Chapter 5: Exploring Forces

When cycling uphill, you need to work against both friction and the downward pull of gravity (gravitational force). On flat ground, you only need to overcome friction. Since the gravitational force adds extra resistance while going uphill, more muscular force is required to pedal, making it feel harder.

Friction arises due to the irregularities on two surfaces locking into each other. Water acts as a lubricant on a wet surface, filling in the tiny irregularities and reducing the interlocking between the two surfaces. This greatly reduces the force of friction, making it much easier to slip.

At the highest point of a swing, when you begin to come down, gravity pulls you downward and the swing also moves downward. For a brief moment, both you and the swing are falling together under gravity, which reduces the normal force between you and the swing. This reduction in the contact force makes you feel weightless or 'floating'.

When you try to move the cardboard box, you can push it forward, pull it toward you, or lift it upward. In all these cases, you are applying a force — either a push or a pull — on the box. This shows that a force is needed to move an object from its position, and a force is always a push or a pull.

From analysing different situations, we conclude that a force can make a stationary object move, change the speed of a moving object, change the direction of a moving object, or change the shape of an object. For example, kicking a football makes it move (force on a stationary object), applying brakes slows a bicycle (changes speed), hitting a moving ball with a bat changes its direction, and pressing a balloon changes its shape.

When the object is pushed, it slides for some distance and then stops on its own. This happens in both directions of pushing. A force called friction acts between the surfaces of the object and the table, opposing the motion of the object. Friction always acts in a direction opposite to the direction of motion, which is why the object slows down and stops.

No, the object does not stop at the same distance on all surfaces. On smooth surfaces like glass or ceramic tile, the object travels farther before stopping, while on rough surfaces like sand or cloth, it stops sooner. This shows that friction depends on the nature of the surfaces in contact — rougher surfaces produce greater friction.

When like poles of the two ring magnets face each other, the second magnet floats above the first because like poles repel each other — this is the magnetic force acting without contact. When you gently push the second magnet down, you can feel the repulsive force pushing back. When the poles are reversed (unlike poles facing each other), the magnets attract and the second magnet does not float; instead it moves toward the first magnet.

The small pieces of paper get attracted to the plastic scale and stick to it even without being touched. This happens because rubbing the plastic scale with polythene builds up static electrical charges on the scale. The charged scale exerts an electrostatic force on the uncharged paper pieces, pulling them toward itself — this is an example of electrostatic force acting without contact.

After rubbing both balloons with the woollen cloth, they acquire similar (same type of) static charges. When brought near each other, the two similarly charged balloons repel each other and move apart. When the woollen cloth is brought near a charged balloon, they attract each other because the cloth and the balloon acquire opposite kinds of charges upon rubbing. This demonstrates that like charges repel and unlike charges attract.

Yes, the ball always comes back down to the ground no matter how hard it is thrown upward. This is because the Earth's gravitational force continuously acts on the ball in the downward direction. Even when the ball is moving upward, gravity pulls it back, causing it to slow down, stop momentarily, and then fall back to the ground.

No, the stretch of the spring is different for objects of different masses. A heavier object causes a greater stretch in the spring than a lighter object. This is because the Earth pulls heavier objects with a greater gravitational force (greater weight), causing the spring to stretch more. This shows that the Earth does not pull all objects with equal force.

Looking at the spring balance in Fig. 5.13, the maximum weight it can measure is 10 N (newtons). This means the range of the spring balance is from 0 to 10 N. Any object heavier than 10 N should not be hung from this balance as it may damage the spring.

Weight difference between two bigger marks: 1 N

Number of smaller divisions between two bigger marks: 5 divisions

Weight indicated by one small division: Formula: Value per small division = Difference between big marks ÷ Number of small divisions Substitution: 1 N ÷ 5 = 0.2 N

So the smallest weight this spring balance can measure is 0.2 N.

To measure weight, suspend each object one by one from the hook of the spring balance. Wait for the spring to stop stretching and read the value on the newton scale carefully at eye level to avoid parallax error. Record the weight in newtons for each object in Table 5.2. Objects with greater mass will show a higher reading, confirming that weight depends on mass.

Yes, when you push the empty bottle into the water, you feel an upward push (resistance) from the water. This upward force exerted by the liquid on a submerged object is called upthrust or buoyant force. When you release the bottle, it bounces back up to the surface because the upthrust from the water is greater than the weight of the empty bottle, pushing it upward.

The correct matches are:

(i) Muscular force → (b) A child lifting a school bag (muscles provide the force to lift).

(ii) Magnetic force → (e) A compass needle pointing North (Earth's magnetic force aligns the needle).

(iii) Frictional force → (a) A cricket ball stopping just before the boundary line (friction with the ground stops it).

(iv) Gravitational force → (c) A fruit falling from a tree (Earth's gravity pulls it down).

(v) Electrostatic force → (d) A balloon rubbed on woollen cloth attracting hair strands (static charges attract hair).

(i) True — A force is always required to change the speed of an object, whether to speed it up or slow it down, since speed change is a change in motion.

(ii) False — Friction always acts opposite to the direction of motion, so it decreases the speed of a rolling ball, not increases it.

(iii) False — Charged objects exert electrostatic force on each other even without contact; like charges repel and unlike charges attract.

Both balloons, when rubbed with the same woollen cloth, acquire the same type (similar) of static electrical charges. Since like charges repel each other, the two balloons will move away from each other when brought close. This happens due to the electrostatic force of repulsion acting between the similarly charged balloons, even without them touching each other.

When an object is placed in water, it experiences two forces: its weight (downward) and upthrust/buoyant force from water (upward). Whether it sinks or floats depends on which force is greater. A coin is made of dense metal — its weight is much greater than the upthrust it receives, so it sinks. A wooden block, even though bigger, is less dense; its weight is less than the upthrust acting on it, so it floats. According to Archimedes' Principle, the upthrust equals the weight of water displaced.

(i) During upward motion:

• Gravity (gravitational force): acts downward, opposing the upward motion and causing the ball to slow down.
• Air resistance (friction due to air): acts downward (opposite to the upward direction of motion), further slowing the ball.

(ii) During downward motion:

• Gravity (gravitational force): acts downward, in the same direction as motion, causing the ball to speed up.
• Air resistance: acts upward (opposite to downward motion), slowing it slightly.

(iii) At the topmost position:

• Gravity (gravitational force): acts downward — this is the only significant force acting at that instant, which is why the ball momentarily stops and then reverses direction.

(i) To stop the ball before point A: Increase the friction on the horizontal surface by placing a rough material like sand, cloth, or carpet between the inclined plane end and point A. Greater friction will slow the ball down faster, making it stop before reaching A.

(ii) To stop the ball after point A: Decrease the friction on the horizontal surface by using a smoother surface like a polished floor or glass sheet. With less friction, the ball travels farther before stopping, going past point A.

Friction is caused by the interlocking of irregularities on two surfaces in contact. Smooth surfaces like ice or polished floors have very few surface irregularities, so there is very little interlocking between our shoes and the floor. This results in very little friction, which means there is insufficient force to prevent our foot from sliding, causing us to slip.

Yes, a force is definitely acting on an object in non-uniform motion. Non-uniform motion means the speed or direction of the object is changing. Since a force is required to change the speed or direction of an object, there must be an unbalanced (net) force acting on the object whenever it undergoes non-uniform motion. For example, a ball rolling on the ground and slowing down has friction acting on it.

The weight of an object depends on the gravitational force pulling it. The Moon's gravity is about one-sixth that of Earth's because the Moon is much smaller and less massive. So the Moon pulls objects with less force, making their weight one-sixth of what it is on Earth. However, mass does not change — mass is the amount of matter in an object and remains the same everywhere, whether on Earth, the Moon, or in space.

✔ Correct Answer: (ii) W₁ > W₂ > W₃

Why (ii) is correct: In Fig. 5.17, Object 1 sinks the deepest, Object 2 sinks to a medium depth, and Object 3 sinks the least. Since the objects have the same size and shape (same volume), the one that sinks deeper is denser and heavier. Object 1 displaces the most water and has the greatest weight (W₁ > W₂ > W₃).

Why other options are wrong:

• (i) The objects dip to different depths, proving their weights are different, not equal.
• (iii) Object 2 does not sink the deepest, so W₂ is not the greatest weight.
• (iv) Object 3 sinks the least, meaning it is the lightest, so W₃ cannot be the greatest.