Solutions-NCERT-Class-8-Science-Curiosity-Chapter-5-Exploring Forces-CBSE

• When we ride a bicycle uphill, we have to work against the pull of gravity, which tries to pull us back down.
• On flat ground, gravity acts straight down and doesn’t affect our forward movement much.
• But when the ground is sloped, part of gravity pulls us backward, so we need to push harder to move upward.

• It is easier to slip on a wet surface because water reduces friction between our feet and the ground.
• Friction is the force that helps us grip the surface while walking.
• When the surface is wet, a thin layer of water acts like a lubricant, making it smooth and slippery, so we lose our grip and can easily fall.

When you're on a swing and it reaches the highest point, your body is changing direction — from going up to coming down. At that moment:

• Your speed becomes almost zero.
• Gravity starts pulling you downward.
• But the swing rope hasn’t started pulling you down strongly yet.

• You feel light because there’s less force acting on your body. Gravity is pulling you, but nothing is pushing back yet.

So for a tiny moment, you’re not being pushed up or pulled down hard — it feels like you're floating!

Yes, force of friction

No, it is not essential for an object applying force on another object to always be in contact with it. Forces can be applied through contact (contact forces) or without direct contact (non-contact forces).

Yes, there are two types of electric charge: positive charge and negative charge.

These charges are fundamental properties of matter.

(i) All objects fall towards the Earth because of a force called gravity.

(ii) Yes, there is a force acting on any object in the universe, and it is called gravity.

(iii) Gravity is a force of attraction that exists between any two objects with mass.

The Earth, due to its large mass, exerts a gravitational force on all objects near it, pulling them towards its center.

Not exactly, — Earth pulls all objects with gravity, but not with equal force. Here's a simple explanation:

• Gravity acts on all objects, but the force depends on their mass.
• Heavier objects (like a big rock) are pulled with more force.
• Lighter objects (like a feather) are pulled with less force.

Mass is the amount of matter in an object. Weight is the downward force acting upon an object due to gravity. On planet Earth, the two quantities are proportional.

Comparison Table :

Gravity pulls all objects down. But whether an object floats or sinks in water depends on two forces:

(i) Gravitational Force : Pulls everything downward towards Earth.

(ii) Buoyant Force (Upward Push by Water) : Water pushes upward on objects placed in it. This is called buoyancy.

• If buoyant force is stronger than gravity → object floats.
• If gravity is stronger than buoyant force → object sinks.

This depends on the density of the object:

• Objects with a density lower than water experience a stronger buoyant force, causing them to float,
• Objects with a higher density experience a weaker buoyant force and sink.

Example :

• A plastic ball floats because it’s light and water pushes it up.
• A stone sinks because it’s heavy and water can’t push it up enough.

• Yes, the object will stop after moving a certain distance when pushed on a table or floor. This is due to the force of friction, which opposes the motion of the object and eventually brings it to rest.
• When the object is pushed in the opposite direction, it will again stop after a certain distance due to the same frictional force.

• When we throw a ball upward, it goes up for a while.

• But gravity is always pulling it downward.
• So after reaching the highest point, the ball slows down, stops for a moment, and then falls back.

Earth’s gravity acts on everything, all the time. It pulls objects towards the ground, no matter which direction they were thrown.

Even if you throw it harder, it just goes higher, but gravity still pulls it back.

Real-Life Examples Around us :

• A cricket ball hit high in the air — it comes down.
• A mango falling from a tree — it drops to the ground.
• A paper plane — it glides but eventually lands.
• A rocket — even it falls back unless it escapes Earth’s gravity.

True: A force is indeed always required to change the speed of motion of an object. If there is no force acting on an object it will maintain its current speed and direction (unless it's already at rest in which case it will stay at rest).

False: Friction opposes motion, so it will actually decreases the speed of a rolling ball.

False: There is a force between charged objects. This force can be attractive or repulsive depending on the charges. But it is always present when charges are close to each other.

When two balloons are rubbed with a woollen cloth, they both get the same type of electric charge (either both positive or both negative).

Since like charges repel each other, the two balloons push away from each other when brought close.

• A coin sinks in water because its density (mass per unit volume) is greater than that of water.
• A wooden block floats because its density is less than that of water, causing it to be buoyed up by the water.

When a ball is thrown upwards, the only force acting on it throughout its entire motion is gravity, which pulls it downwards.

However depending on the motion of the ball, the direction of this force relative to the ball's velocity changes

• During its upward motion: The force of gravity acts downward, opposing the motion of the ball, causing it to slow down.
• During its downward motion: The force of gravity is still downwards, but now it aligns with the ball's direction of motion, accelerating it downwards.
• At its topmost position: The ball has zero velocity, meaning its momentarily stationary. At this point, the force of gravity is still downwards, but since the ball is not moving upwards or downwards, it has no net effect on the ball's motion.

The ball's motion is governed by the forces of gravity and friction.

Ball's Motion on Inclined and Horizontal Surfaces :

• Inclined plane: Gravity pulls the ball downward along the slope, causing it to accelerate.
• Horizontal surface: Gravity acts downward, but it doesn’t move the ball forward. Here, friction is the main force that slows the ball down.

(i) Stopping before A: Increasing friction on the horizontal surface will cause the ball to decelerate more rapidly, meaning it will come to a stop sooner, potentially before reaching point A.

(ii) Stopping after A: Decreasing friction on the horizontal surface will reduce the deceleration, allowing the ball to travel further before losing all its kinetic energy and stopping.

• Friction is the force that helps our feet grip the ground when we walk.
• Smooth surfaces like ice or polished floors have fewer irregularities, so friction is very low. This makes it hard for our shoes to grip — so we slide and may lose balance.
• Role of Water on Ice : A thin layer of water on ice acts like a lubricant. It reduces friction even more, making the surface extra slippery.

Yes for an object to be in non-uniform motion, a force must be acting upon it. Non-uniform motion also known as accelerated motion means the object's velocity is changing, either in speed or direction or both. This change in velocity require a force to be applied.

• The change in object's weight on the moon compared to Earth is due to the difference in gravitational force.
• The moon's gravity is significantly weaker than Earth's, roughly one sixth as strong.
• However the mass of an object remains the same regardless of location; only weight changes with gravitational pull.

The correct answer is: (ii) w₁ > w₂ > w₃

Explanation :

• All three objects are same in size and shape, so their volume is equal.

• They are made of different materials, so their densities and weights are different.
• In water, the deeper an object sinks, the heavier it is (more weight = more gravitational force = more depth).
• Since they dip to different depths, their weights must be different.

In fig.,

• Object 1 is the deepest meaning it displaces the most water.
• Object 2 is less deep than object 1 but deeper than object 3.
• Object 3 is the least deep, meaning it displaces the least amount of water.

So, the object that sinks the most has the highest weight, and the one that floats the most has the least weight.