Explore & Learn

Gravitational Potential Energy (GPE)

Gravitational Potential Energy is the energy an object has because of its height above the ground. The higher the object or the heavier it is, the more GPE it has.

Why Does GPE Matter?

When an object is lifted, work is done against gravity, and this energy is stored as GPE. If the object falls, the GPE gets converted into kinetic energy (energy of motion).

Formula:

GPE = Mass × Gravity × Height

• Mass (kg)

• Gravity (N/kg, usually 9.8 or 10)

• Height (m)

• GPE is measured in joules (J)

  
  

Example:

If a 2 kg object is lifted 5 meters high:GPE = 2 × 10 × 5 = 100 J

Real-Life Uses:

• Water stored high in a dam has GPE and can generate electricity.

• A rollercoaster gains GPE at the top of a track and converts it to kinetic energy on the way down.

• Lifting an object stores energy that can be released later.

Pivot in Physics

A pivot is the fixed point around which an object turns or rotates. It's also known as a fulcrum in levers.

Where Do We See Pivots?

• In seesaws or balance scales

• Hinges of a door

• Scissors or pliers

• Human joints like elbows and knees

In all of these, the pivot is the point the object rotates around.

Why Is the Pivot Important?

The pivot helps us understand how moments (turning forces) work. A moment is the force that causes something to rotate around a pivot.

Formula for Moment:

Moment = Force × Distance from the pivot

• Force is in newtons (N)

• Distance is the perpendicular distance from the pivot in meters (m)

• Moment is measured in newton-meters (Nm)

  
  

Balanced Moments:

For an object to be balanced and not rotate, the total clockwise moment must equal the total anticlockwise moment.This is known as the principle of moments.

Example:

On a seesaw, if two people sit at equal distances from the center (pivot) but one is heavier, the seesaw tips. To balance it, the heavier person must sit closer to the pivot, or the lighter person farther away.

A resultant force is the overall force acting on an object when all the individual forces are combined. It tells us the net effect of all the forces.

Why Is It Important?

If forces acting on an object are balanced (equal and opposite), the resultant force is zero, and the object stays still or moves at a constant speed.If the forces are unbalanced, the object will accelerate, decelerate, or change direction.

How to Find Resultant Force:

• If the forces are in the same direction, add them.

• If the forces are in opposite directions, subtract the smaller force from the larger one.

Example:If 10 N pushes right and 4 N pushes left,Resultant Force = 10 N – 4 N = 6 N to the right

Effect of a Resultant Force:

• Starts or stops motion

• Changes speed

• Changes direction

• Changes shape (in some cases)

  

Friction is a force that opposes motion between two surfaces that are touching. It tries to slow down or stop objects that are moving or trying to move.

What Causes Friction?

Even smooth surfaces have tiny bumps. When two surfaces rub against each other, these bumps catch on each other, causing friction.

Types of Friction:

1. Static Friction – Stops an object from starting to move.

2. Sliding Friction – Acts when an object is already moving over a surface.

3. Rolling Friction – Acts on objects that roll, like wheels.

4. Air or Fluid Resistance – A type of friction when objects move through air or liquid.

   
   

Is Friction Good or Bad?

• Useful: Helps us walk, stops cars with brakes, and keeps objects from sliding.

• Not useful: Causes parts to wear out, makes machines less efficient by wasting energy as heat.

  
  

How Can We Reduce Friction?

• Use lubricants like oil or grease

• Use smooth or polished surfaces

• Use ball bearings or wheels

Refraction in Physics

Refraction is the bending of light when it passes from one material into another, like from air into water or glass. This happens because light changes speed in different materials.

Why Does Light Bend?

When light enters a denser material (like air to glass), it slows down and bends towards the normal (an imaginary line at 90° to the surface).When light goes into a less dense material (like glass to air), it speeds up and bends away from the normal.

Everyday Examples of Refraction:

• A straw looks bent in a glass of water

• Lenses in glasses or cameras use refraction to focus light

• Rainbows form because of refraction in water droplets

  
  

Refraction Index (n):

The refractive index tells us how much a material slows down light.Higher n = more bending.

Everyday Examples of Refraction:

• A pencil looks bent in a glass of water.

• Lenses in glasses and cameras focus light using refraction.

• Diamonds sparkle because of their high refractive index.

• Rainbows form when sunlight refracts through water droplets in the air.

Total Internal Reflection:

If light travels from a denser to a less dense material at a steep angle, it can be completely reflected instead of refracted. This is called total internal reflection and is how optical fibres work.