NCERT Solutions: Chapter5 Exploring Forces (class 8 Science)
Every question and complete textbook solution is fully pre-rendered below, covering Contact & Non-Contact Forces, Friction Dynamics, Gravitational Acceleration, Mass vs. Weight, Electrostatic Charges, and Homemade Instruments.
When you cycle uphill, you must lift both the bicycle and yourself against gravity. A component of your weight acts along the slope and opposes the forward motion, so you must apply extra muscular force to overcome this component and the usual resistances (like friction and air resistance). On flat ground gravity does not produce this opposing component along the direction of motion, so pedalling feels easier.
It is easier to slip on a wet surface because water reduces the friction between your footwear and the surface. A thin layer of water acts like a lubricant, filling the tiny roughnesses of the surfaces and lowering their grip on each other, so your feet can slide more easily.
When the swing reaches its highest point it momentarily changes direction and its upward acceleration reduces. At that instant the support (normal) force from the swing on our body becomes smaller than usual because the swing cannot push up as much while changing motion. Because that upward push decreases, we feel a reduced contact force and so we feel "light" or as if we are "floating" for a brief moment.
Questions that spark scientific curiosity about forces include:
- Why does sliding work better on ice than on sand?
- What would happen if friction did not exist at all?
- Why do heavier objects sink more in water than lighter ones of the same size?
- Does air also provide friction to moving objects?
- Why can some birds fly easily for long periods without getting tired?
Yes. Friction is another contact force. It acts at the surfaces in contact and always opposes the relative motion or tendency of motion between those surfaces.
No. Forces can act without direct contact. For example, gravitational force, magnetic force and electrostatic force act at a distance and can pull or push objects even when they are not touching.
Yes. There are two types of electric charge: positive and negative. Like charges repel each other while unlike charges attract each other.
Objects fall towards the Earth because of gravity, the attractive force between the Earth and masses near it. Near Earth's surface this force causes objects to accelerate downward at nearly the same rate (about $9.8\text{ m/s}^2$), ignoring air resistance.
Yes. Gravity acts on all objects. The Earth, because of its large mass, exerts a gravitational pull on objects near it and that pull is what we call their weight.
No. The gravitational force (weight) on an object is proportional to its mass, so heavier objects experience a larger gravitational force. However, if we ignore air resistance, all objects accelerate downward at the same rate near Earth's surface because acceleration due to gravity is the same for all masses.
| Property | Mass | Weight |
|---|---|---|
| Definition | The amount of matter contained within an object. | The gravitational force exerted on that mass by a celestial body. |
| Formula | $m$ (fundamental quantity) | $W = m \times g$ (product of mass and acceleration due to gravity). |
| SI Unit | Kilogram (kg) | Newton (N) |
| Variability | Remains constant everywhere; does not change with location. | Changes with the value of acceleration due to gravity $g$ (e.g., less on the Moon). |
Although gravity acts on all objects, whether an object floats or sinks depends on the buoyant force from the water and the object's density. If the object's overall density is less than water, the upward buoyant force is large enough to keep it afloat. If the object is denser than water, gravity overcomes buoyancy and it sinks.
Column A (Type of force): (i) Muscular force, (ii) Magnetic force, (iii) Frictional force, (iv) Gravitational force, (v) Electrostatic force.
Column B (Example): (a) A cricket ball stopping on its own just before touching the boundary line, (b) A child lifting a school bag, (c) A fruit falling from a tree, (d) Balloon rubbed on woollen cloth attracting hair strands, (e) A compass needle pointing North.
| Column A (Type of force) | Column B (Example) | Physical Mechanism |
|---|---|---|
| (i) Muscular force | (b) A child lifting a school bag | Action of muscles exerting force through physical contact. |
| (ii) Magnetic force | (e) A compass needle pointing North | Alignment with Earth's magnetic field without contact. |
| (iii) Frictional force | (a) A cricket ball stopping on its own just before touching boundary line | Contact force opposing relative motion between ball and ground. |
| (iv) Gravitational force | (c) A fruit falling from a tree | Attractive non-contact force exerted by Earth pulling mass downward. |
| (v) Electrostatic force | (d) Balloon rubbed on woollen cloth attracting hair strands | Attractive force exerted by charged body on uncharged/oppositely charged hair. |
(i) A force is always required to change the speed of motion of an object.
(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
(iii) There is no force between two charged objects placed at a small distance apart.
(i) True
Explanation: A change in speed (or direction) means the object's velocity is changing. Such a change requires a net force acting on the object; without a net force the object continues with the same speed and direction.
(ii) False
Explanation: Friction opposes relative motion and therefore reduces the speed of a rolling ball on a flat surface; it does not increase the speed.
(iii) False
Explanation: Two charged objects exert an electrostatic force on each other when they are close. This force may be attractive or repulsive depending on whether the charges are unlike or like.
When two balloons are rubbed with a woollen cloth they both acquire the same kind of electric charge (usually negative). Since like charges repel, the balloons will repel each other when brought near.
A coin sinks because its density (mass per unit volume) is greater than that of water, so gravity pulls it down more than the water can push it up. A wooden block has lower overall density than water, so the upward buoyant force on it is sufficient to keep it floating.
(i) During its upward motion
(ii) During its downward motion
(iii) At its topmost position
When a ball is thrown upwards, the principal force acting throughout the motion (neglecting air resistance) is gravity, which acts downward.
- (i) During its upward motion: Gravity acts downward and opposes the upward motion, causing the ball to slow down.
- (ii) During its downward motion: Gravity acts downward and now assists the motion, causing the ball to speed up as it falls.
- (iii) At its topmost position: The ball is momentarily at rest, but gravity still acts downward, which causes it to start moving back towards Earth.
(i) Before the point A
(ii) After crossing the point A
The motion on the inclined plane is driven by gravity, while on the horizontal surface the ball slows down mainly because of friction.
- (i) Stopping before A: Increase the friction on the horizontal surface (for example, place a rougher material such as sandpaper). This makes the ball lose its speed faster so it will stop sooner, before A.
- (ii) Stopping after A: Decrease the friction on the horizontal surface (for example, make it smoother or lubricate it). Lower friction reduces deceleration and the ball will travel further, possibly beyond A.
We slip on smooth surfaces because there is very little friction to hold our shoes to the surface. Smooth surfaces have fewer microscopic bumps to provide grip; a thin film of water (on ice, from melting) further reduces friction by acting as a lubricant. With too little friction, our feet cannot push against the ground to prevent sliding, so we lose balance and slip.
Yes. Non-uniform motion means the object's speed or direction (or both) are changing. Such a change in velocity requires a net force to act on the object. Examples are a car accelerating forward or changing direction while turning; in both cases, forces are present.
The change is caused by the Moon's weaker gravitational field. Weight equals mass $\times$ gravitational acceleration ($W = m \times g$). Since $g$ on the Moon is about one-sixth of that on Earth, the weight becomes one-sixth. The mass of the object, however, remains the same everywhere; mass is independent of location.
(i) $w_1 = w_2 = w_3$
(ii) $w_1 > w_2 > w_3$
(iii) $w_2 > w_3 > w_1$
(iv) $w_3 > w_1 > w_2$
Correct Answer: (ii) $w_1 > w_2 > w_3$
Explanation: All three objects have the same size and shape but are made of different materials. The object that dips deepest in water is the heaviest (has the greatest weight and density).
From the figure:
• Object 1 is dipped the deepest $\rightarrow$ heaviest ($w_1$)
• Object 2 dips less than 1 ($w_2$)
• Object 3 dips the least $\rightarrow$ lightest ($w_3$)
Therefore: $w_1 > w_2 > w_3$
Experiment Steps: Gather materials (plastic scale, wool, silk cloth, rubber balloon, polythene sheet, small pieces of paper, metal key or coin). Rub one material against another, bring it close to tiny paper pieces, and observe whether electrostatic attraction occurs.
| Material Rubbed | Material Used for Rubbing | Attracts Paper Pieces? (Yes/No) |
|---|---|---|
| Plastic scale | Woollen cloth | Yes |
| Rubber balloon | Silk cloth | Yes |
| Plastic scale | Polythene sheet | Yes |
| Metal coin / key | Woollen cloth | No |
| Rubber eraser | Dry paper | No |
Story: The Day Gravity Vanished
One morning, everyone wakes up to find that gravity has vanished! The moment a ball is thrown, it rises up and never comes down. People start floating off their beds and must grab tightly to the doors and furniture. Water in glasses begins to float around in little blobs, and food hovers above plates. Birds try to fly but end up floating higher and higher. Cars and cycles no longer grip the road, and everyone must wear heavy boots to avoid floating away. Animals and plants on Earth start floating as well. Without gravity, life becomes very difficult, and people must come up with clever ways to stay on the ground or inside their houses.
Cartoon Strip Description:
- Panel 1: A boy throws a red ball outside; the ball keeps ascending into outer space with a shocked expression on his face.
- Panel 2: In the kitchen, breakfast cereal and floating spheres of milk hover weightlessly above the dining table.
- Panel 3: A family sitting tied down with ropes to their living room sofa just to watch television together.
- Panel 4: The family pet cat and dog floating upside down near the ceiling alongside floating keys, shoes, and homework sheets.
| Why Friction is an Absolute Necessity | Why Friction is a Persistent Problem |
|---|---|
| Friction prevents slipping and enables us to walk firmly on roads and floors. | Causes mechanical wear and tear of rotating machine parts, vehicle tyres, and shoe soles. |
| Allows vehicles to start, steer, accelerate, and stop safely through tyre-road grip. | Dissipates valuable mechanical energy as wasted heat, reducing industrial machine efficiency. |
| Enables our hands to grip objects, hold pens, write on paper, or hammer nails into walls. | Creates unwanted resistance, making it much harder to drag and push heavy objects. |
| Essential for the operational functioning of automobile brakes and clutch assemblies. | Requires expensive continuous lubrication (oils, greases, ball bearings) to prevent overheating. |
Experimental Procedure: Construct a simple spring balance using an elastic spring and pointer, calibrated against known standard masses (e.g., 100 g, 200 g). Suspend common objects (eraser, pencil box, water bottle), record their weight in Newtons, and measure their true mass in grams using a standard balance.
| Object Tested | Measured Mass ($m$ in g) | Measured Weight ($W$ in N) | Weight/Mass Ratio ($W/m$ in N/g) |
|---|---|---|---|
| Eraser | 50 g | 0.5 N | $0.01\text{ N/g}$ ($9.8\text{ N/kg}$) |
| Pencil box | 150 g | 1.5 N | $0.01\text{ N/g}$ ($9.8\text{ N/kg}$) |
| Water bottle | 300 g | 3.0 N | $0.01\text{ N/g}$ ($9.8\text{ N/kg}$) |
Observed Pattern: The ratio of weight to mass is nearly constant for all objects measured at the same place on Earth. This constant ratio represents the local acceleration due to gravity ($g \approx 9.8\text{ m/s}^2$, which corresponds to $\approx 0.01\text{ N per gram}$). This demonstrates that an object's weight is directly proportional to its mass ($W \propto m$).
Steps to Make an Electroscope:
- Take a clean, transparent, dry glass jar fitted with a cardboard or plastic lid.
- Insert a thick copper wire or unfolded paperclip through a straw fixed in the lid so that one end enters the interior and the other remains exposed outside.
- Cut two thin, identical strips of aluminium foil (leaves) and hang them freely from a small hook bent at the lower end of the wire inside the jar.
- Rub a plastic scale with wool to charge it, then bring it near or in contact with the exposed upper wire terminal.
Other Uses of the Electroscope:
- Detecting Electric Charge: To test whether an unknown body carries an electric charge or is neutral.
- Charge Conduction: To demonstrate that electric charge can be transferred from one material to another via conductors.
- Demonstrating Grounding (Earthing): Touching the top wire with a human finger allows stored charges to flow safely into the Earth, causing the foil leaves to collapse immediately.
- Comparative Charging Investigation: To compare which rubbing materials produce stronger charges based on the degree of foil divergence.