy

Nuclear fusion	Nuclear fission
Lighter nuclei fuse together to form a heavier nucleus.	A heavier nucleus splits into two or more lighter nuclei.
It produces comparatively large amount of energy.	It produces comparatively samll amount of energy.
It is the principle of hydrogen bomb.	It is the principle of atom bomb.

 Transformation of energy:

Energy can be changed from one form to another form. The process of conversion of one form of energy into another form of energy is called transformation of energy.

Examples;

1. Electrical fan: It converts electrical energy into mechanical energy.
2. Burning Candle: It converts chemical energy into heat and light energy.
3. Electric bulb: It converts electric energy into heat and light energy.
4. Electric heater: It converts electric energy into heat energy.
5. Burning wood: It converts chemical energy into heat and light energy.
6. Generator: It converts mechanical energy into electrical energy.
7. Electric motor: It converts electrical energy into mechanical energy.

 Power:

Power is defined as the rate of doing work.

Power = work done / time taken

It's SI unit is Joule / sec or Watt.

If 1 joule work is done in 1 second, then power of the body will be 1 Watt. 

Other units;

Horse power (Hp) = 750 Watt
(more accurately 746 W = 1 Horse power)
1 Kilo Watt = 1000 W 
1 Mega Watt = 1000 KW = 10^6 W

In case of electrical device, power is defined as the rate at which it covers electrical energy into heat and light energy.

Power =  Energy consumer / time taken

Example; power of an electric bulb is 100 W, that means the electric bulbs can connect 100 J of electric energy into heat and light energy in 1 second.

Work	Energy
It is defined as the product of force applied on a body and the distance travelled by a body in the direction of force applied.	It is defined as the capacity to do work.
It is the effect of energy.	It is the cause of work.

 

Energy	Power
It is defined as the capacity to do work.	It is defined as the rate of doing work.
It's SI unit is Joule.	It's SI unit is Watt.

 

Work	Power
It is the product of force applied on a body and distance travelled by the body in the direction of force applied.	It is the rate of doing work.
It's SI unit is Joule.	It's SI unit is Watt.
It does not depend on time.	It depends on time.

Numerical Problems:

1. A stone of mass 0.5 kg is thrown with the velocity 5 m/s. Calculate the kinetic energy.

Kinetic energy(K) = 1/2mv² = 1/2*0.5*5² = 6.250 Joule.

2. How much energy is required to lift 25 kg mass to a height 10m? (g = 9.8 m/s²).

Here,

mass of the substance = 2.5 kg
acceleration due to gravity = 9.8 m/s²
Potential energy = m*g*h = 2.5*9.8*10 = 2450 J.

3. A person can lift 20 bricks. each brick has 10N weight. He carried 20 bricks to a distance of 100m in 50 sec. Calculate its power.

Total weight force of bricks = 20*10 N= 200 N
Distance travelled = 100 m
Time taken = 50 sec
Work done = F*d = 200N*100m =20000 J

Power = Work done / time taken = 20000 J / 50 sec = 400 wat.

4. A man with mass 40 kg climbs up a stair case having 14 steps each of 15 cm height in 10 seconds. Calculate his power. (g=m/s²).

Mass of the man = 40 kg = 40* 9.8 = 892 N
Height of man = 14* 15 cm = 210 cm = 210 / 100 m = 2.10 m
W = F* d = 892*2.10 = 823 J

Now, Power = W / T = 823 J/ 10 sec = 8.23 Watt

5.  A crane lifts 72 kg mass though a height 20 m in 24 seconds. Calculate the power of the crane.

Mass lifted = 72 kg = 72 kg* 10 m/² = 720 N
Work done = F*d = 720 N * 20 m = 14400 J

Power = Work done / Time taken = 14400 J / 24 sec = 600 Watt

6. A crane lifts 3000 N load to a height of 10m in 4 seconds. Calculate its power in watt, horse power, and kilo watt.

Force = 3000 N
W = F* d = 3000 N * 10 m= 30000 J

P = W / T = 30000 J / 4 sec = 7500 W.

In Horse power, P = 7500 W = 7500 / 750 Hp  = 10 Hp ( 1Hp = 750 W)

In Kilowatt, P = 7500 W = 7500 / 1000 W = 7.5 KW (1 KW= 1000 W)