KNOWLEDGE ACADEMY AYODHYA
CLASS 9 – PHYSICS
Chapter: Work, Energy and Simple Machines
1. WORK
Definition: Work is said to be done when a force applied to an object produces displacement in the direction of the force.
Formula:
[
W=F\times s
]
Where:
- W = Work done
- F = Force applied
- s = Displacement
SI Unit: Joule (J)
Example: If a force of 10 N moves an object through 5 m:
W = 10 × 5 = 50 J
Types of Work
1. Positive Work: Force and displacement act in the same direction.
2. Negative Work: Force and displacement act in opposite directions.
3. Zero Work: No displacement occurs, or force is perpendicular to displacement.
2. ENERGY
Definition: Energy is the capacity to do work.
SI Unit: Joule (J)
Types of Energy
A. Kinetic Energy: The energy possessed by an object due to its motion is called kinetic energy.
Formula:
[
KE=\frac{1}{2}mv^2
]
Where m = mass and v = velocity.
Examples: A moving car, a running person and a rolling ball.
B. Potential Energy: The energy possessed by an object due to its position or configuration is called potential energy.
Formula:
[
PE=mgh
]
Where m = mass, g = acceleration due to gravity and h = height.
Examples: Water stored in a dam and a stone held at a height.
3. LAW OF CONSERVATION OF ENERGY
Statement: Energy can neither be created nor destroyed. It can only be transformed from one form into another.
Example: When a ball falls from a height, its potential energy gradually changes into kinetic energy. Neglecting air resistance, the total mechanical energy remains constant.
4. POWER
Definition: The rate of doing work is called power.
Formula:
[
P=\frac{W}{t}
]
Where:
- P = Power
- W = Work done
- t = Time taken
SI Unit: Watt (W)
One watt is the power when one joule of work is done in one second.
[
1W=1J/s
]
5. SIMPLE MACHINES
Definition: A simple machine is a device that makes work easier by changing the magnitude or direction of the applied force.
Types of Simple Machines
1. Lever: A rigid bar that rotates about a fixed point called the fulcrum.
Examples: Seesaw, scissors and crowbar.
Parts of a Lever:
- Fulcrum: Fixed turning point.
- Effort: Applied force.
- Load: Resistance or object to be moved.
Types of Levers:
- First-class lever: Fulcrum lies between effort and load. Example: Seesaw.
- Second-class lever: Load lies between fulcrum and effort. Example: Wheelbarrow.
- Third-class lever: Effort lies between fulcrum and load. Example: Tweezers.
2. Pulley: A grooved wheel used with a rope to lift loads.
Examples: Drawing water from a well and lifting loads with cranes.
3. Inclined Plane: A sloping surface used to move heavy objects with less effort.
Examples: Ramp and sloping road.
4. Wheel and Axle: A simple machine consisting of a wheel attached to an axle.
Examples: Steering wheel and door knob.
5. Screw: An inclined plane wrapped around a cylinder.
Examples: Screw jack and bolt.
6. Wedge: A simple machine consisting of two inclined surfaces joined together.
Examples: Knife, axe and chisel.
6. MECHANICAL ADVANTAGE
Definition: Mechanical advantage is the ratio of the load lifted by a machine to the effort applied.
Formula:
[
MA=\frac{\text{Load}}{\text{Effort}}
]
Example: If the load is 200 N and the effort is 50 N:
[
MA=\frac{200}{50}=4
]
Mechanical advantage has no unit.
7. EFFICIENCY OF A MACHINE
Definition: Efficiency is the ratio of useful work output to work input, expressed as a percentage.
Formula:
[
\text{Efficiency}=\frac{\text{Useful work output}}{\text{Work input}}\times100%
]
Example: If the input work is 500 J and useful output work is 400 J:
[
\text{Efficiency}=\frac{400}{500}\times100=80%
]
The efficiency of a real machine is less than 100% because some energy is lost, for example, due to friction.
8. IMPORTANT FORMULAS
1. Work: (W=Fs)
2. Kinetic Energy: (KE=\frac12mv^2)
3. Potential Energy: (PE=mgh)
4. Power: (P=\frac{W}{t})
5. Mechanical Advantage: (MA=\frac{Load}{Effort})
6. Efficiency: (\frac{Output}{Input}\times100%)
9. IMPORTANT QUESTIONS
1. Define work and write its SI unit.
2. What is energy? Name its two main forms.
3. State the law of conservation of energy.
4. Define power and write its SI unit.
5. What is a simple machine? Give three examples.
6. Explain the three types of levers with examples.
7. Define mechanical advantage.
8. What is the efficiency of a machine?
9. Differentiate between kinetic energy and potential energy.
10. Why is the efficiency of a real machine less than 100%?
Knowledge Academy Ayodhya
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