A. Choose the correct option.
- The unit of force is
a. Metre
b. Centimetre
c. Newton
d. Second
Answer: c. Newton - Force can change the ______ of an object.
a. Speed
b. Direction of motion
c. Shape
d. All of these
Answer: d. All of these - Friction between moving parts can be reduced by
a. Force
b. Lubrication
c. Treading
d. Adhesive
Answer: b. Lubrication - Which of these can be used to reduce friction?
a. Oil
b. Grease
c. Ball bearings
d. All of these
Answer: d. All of these - Which shape will you give to a moving object in a fluid to reduce the drag it experiences?
a. Convex
b. Concave
c. Bulging
d. Streamlined
Answer: d. Streamlined
B. Fill in the blanks with the correct words.
- Magnetic force is a non-contact force.
- A frictional force can act only through contact.
- Sliding friction is greater than rolling friction for the same set of surfaces in contact.
- Oil and grease are used to reduce friction.
- The process of lubrication is used to reduce friction.
- Ball bearings are used to reduce friction.
- Frictional force depends on the nature of the surfaces in contact.
- Bodies are given a streamlined shape to reduce drag while moving in a fluid.
C. Match the following.
| Column A | Column B | Answer |
| 1. The force that keeps us bound to the Earth | a. Electrostatic force | d. Gravitational force |
| 2. Disadvantage of friction | b. Oil and grease | c. Wear and tear |
| 3. Acts between charged objects | c. Better grip | a. Electrostatic force |
| 4. Lubrication | d. Gravitational force | b. Oil and grease |
| 5. Treads | e. Wear and tear | c. Better grip |
D. Answer the following questions.
1. What is force? Give its SI unit.
Force is a push or pull that can change the shape, speed, or direction of an object. It can make an object start moving, stop, or move faster or slower. For example, pushing a door or pulling a cart. The SI unit of force is Newton (N).
2. Give an example of a non-contact force.
A non-contact force is a force that acts on objects without touching them. Examples include gravitational force, which pulls all objects toward the Earth, and magnetic force, which attracts or repels magnetic materials like iron from a distance without physical contact.
3. What are the factors on which the frictional force depends?
The frictional force depends on two main factors: the nature of the surfaces in contact and the force pressing them together. Rough surfaces create more friction, while smooth surfaces create less. Also, heavier objects experience more friction because they press the surfaces together more strongly.
4. Why are ball bearings used?
Ball bearings are small metal balls placed between moving parts to reduce friction. They change sliding friction into rolling friction, which is much smaller. This helps machines, bicycles, and fans move smoothly, reduces wear and tear, and saves energy by allowing parts to rotate easily.
5. Give two examples where friction is an advantage.
Friction is useful in many situations. It helps us walk safely without slipping because it provides grip between our feet and the ground. It also helps us write or draw with pencils or pens, as friction between the tip and paper prevents slipping and allows clear writing.
6. Give two examples of situations when we need to increase friction.
Friction is increased to make movement safer. For example, grooves are made on tyres to get a better grip on the road and prevent skidding. Similarly, sand is spread on icy roads to make the surface rough, reducing slipping and helping vehicles and people move safely.
7. What are the disadvantages of friction?
Although useful, friction also has drawbacks. It causes wear and tear on machines, shoes, and tyres, reducing their life. It also produces unwanted heat, which wastes energy and lowers efficiency in engines or moving machines. Too much friction can make movement difficult and energy-consuming.
8. Why are objects given a streamlined shape?
Objects are given a streamlined shape so they can move smoothly through air or water. This special shape reduces air or water resistance (drag) and helps them move faster using less energy. For example, cars, airplanes, and ships are streamlined to move easily and save fuel.
9. Name three objects and one marine mammal that have a streamlined shape.
Many objects and animals have a streamlined shape to move easily through air or water. Examples include an airplane, a ship, and a car, all designed to reduce drag. A dolphin, a marine mammal, also has a streamlined body that helps it swim quickly and smoothly through water.
Think and Answer
Question:
To increase the friction, road workers usually make grooves in the plaster. Can you explain why?
(Hint: In a way, the surface is being made rough. Why?)
Answer:
Grooves are made in the plaster to increase friction. When the surface is rough, it provides a better grip between the tyres and the road. This helps vehicles move safely without slipping, especially when the surface is wet or smooth. Rough surfaces create more resistance to motion, which means more friction.