Energy, Work, and Simple Machines

 Work

When we push a solid wall, people commonly say we are doing work on the wall. But in physics, pushing a rigid wall that does not move is considered no work, even though the person may use a lot of energy to push. Why is that?

In physics, work is the process of moving the position of an object by a force. In formula form:

 

    Explanation :

$W$ = work (joule, J)

$F$ = force (newton, N)

$s$ = displacement of the object (meter, m)

 Examples

• Pushing a table until it moves.

• Lifting a load onto a desk.

   

If no displacement occurs, then work = 0, even if force is applied.

So if the displacement $s = 0$,  then the work $W = 0$

$As$The Qur’an ( Surat An-Najm : 39) reminds us: 

  

And indeed, a person will not gain anything except what he strives for.

It remind us about effort and reward. 

 

 

 Work is closely related to energy. Energy is the ability to do work. Work is energy transferred by a force to move an object.

 

Energy

As discussed above, energy is the ability to perform work. In the International System (SI), energy is measured in joules (J) or sometimes calories (cal).

1. Types of Energy

Energy exists in various forms. Based on form, energy can be classified into:

• Kinetic energy

• Potential energy

• Thermal (heat) energy

• Chemical energy

• Light (radiant) energy

• Sound energy

• Nuclear energy

• Electrical energy, and others.

   

2. Energy Transformations

Energy can change from one form to another. Examples:

a. Electrical → Light

• Example: a lamp.

b. Light → Electrical

• Example: a solar cell.

c. Chemical → Kinetic

• Example: food is converted to energy used for movement.

d. Electrical → Sound

• Example: radio or doorbell.

e. Electrical → Kinetic

• Example: a fan.

f. Nuclear → Electrical

• Example: nuclear power plant.

3. Energy Sources

Two main categories:

a. Renewable (can be replenished)

• Examples: sunlight, wind, water (waterfalls, ocean waves).

b. Non-renewable

• Examples: nuclear fuel, fossil fuels (oil, natural gas).

   

Energy can also be grouped into three useful categories: kinetic energy, potential energy, and mechanical energy.

Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion (speed). 

 

$m$ is mass (kg) and $v$ is velocity (m/s).

Potential Energy

Potential energy is the energy an object possesses because of its position, commonly height in a gravitational field. The gravitational potential energy:

$m$ is mass (kg), $g$ is gravitational acceleration (≈9.8 m/s²), and $h$ is height (m)

 

Mechanical Energy

Mechanical energy is the total energy of an object due to motion and position:

 

 Law of Conservation of Energy

Energy cannot be created or destroyed; it can only change from one form to another.

Example: Chemical energy in food → kinetic energy when we are running.

another example: 

A coconut falls from a tree. While it is on the tree it has gravitational potential energy. As it falls, potential energy converts into kinetic energy. At the moment just before hitting the ground kinetic energy is maximum. After impact, the mechanical energy is dissipated into sound and heat, etc.

 

In ideal (no loss) situations:

 

 or

 

 

Simple Machines

Definition

A simple machine is a tool that helps make work easier. It works by changing the size of a force, the direction of a force, or the distance over which the force is applied.

Although simple machines make work easier, the amount of work (work = force × distance) remains the same.

 

Types of Simple Machines

 

1. Lever

A lever is a rigid bar that rotates around a fixed point called the fulcrum. It has three parts:

• Fulcrum (F): the pivot point

• Load (W): the object to be moved

• Effort (E): the force applied

Types of levers:

1. First-class lever: Fulcrum between load and effort (e.g., scissors, seesaw).

2. Second-class lever: Load between fulcrum and effort (e.g., wheelbarrow, nutcracker).

3. Third-class lever: Effort between fulcrum and load (e.g., tweezers, shovel, human arm).

    

   Mechanical Advantage (MA) 

 or 

 

Fr  : resistance force

Fe : effor Force

2. Pulley

A pulley is a wheel with a groove around it, through which a rope or belt can move. It is used to change the direction of force or reduce the effort needed.

• Fixed pulley: Changes direction of force, MA = 1 (e.g., well pulley).

• Movable pulley: Reduces effort, MA = 2 (e.g., pulley for lifting loads).

• Compound pulley: Combination of fixed and movable pulleys, larger MA.

   

3. Inclined Plane

An inclined plane is a sloping surface used to raise objects to a higher level with less effort.

Mechanical Advantage (MA):

 

 Examples: ramps, screw, wedge.

 

4. Wheel and Axle

Consists of a large wheel attached to a smaller axle that rotate together. It reduces the effort needed to move or lift a load.

Mechanical Advantage (MA):

 

Examples: steering wheel, well wheel, spinning top.

 

5. Gears

Gears are wheels with teeth that interlock with each other. They transfer motion and force between machine parts.

Examples: clock, bicycle, car engine.

 

Assesmen

1. Test questions for  Energy, Work, and Simple Machines