Potential And Kinetic Energy JSS2 Basic Science Lesson Note

 POTENTIAL AND KINETIC ENERGY

Potential energy is the type of energy possessed by a body due to its position. A stone on the ground does not have energy so long as it is lying on the ground. The stone cannot be seen doing any work. However, if a stone is on a table and it is placed on a table and it falls off, it can break a lamp on which it falls, the stone has done some work by its position. Therefore, when the stone is on the table, it has energy stored up as a result of its position. This energy increases as the height of the table increases and it decreases as it falls towards the ground. When it reaches the ground, it has zero potential energy.

KINETIC ENERGY

This is the energy possessed by a moving body. For example, a moving car, a man running, a falling orange, a fired bullet, a rolling ball and so on all possess kinetic energy. Both forms of energy are examples of a more general form of energy called MECHANICAL ENERGY.

KINETIC ENERGY

Kinetic energy(K.E) is the energy possessed by an object due to its motion i.e movement energy √K.E=1/2mv2

Where M is the mass of  the moving object

V is the velocity of the object.

Potential energy is the energy possessed by an object by its position at rest ( its height). Potential energy is stored energy.

P.E=m.g.h

M is the mass of the object;

g is the acceleration due to gravity; and

h is the height of the object from the ground.

Example1                                                                                                        

If the mass of an object is 6kg and the height is 5m.Calculate the potential energy.(Assuming g=10m/s2)

Solution

P.E=m.g.h

M=6k.g

h=5m

g=10m/s2

P.E=6x5x10

=300J 

Example 11

If the kinetic energy of a ball moving at a velocity of 10m/s2 is 50KJ.What is the mass of the ball?

Solution

K.E=1/2mv2

K.E=50KJ=50,000J

Velocity=10m/s2

50,000=1/2x mx102                               

i.e 50,000=1/2x 100m

=1/2x100m

50,000=50m

50m==50,000

M=50,000/50

=1000Kg

ENERGY TRANSFER WHEN WORK IS DONE

When an object is dropped from above the ground, work is done as the object is pulled to the ground. As the object is falling and work is done, the potential energy of the body is changed to kinetic energy. In principle, the quantity of potential energy stored in a body is always equal to kinetic energy produced when the body is released to do work. In other words, when energy changes, for example from potential to kinetic, there is always an accompanying work done.

EXAMPLE

What work is done when  a mass of 6kg is raised through a vertical height of 3.5m(acceleration due to gravity is 10m/s2)

Mass=6kg

Acceleration( due to gravity)=10m/s2

Work done W=Mass x acceleration x distance

= 6kgx10x 3.5

=210 joules.

 POWER

Power is also related to the concepts of energy and work. Power is defined as the rate of doing work, i.e. work done divided by time.

Power=Workdone/Time taken.

The unit of power is Watt(w)

Example  1

What is the power of a child that has done work of 60 J in 20 seconds?

Solution

Power P=Work/Time

=60/2=3 watts 

Example 2

Calculate the power of a pump that can lift 500kg of water through a vertical height of 12m in 0.3 minutes. Assuming g=9.8m/s2

Force=mass x acceleration(g)

Force=500kg x9.8N

Distance=12m

Workdone=500 x9.8 x12m

Time taken= 0.3minutes x60

=18 secs

Workdone=mass x acceleration x distance

Workdone=Force x distance

Since force= mass x acceleration

Workdone=500kg x 9.8N x12m

=58800J

 Power=workdone/Time taken

=58800/18

Power=3266.67 watts

Convert this to kilowatts by dividing by 1000

=3266.67/1000

=3.27kilowatts