Force and Pressure Class 8 Notes - Science Chapter 11

1. Contact Forces

As we learn in Force and Pressure Class 8, contact forces are exactly what they sound like—forces that work only when objects are in direct physical contact or "touching" each other. Think of it as a force that needs a handshake to work!

• Muscular Force: This is the force that results from the action of muscles in living beings. Every time you push or pull something, like lifting your heavy school bag, riding a bicycle, or even just chewing your food, you are using muscular force. Animals use it too, like an ox pulling a cart. It's the force of life and movement!
• Frictional Force (Force of Friction): This is a very important contact force that opposes, or works against, the state of motion of an object. It's like a "stopping force."
  • Example: When you roll a ball on the ground, it eventually slows down and stops. This is because the force of friction between the ball's surface and the ground acts in the opposite direction of its movement.
  • Why we can walk: We can walk without slipping because of the friction between the soles of our shoes and the ground. On a wet, slippery floor, friction is very low, which is why it's easy to fall.

2. Non-Contact Forces

Non-contact forces are the "magical" forces of nature that can act on an object from a distance, without any physical touch. These forces can push or pull an object from across a space.

• Magnetic Force: This is the force exerted by a magnet. You have likely seen how a magnet can pull iron pins or paper clips towards itself without touching them. This is the magnetic force at work. This force can be a pull (attraction) or a push (repulsion) if you bring two magnets close to each other.
• Electrostatic Force: This force is created by charged objects. An object can get an "electric charge" by being rubbed.
  • Example: If you rub a plastic comb on your dry hair, it gets charged. When you bring this comb near tiny, uncharged pieces of paper, it pulls them up. This pull is the electrostatic force.
• Gravitational Force (Gravity): This is a force of attraction that every object in the universe exerts on every other object. The Earth has a very large mass, so its gravitational force is very strong. We call this force gravity.
  • Example: When a fruit breaks from a tree, gravity pulls it straight down to the ground. When you throw a ball up into the air, it is the Earth's gravitational force that pulls it back down. This force keeps us, and everything else, firmly on the ground!

Understanding Pressure

What is Pressure? (Force Acting on an Area)

In our study of Force and Pressure Class 8, the next key concept is pressure. While force is a push or a pull, pressure describes how concentrated that force is on a surface.It is defined as the force acting per unit area.

The relationship between pressure, force, and area is given by the formula:

This formula tells us that pressure depends on two things: how much force is applied and the area over which it is applied.

The SI unit of Pressure is the Pascal (Pa).One Pascal is the pressure produced when a force of 1 Newton acts on an area of 1 square meter.

The formula for pressure reveals a very important relationship: for the same amount of force, the pressure is high on a small area and low on a large area. This principle is used everywhere in our daily lives.

• Why is it easier to cut with a sharp knife? A sharp knife has a very thin edge, which means its surface area is extremely small. When you apply force, this small area concentrates the force, creating very high pressure. This high pressure is what allows the knife to easily cut through vegetables or fruits. A blunt knife has a larger area, so it creates less pressure and doesn't cut well.
• Why do school bags have wide straps?Your school bag can be heavy, which means it exerts a large force.The wide straps have a larger area of contact with your shoulders. This large area spreads the force out, which reduces the pressure on your shoulders, making the bag more comfortable to carry. Narrow straps would have a smaller area, creating high pressure and causing pain.
• Why do porters place a round piece of cloth on their heads? When porters carry heavy luggage, they place a coiled piece of cloth (a pagri) on their heads. This increases the surface area on which the luggage rests. By increasing the area, the force from the luggage is spread out, which decreases the pressure on their head, making it easier to carry the load.

Pressure Exerted by Liquids and Gases

Pressure isn't just exerted by solid objects.Fluids, which include both liquids and gases, also exert pressure.

Pressure Exerted by Liquids

When a liquid is kept in a container, it exerts pressure on the bottom and also on the walls of the container. This liquid pressure has a very important property:

• Key Fact:Liquid pressure increases with depth.The deeper you go into a liquid, the greater the weight of the liquid above you, and therefore, the greater the pressure.
• Example: This is why the walls of a dam are built to be much thicker at the bottom than at the top.The water pressure is greatest at the bottom, so the wall needs to be stronger there to withstand it. It's also why deep-sea divers have to wear specially designed, strong suits to protect themselves from the immense pressure deep in the ocean.