Chap 5 : Exploring Forces

Page 1: Probe and ponder

Why does it feel harder to pedal a bicycle when going uphill than on flat ground?

Answer: It feels harder to pedal uphill because you are working against the force of gravity, which is trying to pull you and your bicycle back down the hill. On flat ground, you mainly have to work against wind and friction, but uphill you must also overcome gravity, which requires much more force.

Why is it easier to slip on a wet surface?

Answer: It is easier to slip on a wet surface because the water reduces the friction between your shoes and the ground. Friction is the force that grips the ground and allows you to walk without slipping. When friction is low, your feet can slide easily.

Why do we feel ‘light’ or like we are ‘floating’ just after our swing reaches its highest point and begins to come down?

Answer: At the very top of the swing’s path, you stop for a tiny moment. As you start to come down, you are moving with the force of gravity, rather than against it. This feeling of moving along with gravity’s pull is what makes you feel ‘light’ or like you are ‘floating’ for a second.

Page 2: Activity 5.1: Let us explore

Did you move the box in any other way than shown in Fig. 5.1?

Answer: Yes, besides pushing (a), pulling (b), and lifting (c), the box could also be moved by:

Tilting it on one edge and spinning it.
Sliding it sideways without turning.
Dragging it with a rope tied to a corner. In all these actions, a push or a pull (a force) was applied to move the box.

Page 3: Activity 5.2: Let us analyse

Think of situations where a force (push or pull) is applied and list them in Table 5.1.

Answer: Here are some situations:

Kicking a football (Push)
Opening a drawer (Pull)
Lifting a school bag (Pull)
Stretching a rubber band (Pull)
Squeezing a tube of toothpaste (Push)

Analyse each situation and write the effect of the force in Table 5.1.

Answer: Here are the effects for the situations above:

Kicking a football: Makes the football move from rest and changes its speed.
Opening a drawer: Makes the drawer move from rest.
Lifting a school bag: Makes the bag move upwards from rest.
Stretching a rubber band: Changes the shape of the rubber band.
Squeezing a tube of toothpaste: Changes the shape of the tube.

What do you conclude from these examples? Does a force cause a moving object to stop? Can it change speed, or direction of motion, or change the shape of an object?

Answer: Yes, we can conclude that a force can do all of those things. A force can:

make an object move from rest.
stop a moving object.
change the speed of a moving object (make it faster or slower).
change the direction of a moving object.
change the shape of an object.
It can also cause some or all of these effects at the same time.

Page 4: In-text Questions

(Bubble): Does this mean that whenever there is a change in speed or direction, or change in shape, a force is acting on the object?

Answer: Yes, that is correct. None of these effects (a change in speed, direction, or shape) can happen unless a force is acting on the object.

(Bubble): Suppose an object is at rest. Does it mean that no force is acting on this object?

Answer: No, it does not necessarily mean no force is acting on it. It could mean that all the forces acting on the object are balanced. For example, a book on a table has gravity pulling it down and the table pushing it up. The forces cancel each other, so the book stays at rest.

Think of all the actions listed in Table 5.1. How many objects are involved in each of the actions? Do you notice that forces result only when two objects are interacting in some way or the other?

Answer: In every action, there are at least two objects involved. For example, in “Hitting a moving ball with a bat,” the two objects are the ball and the bat. Yes, we notice that forces only happen when two or more objects interact with each other.

Page 5: In-text Question (A step further)

When you pushed the table with your hand, did you feel a force on your hand too?

Answer: Yes. When I pushed the table, my hand also felt a force from the table pushing back on it. This shows that when two objects interact, both of them experience a force.

Page 6: In-text Question

(Bubble): Is there any other contact force?

Answer: Yes, besides muscular force, another very common contact force is the force of friction.

Page 6: Activity 5.3: Let us investigate

Take an object with a flat base (such as an empty lunch box/ geometry box/ a notebook) and place it on a table or floor. Gently push it and observe. Does it stop after travelling some distance? Is there a force acting on it which brings it to rest?

Answer: Yes, after being pushed, the object moves for a short distance and then stops on its own. Yes, there must be a force acting on it that slows it down and brings it to rest. This force is called friction.

Now repeat by pushing the object in the opposite direction. Does it stop again after travelling some distance?

Answer: Yes, even when pushed in the opposite direction, the object still slows down and stops after travelling some distance, again because of the force of friction.

Page 7: In-text Question

(Bubble): Does this mean that the force of friction will be greater if the surfaces are rough?

Answer: Yes, that is correct. The force of friction is greater between rough surfaces. Smooth surfaces have less friction.

Page 7: Activity 5.4: Let us explore

Try Activity 5.3 again, but this time place the same object on different surfaces, such as glass, cloth, wood, ceramic tile, and sand. Does the object stop after travelling the same distance as in Activity 5.3?

Answer: No, the object does not stop after travelling the same distance. It travels different distances on different surfaces.

Does the object stop at the same distance on all surfaces?

Answer: No, the object stops at different distances. It will travel farther on smooth surfaces (like glass or tile) and stop much sooner on rough surfaces (like cloth or sand). This shows that the amount of friction is different for different surfaces.

Page 7: In-text Question (A step further)

Does the force of friction act only if the objects are moving on solid surfaces? What about objects moving through liquids and gases?

Answer: No, friction does not only act between solid surfaces. Liquids (like water) and gases (like air) also exert a force of friction on objects that move through them. This is sometimes called ‘drag’ or ‘air resistance’.

Page 8: In-text Question

(Bubble): Is it essential for an object applying force on another object to always be in contact with it?

Answer: No, it is not essential. There are some forces, called non-contact forces, that can act even when the objects are not touching each other.

Page 8: Activity 5.5: Let us test

…insert the second ring magnet above it such that the like poles of the two magnets face each other. Does the second magnet stay floating above the first magnet?

Answer: Yes, the second magnet stays floating above the first magnet. This happens because the like poles (e.g., North pole facing North pole) repel, or push each other away, with a magnetic force.

Try pushing the second magnet down gently. Do you feel a force on it?

Answer: Yes, when pushing the second magnet down, you can feel an upward force pushing back against your hand. This is the magnetic repulsion.

Now, reverse the poles of both the magnets. Does the second magnet still remain floating?

Answer: Yes, it will still float. If you reverse both magnets, their like poles will still be facing each other (e.g., it will change from North-facing-North to South-facing-South). Since like poles always repel, the magnets will still push each other apart, and the top magnet will float.

(Bubble): Are there more such forces which act from a distance?

Answer: Yes, besides magnetic force, other non-contact forces are electrostatic force and gravitational force.

Page 9: Activity 5.6: Let us experiment

…bring it [the rubbed plastic scale] close to the small pieces of paper… Do you notice something surprising?

Answer: Yes. The small pieces of paper are pulled towards the plastic scale and stick to it, even though the scale is not touching them. This happens because rubbing the scale gave it an electrostatic charge.

Page 9: Activity 5.7: Let us experiment

Rub both balloons with the woollen cloth and release them. … What do you observe?

Answer: We observe that the two balloons move away from each other; they repel.

Now bring the woollen cloth used for rubbing the balloons close to one of the rubbed balloons. What happens?

Answer: The balloon and the woollen cloth move towards each other; they attract.

Page 10: In-text Questions

(Bubble): Does it mean that there are two kinds of electrical charges?

Answer: Yes, this indicates that there are two kinds of electrical charges.

We found that the two similarly charged balloons repel each other whereas a charged balloon and the woollen cloth (with which the balloon was rubbed) attract each other. Does this indicate that the charge on the balloon is of a different kind from the charge on the woollen cloth?

Answer: Yes, this indicates that the charge on the balloon is of a different (opposite) kind from the charge on the woollen cloth. We can infer from this that ‘like’ charges (which were on the two balloons) repel each other, and ‘unlike’ (opposite) charges (on the balloon and the cloth) attract each other.

Page 10: Activity 5.8: Let us observe

Take a ball and throw it vertically up. Does it come down?

Answer: Yes, the ball goes up, slows down, stops for a moment, and then comes back down.

Now throw it again, but this time harder. Does it still fall back down to the ground?

Answer: Yes, even if it is thrown harder, it still slows down, stops, and falls back down to the ground. It will just go higher before coming down.

Think about different situations around you where any object thrown up in any direction, finally falls or comes back to the ground or floor.

Answer: Some examples are:

A coin tossed in the air.
A key thrown to a friend.
A fruit that breaks off a tree branch.
Raindrops falling from the sky.
A basketball thrown towards the hoop.

Page 11: In-text Questions

(Bubble): Why do all the objects fall towards the Earth?

Answer: All objects fall towards the Earth because the Earth attracts (pulls) them.

(Bubble): Is there any force which acts on them? What exerts this force?

Answer: Yes, there is a force acting on them. This force is called gravitational force, or gravity. The Earth exerts this force on all objects near it.

(Bubble): Does the Earth pull every object with equal force?

Answer: No, the Earth does not pull every object with equal force. It pulls heavier objects (objects with more mass) with a stronger force than lighter objects.

Page 12: Activity 5.9: Let us explore

Hang one end of the spring from a nail. From the other end, hang an object and observe the spring. Does the spring stretch?

Answer: Yes, the spring stretches.

Now hang the other objects, one by one and notice the stretch in the spring each time. Is the stretch caused by each object the same?

Answer: No, the stretch caused in the spring is different for different objects. Heavier objects cause the spring to stretch more.

Can we use the spring to measure the weight of an object?

Answer: Yes, we can use the spring to measure weight. Since the amount of stretch depends on the weight (the force of gravity) pulling the object, we can make a scale next to the spring to read the weight. This is how a spring balance works.

Page 12: Activity 5.10: Let us observe

Look at the spring balance shown in Fig. 5.13 carefully. What is the maximum weight it can measure?

Answer: The maximum weight this spring balance can measure is 10 N (ten newtons).

Page 13: Activity 5.11: Let us calculate

How much is the weight difference indicated between the two bigger marks?

Answer: The weight difference between two bigger marks (like 01 N and 02 N) is 1 N.

How many divisions (shown by smaller marks) are there between these two bigger marks?

Answer: There are 5 divisions between these two bigger marks.

How much weight does one small division indicate?

Answer: One small division indicates 1 N ÷ 5 = 0.2 N.

…calculate the smallest value of weight that can be measured with the spring balance given to you.

Answer: Based on the spring balance shown in Figure 5.13, the smallest value that can be measured is one small division, which is 0.2 N.

Page 14: In-text Question

(Bubble): What is the difference between weight and mass?

Answer:

Mass is the amount of matter (or “stuff”) in an object. It is measured in grams (g) or kilograms (kg). An object’s mass is the same everywhere, whether on Earth or on the Moon.
Weight is the force of gravity pulling on an object. It is a force, so it is measured in newtons (N). An object’s weight can change depending on where it is (for example, you weigh less on the Moon because the Moon’s gravity is weaker).

Page 15: In-text Questions

(Bubble): If we place some objects on water, some of them float, while others fall to the bottom. The gravitational force of the Earth is acting on all objects, then why don’t all objects fall to the bottom?

Answer: All objects don’t fall to the bottom because water (and other liquids) exerts an upward force on any object placed in it. This upward force is called the buoyant force or upthrust. If the buoyant force pushing up is greater than or equal to the object’s weight (gravity pulling down), the object will float. If the weight is greater than the buoyant force, the object will sink.

(Bubble): While taking out water from a bucket filled with water using a mug, do you notice that the mug feels lighter when it is inside water?

Answer: Yes, the mug feels lighter when it is inside the water. This is because the water is helping to lift the mug by pushing up on it with a buoyant force.

Page 15: Activity 5.13: Let us investigate

Push the bottle [empty plastic bottle with lid on] in the water (Fig. 5.15). Do you feel an upward push?

Answer: Yes, when you push the bottle into the water, you feel a strong upward push from the water.

Release the bottle. Does it bounce up?

Answer: Yes, when the bottle is released under water, it bounces up to the surface. This demonstrates the upward buoyant force from the water.

Page 16 & 17: Keep the curiosity alive (Exercises)

Question 1: Match items in Column A with the items in Column B.

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

Answer 1:

(i) Muscular force — (b) A child lifting a school bag
(ii) Magnetic force — (e) A compass needle pointing North
(iii) Frictional force — (a) A cricket ball stopping on its own just before touching the boundary line
(iv) Gravitational force — (c) A fruit falling from a tree
(v) Electrostatic force — (d) Balloon rubbed on woollen cloth attracting hair strands

Question 2: State whether the following statements are True or False.

(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.

Answer 2:
(i) True. (A force is needed to make an object speed up, slow down, or stop).
(ii) False. (Friction opposes motion, so it causes the speed of the rolling ball to decrease).
(iii) False. (There is always an electrostatic force between two charged objects. It will be a force of attraction or repulsion).

Question 3: Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?

Answer: The two balloons will move away from each other; they will repel.
Why: When both balloons are rubbed with the same woollen cloth, they both get the same kind of electrical charge. Since “like” charges repel each other, the balloons push each other away.

Question 4: When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.

Answer: This happens because of the balance between the object’s weight (pulling it down) and the water’s buoyant force (pushing it up).

Coin: The weight of the coin is greater than the buoyant force the water can apply to it. The downward force of weight wins, so the coin sinks.
Wooden Block: The weight of the wooden block is less than the buoyant force the water can apply to it. The upward buoyant force is strong enough to balance the block’s weight, so the block floats.

Question 5: If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.
(i) During its upward motion
(ii) During its downward motion
(iii) At its topmost position

Answer: The two main forces acting on the ball are gravitational force (gravity) and frictional force (from the air, or air resistance).

(i) During its upward motion:
* Gravitational force acts downwards.
* Frictional force (air) also acts downwards (because it opposes the upward motion).

(ii) During its downward motion:
* Gravitational force acts downwards.
* Frictional force (air) acts upwards (because it opposes the downward motion).

(iii) At its topmost position:
* For the split second the ball is stopped, the frictional force is zero.
* Gravitational force still acts downwards (this is the force that makes it start to fall).

Question 6: A ball is released from the point P and moves along an inclined plane and then along a horizontal surface as shown in the Fig. 5.16. It comes to stop at the point A on the horizontal surface. Think of a way so that when the ball is released from the same point P, it stops (i) before the point A (ii) after crossing the point A.

Answer: The ball stops at point A because of the force of friction on the horizontal surface.

(i) To make it stop before point A: We must increase the friction on the horizontal surface. We could do this by spreading a thin layer of sand or by placing a piece of cloth on the surface.

(ii) To make it stop after crossing point A: We must decrease the friction on the horizontal surface. We could do this by making the surface smoother, or by applying a lubricant like oil or soap water.

Question 7: Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.

Answer: We are able to walk because of friction, which is the grip between our shoes and the ground. This friction stops our feet from sliding. Smooth surfaces like ice or polished floors (especially if they are wet) have very little friction. When we try to walk, there is not enough grip (friction) to stop our feet from sliding, which causes us to slip.

Question 8: Is any force being applied to an object in a non-uniform motion?

Answer: Yes. Non-uniform motion means the object’s speed or its direction of motion (or both) is changing. Any change in speed or direction can only be caused by a force. Therefore, an object in non-uniform motion must have a force acting on it.

Question 9: The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?