Force and Pressure — NCERT Solutions, AI Tutor & Practice

Force is a push or a pull acting on an object. It arises due to the interaction between two objects.

Two effects of force:

1. A force can change the state of motion of an object — it can make a resting object move, speed up or slow down a moving object, or change its direction.
2. A force can change the shape and size of an object — for example, pressing a rubber ball or stretching a spring.

Contact forces act only when two objects are physically in touch with each other.

• Example: Muscular force (lifting a bag) and frictional force (a moving cycle slowing down).

Non-contact forces act even without physical contact between objects.

• Example: Magnetic force (a magnet attracting iron nails from a distance) and gravitational force (an apple falling to the ground).

Key difference: Contact forces require touching; non-contact forces act across a distance.

When two forces act along the same direction, the net force is their sum.

$F_{net} = 30\ \text{N} + 50\ \text{N} = 80\ \text{N}$

The net force is $80\ \text{N}$ acting in the direction of the push.

When forces act in opposite directions, the net force is their difference, in the direction of the larger force.

$F_{net} = 250\ \text{N} - 200\ \text{N} = 50\ \text{N}$

The net force is $50\ \text{N}$ directed toward Team B, so Team B wins.

Pressure is the force acting perpendicularly on a unit area of a surface.

$\text{Pressure} = \frac{\text{Force}}{\text{Area}}$

The SI unit of pressure is the pascal (Pa), where $1\ \text{Pa} = 1\ \text{N/m}^2$.

Given: Force $F = 150\ \text{N}$, Area $A = 3\ \text{m}^2$.

Formula: $P = \frac{F}{A}$

Substituting: $P = \frac{150\ \text{N}}{3\ \text{m}^2} = 50\ \text{Pa}$

The pressure exerted is $50\ \text{Pa}$.

Understand: Pressure depends on both force and the area over which it acts, $P = F/A$.

Analyze:

• The pointed tip has a very small area. For the same hammering force, a small area produces a large pressure, allowing the nail to pierce the wood easily.
• The broad head has a large area, so the hammer's force spreads over a wide region, making it easy and safe to strike.

Conclude: The shape is designed to increase pressure at the tip (for penetration) and reduce pressure at the head (for convenient striking).

Atmospheric pressure is the pressure exerted by the weight of the air (atmosphere) surrounding the Earth on all objects and surfaces.

Everyday observation: When you suck air out of an empty plastic bottle, it gets crushed. This happens because the atmospheric pressure outside becomes greater than the reduced pressure inside, pushing the bottle inward.

(Another example: a rubber sucker sticks to a smooth wall because atmospheric pressure holds it in place.)

Given: Pressure $P = 40\ \text{Pa}$, Area $A = 5\ \text{m}^2$.

Rearranging the formula $P = \dfrac{F}{A}$: $F = P \times A$

Substituting: $F = 40\ \text{Pa} \times 5\ \text{m}^2 = 200\ \text{N}$

The force applied is $200\ \text{N}$.

Understand: Liquids exert pressure, and this pressure increases with depth.

Analyze:

• Near the surface of the water, the depth is small, so the water pressure on the wall is low.
• Near the bottom, the depth is large, so the water exerts much greater pressure on the wall.

Conclude: To withstand the higher pressure at greater depths, the dam is built thicker at the bottom and thinner at the top, giving it strength and stability.

Understand: Pressure $= \dfrac{\text{Force (weight)}}{\text{Area}}$.

Analyze:

• A camel has broad, flat feet with a large area of contact with the sand.
• For the same weight, a larger area of contact gives a smaller pressure on the sand.
• A human's feet have a much smaller area, so the pressure exerted on the sand is larger, causing sinking.

Conclude: The camel's broad feet reduce the pressure on soft sand, allowing it to walk without sinking.

Analyzing each case:

(a) Falling coconut → caused by gravity → non-contact force

(b) Kicking a football → foot touches the ball (muscular force) → contact force

(c) Iron pins attracted to a magnet → acts from a distance → non-contact force (magnetic)

(d) Pushing a trolley → hands touch the trolley (muscular force) → contact force