Energy is the ability to do work or make a change.

4. A body which is moving has kinetic energy.

5. Raise a book & put it on a high shelf. Your energy is stored in the book. The stored energy is potential energy.
When the book falls down, its potential energy is converted into kinetic energy.

Mechanical energy = potential energy only.

Mechanical energy of the ball at B = Mechanical energy only.

Potential energy increases when the object’s weight or height increase.

Potential energy = weight x height.

Example: compare the potential energy of 2 balls, the weight of each is 1 Newton & the first is placed at a height of 1m & the 2^nd is placed at a height of 2 m.

Solution:

Potential energy (1) = 1 Newton x 1 m

= 1 Joule

Potential energy (2) = 1 Newton x 2 m

= 2 Joules

Potential energy of the 2^nd object is double the potential energy of the 1^st object.

Kinetic energy increases if the mass or the speed of the moving object increase.

The following rule calculates kinetic energy .

A solved example:

Calculate the kinetic energy of a ball that has

a mass =0.5 kg & moves with a speed 3 m/s.

Solution:

Kinetic energy = ½ x 0.5 x (3)²

= 2.25 Joules.

I. Choose the correct answer:

1. An object of 20 N weight is placed on a shelf of height 5 m. its potential energy = …….J

(50 – 100 – 150)

2. A truck & a car move with the same speed. The car has ……………kinetic energy than/ as truck.

(less – more – the same)

3. The …………………..energy of water at the top of waterfalls is maximum.

(kinetic – potential – mechanical)

4. The kinetic energy of a falling ball……………………gradually.

(doesn’t change – increases – decreases)

5. An object is thrown upwards, therefore its velocity……………...

(increases – decreases – doesn’t change)

II. Solve the following problems:

1. Calculate the mechanical energy of a book whose weight = 10 Newton & is placed on a shelf 3m high.

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

2. Calculate the mechanical energy of a ball of weight 3 Newton , its mass = 0.3 kg , it’s thrown up with speed 3 m/s to a height 4m.

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

A pendulum is a ball tied with a string & fixed at a point. When the ball is pushed, it moves to the right & then back to the beginning point, then to the left.

A B

At points A&B , the potential energy of the pendulum is maximum.

As it moves away from point A, its energy is changed to kinetic energy.

The mechanical energy of the pendulum is the same at each point.

Forms of energy:

Activity 1:

Simple cell

1. In a glass beaker, put 2 metal plates separately
(zinc & copper).

2. Fill half the beaker with diluted acid.

3. Connect the 2 plates externally with an electric current.

4. Put a voltammeter in the circuit to measure the electric current.

Ø Observation :

The pointer of the voltammeter moves to indicate the electric current is passing through the circuit.

Ø Conclusion:

Chemical energy in the simple cell is transformed into electric energy.

Activity 2:

Prepare an electric circuit like the one in the figure, close the circuit using the switch , open the circuit using the switch.

Ø Observation :

The bulb light up when the circuit is closed. The bulb is turned off when the circuit is open & electricity isn’t passing in the circuit.

Ø Conclusion:

In the electric bulb (lamp) electric energy is converted into light energy.

Energy is transformed from one form to another as in the following table.

Device	Energy used	Energy produced
1. Electric bell	Electric	Sound
2. Burning fuel in a car	Chemical	Mechanical & heat
3. Electric generator (dynamo)	Mechanical(kinetic)	Electric
4. Radio	Electric	Sound
5. Electric heater	Electric	Heat

Modern technology has many harms, some are mentioned in the following table.

Nuclear weapons

I. Fill in the blanks:

1.

2.

Device	Energy used	Energy produced
1. Television
2. Cellular phone (mobile)
3. Sewing machine
4. Alarm clock

II.