1. What is a force? What changes can be produced by a force? (AS1)

Answer: A force is a push or a pull acting on an object. Changes produced by a force include: changing the state of rest or motion, changing speed, changing direction, and changing the shape temporarily or permanently.

2. Give two examples each for a contact force and a force at a distance. (AS1)

Answer: 1. Contact Forces: Muscular force (lifting a bucket), Frictional force (walking on a floor).
2. Forces at a distance: Magnetic force (attracting iron nails), Gravitational force (apple falling down).

3. Explain Gravitational Force by giving a suitable example. (AS1)

Answer: Gravitational force is the attractive force that exists between any two masses (or bodies) in the universe. Since the earth is massive, it pulls objects toward itself. Example: When a pen slips from our hand, it falls down to the floor due to Earth's gravitational pull.

4. Draw and explain a free body diagram (FBD) to show all the forces acting on a car. (AS5)

Answer: When a car moves with non-uniform speed along a road, the forces acting on it are: 1. Forward force applied by the engine ($F$). 2. Frictional force applied by the road ($f$). 3. Normal forces ($N_1$ and $N_2$). 4. Downward gravitational force / weight ($W$). Net force along x-axis: $F_{net,x} = f - F$; Net force along y-axis: $F_{net,y} = N_1 + N_2 - W$.

5. Why do tools meant for cutting always have sharp edges? (AS1)

Answer: Cutting tools have sharp edges to have a very small area of contact. According to the pressure formula ($\text{Pressure} = \frac{\text{Force}}{\text{Area}}$), for a given force, a smaller surface area produces greater pressure, making cutting much easier.

1. How can you differentiate between a contact force and a force at a distance? (AS1)

Answer: - Contact Force: Requires direct physical contact or interaction between two objects (e.g., pushing a table, muscular force).
- Force at a distance (Field Force): Acts without any physical contact between objects through a field (e.g., magnetic attraction, gravity).

2. Find the net forces from the given diagrams (Textbook diagrams a, b, c, d). (AS4)

Answer: - When forces act in the same direction, they are added ($F_{net} = F_1 + F_2$).
- When forces act in opposite directions along a straight line, the net force is their difference ($F_{net} = F_1 - F_2$).

3. A man stands still on a level floor. What forces act on him? Draw a free body diagram (FBD) to show all forces acting on him. (AS5)

Answer: The forces acting on a man standing still on a level floor are: (1) Gravitational force (weight $W$) acting vertically downwards, and (2) Normal force ($N$) acting vertically upwards from the floor. Since he is at rest, $N = W$ and the net force is zero.

4. The surface area of an object is $20\text{ m}^2$ and a force of $10\text{ N}$ is applied on it, then what is the pressure? (AS7)

Answer: $\text{Force} (F) = 10\text{ N}$, $\text{Area} (A) = 20\text{ m}^2$.
$\text{Pressure} = \frac{\text{Force}}{\text{Area}} = \frac{10}{20} = 0.5\text{ N/m}^2$ (Pascals).

5. How do you appreciate the role of friction in facilitating our various activities? (AS6)

Answer: Friction is vital in daily life. Without friction, we cannot walk, run, hold a pen, write on a blackboard, drive cars with brakes, or construct buildings. It provides the necessary grip for all motions.

6. Identify and draw all forces acting on the body shown in the diagram. (AS5)

Answer: The forces generally include gravitational weight ($W$ downwards), normal reaction ($N$ upwards), applied force ($F$), and opposing friction ($f$).

1. If you push a heavy box which is at rest, you must exert some force to start its motion. However once the box is sliding you apply a lesser force to maintain that motion. Why? (AS1)

Answer: Static friction (which resists motion from rest) has a maximum limit that is higher than sliding friction. Once the box starts sliding, sliding friction comes into play, which is lesser than maximum static friction, requiring less force to maintain motion.

2. How do you increase the pressure by keeping (a) area unchanged (b) force unchanged? (AS1)

Answer: (a) If area is unchanged, increase the applied force.
(b) If force is unchanged, decrease the surface area of contact.

3. Imagine that friction disappeared from the earth. What will happen? Explain. (AS2)

Answer: If friction disappeared, we could not walk or run without slipping, cars could not stop or start, writing with a pen or chalk would be impossible, and no buildings could be constructed.