1. What do we mean by force?
Answer:
Force is a push or a pull resulting from an interaction between objects.
2. Mention any three changes that a force can produce in an object.
Answer:
A force can:
a) Change the speed of an object
b) Change the direction of its motion
c) Change its shape
3. What is meant by balanced forces?
Answer:
Forces are said to be balanced when they cancel each other, producing zero net force. In this situation, there is no change in the state of motion due to the resultant force.
4. Two opposite forces of $12\text{ N}$ each act on an object. What is the net force?
Answer:
$$F_{\text{net}} = 12 - 12 = 0\text{ N}$$
Therefore, the forces are balanced.
5. A box is pulled with $15\text{ N}$ towards the right and $9\text{ N}$ towards the left. Find the resultant force.
Answer:
$$F_{\text{net}} = 15 - 9 = 6\text{ N}$$
The resultant force is $6\text{ N}$ towards the right. Therefore, the forces are unbalanced.
6. Why is friction classified as a contact force?
Answer:
Friction acts when two surfaces are in physical contact. It opposes the motion or attempted motion between the surfaces.
7. Give two examples of non-contact forces.
Answer:
* Magnetic force: A magnet attracts an iron pin without touching it.
* Gravitational force: A fruit falls from a tree towards the Earth.
8. How does electrostatic force act without direct contact?
Answer:
When an object becomes electrically charged, it can attract or repel another object from a distance. For example, a charged comb can attract small pieces of paper without touching them.
9. What is mass?
Answer:
Mass is the amount of matter present in an object. Its value remains constant when the location of the object changes.
10. What is weight?
Answer:
Weight is the gravitational force acting on an object.
$$W = mg$$
where $m$ is the mass of the object and $g$ is the acceleration due to gravity.
11. State two differences between mass and weight.
| Mass | Weight |
| - | |
| It is the amount of matter in an object. | It is the gravitational force acting on an object. |
| It is measured in kilogram $(\text{kg})$. | It is measured in newton $(\text{N})$. |
| It remains essentially constant at different locations. | It can change when the gravitational field changes. |
Friction and Motion
12. A ball is pushed across a rough floor. Why does it eventually stop after the push is removed?
Answer:
The rough surface produces friction, which acts opposite to the direction of motion. This opposing force reduces the speed of the ball until it eventually comes to rest.
13. Why does an object generally travel farther on a smooth surface than on a rough surface?
Answer:
A smooth surface produces less friction, so the opposing force is smaller. Therefore, the object can continue moving for a greater distance. A rough surface produces greater friction and stops the object sooner.
14. Name factors that can influence the frictional force between two surfaces.
Answer:
The magnitude of friction can depend on:
* The nature or roughness of the surfaces
* The condition of the surfaces in contact, such as the presence of dust or other material
15. A student says, "If an object is not moving, no force is acting on it." Is this statement correct? Explain.
Answer:
No. The statement is incorrect. An object may remain at rest even when forces are acting on it, provided the forces are balanced and the net force is zero.
For example, a book resting on a table experiences gravitational force downward and an upward supporting force from the table.
16. Why do we say that force is associated with interaction?
Answer:
A force arises because of an interaction between two objects or systems. For example:
* Earth and an object interact gravitationally.
* A magnet interacts with a magnetic material.
* Charged objects interact through electrostatic force.
Thus, force does not arise independently; it is associated with interaction.
17. Which two forces are important in deciding whether an object floats or sinks in a liquid?
Answer:
* Weight or gravitational force acting downward
* Buoyant force or upthrust acting upward
18. When will an object sink in a fluid?
Answer:
An object tends to sink when its downward weight is greater than the upward buoyant force.
$$W > F_B$$
where $F_B$ is the buoyant force.
19. Under what condition can an object float in a liquid?
Answer:
An object can float when the upward buoyant force balances its weight.
$$F_B = W$$
20. A closed hollow metal cube floats partially in water. Holes are made in it, allowing water to enter. What is likely to happen? Explain.
Answer:
As water enters the hollow cube, its total mass and weight increase. If the downward weight becomes greater than the upward buoyant force, the cube will sink.
Initially:
$$F_B = W$$
After water enters, the weight increases. If:
$$W > F_B$$
the cube will sink.
Electrostatic Force
21. A plastic comb is rubbed through dry hair and then brought near tiny pieces of paper. The paper pieces move towards the comb. Why?
Answer:
Rubbing causes the comb to become electrically charged. The charged comb produces an electrostatic force that attracts the small paper pieces.
22. What would happen when two similarly charged objects are brought close to each other?
Answer:
Similarly charged objects repel each other due to electrostatic force.
Gravity
23. Why is gravity called a universal force?
Answer:
Gravity acts between objects having mass. It is observed in many situations throughout the universe, such as:
a) The Earth attracting objects towards its surface
b) The Moon remaining under the gravitational influence of the Earth
c) Planets moving under the gravitational influence of the Sun
Therefore, gravity is considered a universal force.
24. A trolley is acted upon by two forces: $20\text{ N}$ towards the east and $20\text{ N}$ towards the west. Will the forces cause a change in its motion?
Answer:
The two forces are equal in magnitude and opposite in direction.
$$F_{\text{net}} = 20 - 20 = 0\text{ N}$$
Therefore, the forces are balanced and produce no change in motion due to the resultant force.
25. A toy car is pushed with the same force on a tiled floor and then on sandpaper. On which surface will it travel farther? Give a reason.
Answer:
The toy car will travel farther on the tiled floor because it is smoother and offers less friction. Sandpaper is rougher and produces greater friction, causing the car to stop sooner.
26. A stone is thrown vertically upward. After leaving the hand, which force mainly pulls it back towards the Earth?
Answer:
The gravitational force exerted by the Earth pulls the stone back downward.
27. A magnet attracts an iron nail even though the nail does not touch the magnet. What does this demonstrate?
Answer:
It demonstrates that magnetic force is a non-contact force because it can act from a distance.
28. Why can two objects experience forces and still remain at rest?
Answer:
If the forces acting on the object are balanced, they cancel each other. The net force becomes zero, so the object can remain at rest even though individual forces are present.
29. Design a simple experiment to compare the effect of different surfaces on the stopping distance of an object.
Answer:
a) Take the same block or toy car.
b) Use surfaces such as tile, wood, cloth and sandpaper.
c) Give the object approximately the same initial push on each surface.
d) Measure the distance travelled before it stops.
e) Repeat the experiment several times.
Conclusion:
The object will generally travel farther on smoother surfaces and a shorter distance on rougher surfaces because rough surfaces offer greater friction.
30. A floating object has its weight increased while the buoyant force remains insufficient to balance the increased weight. Predict what will happen.
Answer:
The object will move downward and may sink because the downward gravitational force becomes greater than the upward buoyant force.
$$W > F_B$$
1. Which of the following best describes a force?
A. Energy stored in an object
B. A push or pull resulting from an interaction
C. The amount of matter in a body
D. The speed of a moving object
Answer: B. A push or pull resulting from an interaction
2. A force of $18\text{ N}$ acts towards the right and another force of $18\text{ N}$ acts towards the left. The forces are:
A. Unbalanced
B. Frictional
C. Balanced
D. Magnetic
Answer: C. Balanced
3. Which of the following is not necessarily an effect of applying a force?
A. Changing the shape of an object
B. Changing its direction of motion
C. Changing its speed
D. Increasing its mass
Answer: D. Increasing its mass
4. Why is friction considered a contact force?
A. It acts only on charged objects
B. It requires surfaces to interact physically
C. It always increases the speed of an object
D. It acts only in liquids
Answer: B. It requires surfaces to interact physically
5. Which example demonstrates a non-contact force?
A. A person pushing a trolley
B. A book resting on a table
C. A magnet attracting an iron pin
D. A football being kicked
Answer: C. A magnet attracting an iron pin
6. Which statement correctly distinguishes mass from weight?
A. Both are measured in newtons
B. Mass changes from planet to planet, but weight remains constant
C. Mass is measured in kilograms, whereas weight is measured in newtons
D. Weight is the amount of matter present in an object
Answer: C. Mass is measured in kilograms, whereas weight is measured in newtons
7. A toy car is pushed across a rough surface and then allowed to move freely. Why does it eventually stop?
A. Its mass gradually decreases
B. Friction acts opposite to its motion
C. Gravity disappears after the push
D. The object loses all its weight
Answer: B. Friction acts opposite to its motion
8. An object will tend to sink in a liquid when:
A. The buoyant force is greater than its weight
B. Its weight is greater than the upward buoyant force
C. There is no gravitational force
D. The object has zero mass
Answer: B. Its weight is greater than the upward buoyant force
9. A charged plastic comb attracts small pieces of paper without touching them. This is due to:
A. Frictional force
B. Muscular force
C. Electrostatic force
D. Buoyant force
Answer: C. Electrostatic force
10. A book is lying motionless on a table. Which statement is correct?
A. No force is acting on the book
B. Only gravitational force acts on the book
C. Forces acting on the book may be balanced, giving zero net force
D. The book has no weight
Answer: C. Forces acting on the book may be balanced, giving zero net force