TOPIC 8: WORK ENERGY AND POWER | PHYSICS FORM 1
S.I Unit of Energy
Different Forms of Energy
There are different forms of energy such as:
- Chemical energy
- Heat energy
- Light energy
- Sound energy
- Electrical energy
- Nuclear energy
- Solar energy
Difference between Potential Energy and Kinetic Energy
Distinguish between potential energy and kinetic energy
There are two types of chemical energy, which are:
- Potential energy:It is the energy possessed by a body mass in its position or state.
- Kinetic energy: It is the energy possessed by a body due to its motion.
Consider when the body is vertically thrown upwards with an initial velocity ‘u’ from the ground.
At the ground
Neglecting the air resistance, as the body moves upwards its velocity decreases it also experiences gravitational force (g) pulling downwards towards the earth’s centre.
The maximum Height Attained
The final velocity of the body will be zero (V=0)
That the object drops from Hmax that is; it leaves with zero Kinetic Energy. At position A in fig. 8. The conservation of mechanical energy (M.E) is given as:
The Transformation of Energy
Transducer is a device used to transform energy from one form to another.
- Battery converts chemical energy into electrical energy.
- A generator converts mechanical energy into electrical energy.
- A motor converts electrical energy into mechanical energy.
The Table Summarising Energy Transformation
|Chemical energy||Battery||Electrical energy|
|Chemical energy||Motor||Chemical energy|
|Mechanical energy||Generator||Electrical energy|
|Solar energy||Solar panel||Electrical energy|
|Chemical energy||Motor car||Mechanical energy|
|Electrical energy||Microphone||Sound energy|
|Electrical energy||Heater||Heat energy|
To demonstrate pressure of potential energy.
- A heavy stone
- A bucket full of water
- A strong inelastic rope; and
- Smooth pulley
- Collect the heavy stone, using a strong rope tie it to a bucket of water
- Pass the rope over smooth pulley fixed to a support.
- Hold stationary the heavy stone at height “h” above the ground
- Release the stone
- When the stone released the bucket of water will start to rise.
- The stone is said to have potentials energy because of its position above the ground.
- Lifting a body of mass “m” to a height “h” above the ground requires work to be done against gravity. Work = Mgh
A ball of mass 0.5 kg is kicked vertically upwards and rises to a height of 5m. Find the potential energy by the ball.
- Pull the bob of a simple pendulum in position A so that it is at height “h” above position B.
- Release the bob so that it swings to position C via the lowest position B and back to A.
- Consider the figure below:
As it swings downwards to position B, the height decreases, and as the result it loses potential energy.
- The bob has Vmax and hence K.Emax at B. The height at B is zero, thus the P.E is zero.
- As it swings towards C, the P.E increases and reaches its maximum again in position C, where the Kinetic Energy is zero. At position D, the energy of the bob is party potential and party Kinetic.
The Principle of Conservation of Energy
A stone of mass 2kg is released form a height of 2m above the ground.
Find the potential energy of the stone when it is at the height of 0.5m above the ground.
According to conservation of energy the loss of P.E should be equal to the gain in K.E, when the air resistance is neglected.
K.E of the stone at 0.5 above the ground = 30 Joules
A ball of mass 0.21kg is dropped from a height of 20m. on impact with the ground it loses 30J of energy. Calculate the height which it reaches on the rebound.
At the top of rebound the energy of the ball = potential energy (P.E)
Uses of Mechanical Energy
> It enables our body to do work,
> it makes work easier and faster, it is used to transport goods and people from one place to another, many transport vehicles uses the knowledge of mechanical energy. Examples of vehicles which uses mechanical energy are aeroplanes and motor cars.
The Concept of Power
Explain the concept of power
- It is a measure of the rate at which energy changes.
- This means that whenever work is done energy changes into a different form.
The S.I Unit of Power
State the S.I unit of power
The SI unit of power is Jules per second J/S or watts, W.
The Rate of Doing Work
Determine the rate of doing work
Suppose that two cranes each lift objects having masses of 200kg to a height of 12m. Crane A lifts its object in 10sec while crane B requires 15sec to lift its object. Assume they lift the objects at a constant velocity they do the same amount of work.
Each did a work that was equivalent to 23520J.
What is different for the two cranes is the rate at which they did the work or their generation of power.
The power of crane A can be calculated by;
How much power is required to accelerate a 1000kg car from rest to 26.7m/s in 8sec?
The work done on the car increases its Kinetic energy.
Car engine is rated in horsepower (hp) where 1hp = 746watts. What is the required power measured in horsepower?
Since work causes a change in energy. DE power can be considered as the rate of change of energy.