The chapter on “Force” for the 6th ICSE board introduces the fundamental concept of force as a push or a pull. It explains that a force can make a stationary object move, stop a moving object, change its speed, or alter its direction and shape.
The chapter further elaborates on the effects of force with real-life examples. It also introduces the SI unit of force, which is the Newton (N).
Crucially, it distinguishes between contact forces (like muscular force, mechanical force, and friction) that require direct interaction, and non-contact forces (like gravitational force, magnetic force, and electrostatic force) that can act from a distance.
In essence, the chapter lays the groundwork for understanding how forces interact with objects and cause changes in their state of motion or shape.
Test yourself
A. Objective Questions
1. Write true or false for each statement
(a) The frictional force acts in the direction of motion of body
Answer. False
(b) The unit of weight is kilogram
Answer. False
(c) A force can change the direction of motion of a moving body.
Answer. True
(d) A force increases the mass of the body when applied on it.
Answer. False
(e) The force of friction is always disadvantageous.
Answer. False
(f) The sliding friction is more than the rolling friction.
Answer. True
(g) Liquids offer more friction than the gases.
Answer. True
(h) A wet oily road offers more friction than a dry rough road.
Answer. False
2. Fill in the blanks
(a) Force is applied as ——– or——– .
Ans : Push , Pull
(b) On squeezing a gum tube, its ——– changes.
Ans : Shape
(c) On pulling a string, its ———increases.
Ans : Length
(d) A moving football when kicked, its ———– changes.
Ans : Direction of motion
(e) On applying brakes on a moving car, its speed——–.
Ans : Slows down
(f) We use ball bearings to ——— the friction.
Ans : Reduce
(g) Friction ——— the motion.
Ans : Opposes
(h) Lubricants are used to ——– friction.
Ans : Reduce
3. Match the following columns
Answer.
4. Select the correct alternative
(a) A body falls downwards because of
- electrical force
- gravitational force
- mechanical force
- magnetic force.
Answer: Gravitational force
(b) A force does not change
- mass
- length
- shape
- state of motion.
Answer : Mass
(c) A force to be expressed correctly requires
- only the magnitude
- only the direction
- both the magnitude and direction
- none of the above.
Answer : Both the magnitude and direction
(d) Friction
- promotes motion
- opposes motion
- acts in the direction of motion
- is always a nuisance
Answer : Opposes motion.
(e) Friction is reduced by
- making the surfaces wet
- making the surfaces dry
- making the surfaces rough
- sprinkling sand on the surface.
Answer : Making the surfaces wet
(f) Friction
- causes wear and tear
- produces heat
- stops a moving body
- has all the above disadvantages
Answer : Has all the above disadvantages
(g) Friction is increased if
- an oil is sprayed
- the surfaces are made wet
- the surfaces are made dry
- the surfaces are polished
Answer : The surfaces are made dry
B. Short/Long Answer Questions
1)Name the term used for the push or pull ?
Answer:
The term used for a push or a pull is force.
2)Give one example each of a force as
- a push
- as pull
- a stretch and
- a squeeze.
Answer:
Here are examples of force as:
A push: Pushing a door to close it.
A pull: Pulling open a drawer.
A stretch: Stretching a rubber band.
A squeeze: Squeezing a sponge to get water out.
3)Explain the meaning of the term force.
Answer : Force is a push or pull on an object that can change its shape, speed, or direction of motion.
4)What effect can a force have on a stationary body ?
Answer : Force can make a stationary body move.
5)What effects can a force have on a moving body ?
Answer : A force can make a moving body speed up, slow down, change direction, or even stop. It can also change the shape or size of the body.
6)What effect can a force produce on a body which is not allowed to move ?
Answer `: Even if something stops a body from moving, a force can still act on it. Imagine pushing on a wall – you’re definitely using force, and even though the wall doesn’t move, it might get squeezed or change shape a tiny bit.
7)Give one example each to indicate that the application of a force
- produces motion
- stops motion
- slows down motion
- changes the direction of motion
- deforms a body
Answer:Here are examples of how force can:
- Produce motion: Pushing a stationary bicycle pedal makes the wheels turn.
- Stop motion: Applying brakes to a moving car brings it to a halt.
- Slow down motion: Dragging your feet while walking reduces your speed.
- Change the direction of motion: Hitting a moving cricket ball with a bat alters its path.
- Deform a body: Squeezing a clay ball changes its shape.
8)State the effect produced by a force in the following cases :
(a) The sling of a rubber catapult is stretched
(b) A man pushes a heavy cart
(c) A player uses his stick to deflect the ball .
(d) A cyclist applies brakes
(e) A spring is compressed.
Answer:(a) The shape of the sling changes.
(b) The cart starts moving.
(c) The direction of the ball changes.
(d) The speed of the cycle decreases.
(e) The shape of the spring change
9)Name the two kinds of forces in nature.
Answer:There are four fundamental forces in nature, but if we broadly categorize them based on their range and how they manifest in our daily lives, we can think of them in two main ways:
- Long-range forces: TThe most familiar example is gravitational force, which keeps planets in orbit and holds us to the Earth. Electromagnetic force is another long-range force, responsible for interactions between charged particles and magnetic phenomena; it underlies much of how matter behaves.
- Short-range forces: These forces operate only at very small distances, typically within the nucleus of an atom.
10)Name the type of force which acts in the following cases:
Answer: (a) A coolie lifts a luggage
Answer. Muscular force (applied by the coolie) acting against gravitational force.
(b) A bicycle comes to rest slowly when the cyclist stops pedalling
Answer. Frictional force (opposing the motion of the tires on the road and air resistance)
(c) A stone falls from a roof
Answer. Gravitational force.(pulling it towards the Earth).
(d) A comb rubbed with silk attracts the bits of paper
Answer. Electrostatic force(attractive force between the charged comb and the neutral paper bits due to polarization).
(e) A string hangs with a load
Answer. Force of tension.(along the string, pulling upwards) balanced by gravitational force (pulling the load downwards)
(f) A horse moves a cart
Answer. Muscular force(exerted by the horse) and frictional force (between the cart’s wheels and the ground, allowing movement).
(g) A magnet attracts an iron pin
Answer. Magnetic force(attractive force between a magnet and a ferromagnetic material).
(h) A boy opens the door
Answer. Muscular force(muscular force exerted by the boy).
(i) An apple falls from a tree
Answer.Gravitational force(pulling the apple towards the Earth).
(j) A man rows a boat.
Answer. Muscular force.(applied by the man on the oars) and the reaction force from the water propelling the boat forward.
11) What do you mean by the gravitational force ? Give an example to illustrate it.
Answer : Gravitational force is a fundamental force of nature that exists between any two objects that have mass. . For example, the Earth pulls objects toward it, which is why an apple falls to the ground.
12) Define the term “weight of a body”
Answer : The weight of a body is the force of gravity acting on its mass. Simply put, it’s how strongly gravity pulls on an object
13) What do you understand by the term friction ?
Answer : Friction is a force that resists the motion of one surface sliding over another. It helps us walk without slipping.
14) Give an example to illustrate the existence of the force of friction.
Answer : Friction is a force that resists the motion of one surface sliding over another. It helps us walk without slipping.
15) What is the cause of friction ?
Answer :The cause of friction is primarily due to the interactions between the surfaces of two objects at a microscopic level. When these surfaces come into contact, these irregularities interlock or rub against each other, opposing motion.
16) State two factors which directly affect the force of friction.
Answer :
How rough the surfaces are: Rougher surfaces = more friction. Smoother surfaces = less friction.
How hard the surfaces are pressed together (Normal Force): Harder press = more friction. Lighter press = less friction.
17) In which case will there be more friction between the truck and the road : when the truck is empty or when it is loaded ?
Answer : There will be more friction when the truck is loaded. The increased weight presses the tires more firmly against the road, leading to a greater frictional force.
18) Which offers more friction on a body : a glass surface or a wooden surface ?
Answer : A wooden surface typically offers more friction than a glass surface. This is because wood generally has a rougher texture at a microscopic level compared to the smoother surface of glass. The irregularities on the wooden surface interlock more with the irregularities of the object moving over it, leading to greater resistance to motion.
19) Name the three kinds of friction.
Answer:There are three main kinds of friction:
- Static friction: This acts on objects that are not moving relative to each other. It’s the force that prevents a stationary object from starting to move.
- Sliding friction (or kinetic friction): This occurs when two surfaces slide past each other. It’s the force that opposes the motion of an object already in motion across a surface.
- Rolling friction: It’s generally much weaker than static or sliding friction.
20) List three disadvantages of friction.
Answer : Here are three downsides of friction:
- It causes wear and tear on moving parts, reducing their lifespan.
- It generates heat, which can lead to energy loss and potential damage.
- It opposes motion, requiring extra effort or energy to overcome.
21) When you apply the brakes, the bicycle stops and the rim of the wheel becomes hot. Explain the reason.
Answer :
When you apply the brakes on a bicycle, the bicycle stops due to the force of friction acting between the brake pads and the rim or the brake disc . Here’s a step-by-step explanation of why the rim of the wheel becomes hot:
- Applying the Brakes: When you squeeze the brake levers, it causes the brake pads (made of a friction material like rubber or a composite) to be pressed firmly against the rim of the bicycle wheel.
- Friction is Generated: As the wheel continues to rotate and the brake pads are held against it, there is a rubbing action between the two surfaces. This rubbing creates friction.
- Conversion of Energy: The kinetic energy of the moving bicycle (energy due to its motion) is converted into heat energy due to the work done by the frictional force. This is a direct consequence of the law of conservation of energy – energy cannot be destroyed, only transformed from one form to another.
- Slowing Down: The frictional force opposes the rotation of the wheel, causing it to slow down. The harder you apply the brakes, the greater the frictional force, the more kinetic energy is converted into heat per unit time, and the faster the bicycle decelerates and the hotter the rim becomes.
22) The eraser gets smaller and smaller as you use it more and more. Explain the reason.
Answer : As you rub an eraser against paper to remove pencil marks, friction acts between the eraser and the paper’s surfaceThese tiny bits, along with the graphite from the pencil, get dislodged, leading to the eraser gradually shrinking in size with use.
23) List three ways of reducing friction.
Answer:Here are three common ways to reduce friction:
Using Lubricants: Introducing a lubricant (like oil, grease, or graphite) between two surfaces in contact creates a thin layer that separates them. This layer reduces direct contact between the solid surfaces, replacing it with friction within the fluid lubricant, which is generally much lower.
Making Surfaces Smoother: Reducing the roughness of the surfaces in contact minimizes the interlocking of microscopic irregularities.
Using Rollers or Ball Bearings: Replacing sliding friction with rolling friction significantly reduces the opposing force. Rollers or ball bearings allow objects to move by rolling instead of sliding.
24) It is difficult to open an inkpot with greasy or oily hands. Explain.
Answer : Greasy or oily hands reduce friction between the fingers and the inkpot lid. This makes it harder to get a good grip and apply the necessary force to twist and open the lid.
25) It is difficult to walk on a wet road. Explain.
Answer: The water essentially acts as a lubricant, minimizing the grip between our shoes and the ground. This reduced friction makes it much easier for our feet to slide, disrupting our balance and making each step a bit of a gamble. 1 Stay safe on those wet surfaces!.
26) Give three examples to illustrate that friction is a necessary evil.
Answer : Here are three examples showing why friction is considered a “necessary evil”:
Walking and Running:
- Necessary: Friction between the soles of our shoes and the ground is crucial for us to walk or run. Without it, our feet would slip backward, and we wouldn’t be able to propel ourselves forward. The grip provided by friction allows us to exert a force on the ground and receive an equal and opposite reaction force that moves us.
- Evil: However, friction between our shoes and the ground also causes wear and tear on the soles over time. Additionally, excessive friction can lead to blisters or injuries during strenuous activity.
Braking in Vehicles:
- Necessary: Friction is the fundamental principle behind how brakes in cars, bicycles, and other vehicles work. When you apply the brakes, brake pads (or discs) are forced against the wheels, and the friction generated slows down or stops the rotation of the wheels, thus stopping the vehicle. Without sufficient friction, brakes would be ineffective, leading to dangerous situations.
- Evil: The friction in the braking system causes wear and tear on the brake pads and discs, requiring periodic replacement. It also generates heat, which can sometimes lead to brake fade (reduction in braking power) if the brakes overheat.
Writing with a Pen or Pencil:
- Necessary: Friction between the tip of a pen or pencil and the paper is essential for leaving a mark. The friction allows tiny particles of ink or graphite to be deposited onto the paper’s surface as the writing instrument moves. Without this friction, the pen or pencil would simply slide across the paper without leaving a trace.
- Evil: The continuous friction between the pen/pencil tip and the paper causes wear on the tip, eventually requiring sharpening or replacement. It also causes some wear on the paper itself, although usually minimal.
27) Define
- static friction
- sliding friction and
- rolling friction
Answer:
Static Friction:
Static friction is the force that opposes the initiation of motion between two surfaces in contact and at rest relative to each other. It’s a self-adjusting force that can vary in magnitude, up to a maximum value, to keep the objects from moving. Once the applied force exceeds this maximum static friction, motion begins, and static friction is replaced by kinetic friction.
Sliding Friction:
Sliding friction, also known as kinetic friction, is the force that opposes the motion of two surfaces sliding past each other. It acts in the opposite direction to the motion and is generally less than the maximum static friction between the same surfaces. The magnitude of sliding friction is approximately proportional to the normal force pressing the surfaces together.
Rolling Friction:
Rolling friction is the force that opposes the motion when a body rolls on a surface. It arises due to the deformation of both the rolling object and the surface at the point of contact. Rolling friction is typically much smaller than static or sliding friction for the same materials because the area of contact is smaller, and the nature of the motion reduces interlocking between the surfaces
28) Arrange the following in descending order :
- static friction
- sliding friction and
- rolling friction?
Answer : The arrangement of the given types of friction in descending order is:
Static friction > Sliding friction > Rolling friction
Static friction is the strongest because it’s the force that needs to be overcome to start moving a stationary object. Sliding friction occurs when an object is already moving and sliding over a surface, and it’s generally weaker than static friction. Rolling friction is the weakest, as it occurs when an object rolls, significantly reducing the contact area and thus the resistance to motion.
29) A body needs a force F1 just to start motion on a surface, a force F2 to continue its motion and a force F3 to roll on the surface. What is
- the static friction
- sliding friction and
- rolling friction ? State whether F2 is equal, less than or greater than (1) F1 and (2) F3.
Answer :
- Static friction is the force that needs to be overcome to start the motion of a body at rest on a surface. Therefore, F1 represents static friction.
- Sliding friction (also known as kinetic friction) is the force that opposes the motion of a body already sliding across a surface. So, F2 represents the sliding friction.
- Rolling friction is the force that opposes the motion of a body that is rolling on a surface. Hence, F3 represents the rolling friction.
Now, let’s compare the magnitudes of these forces:
- F2 compared to F1: It generally takes more force to get an object moving from a standstill than it does to keep it moving. Once the initial “stickiness” is overcome, less force is usually required to maintain motion. Therefore, F2 is usually less than F1.
- F2 compared to F3: Rolling friction is significantly less than sliding friction. Think about how much easier it is to move something with wheels compared to dragging it. Thus, F2 is greater than F3.
