Force and Laws of Motion Class 9 Worksheet Science Chapter 8

Q1. The inability of the body to change its state of rest or motion is:

Ans: The inability of a body to change its state of rest or motion is known as inertia. This property means that an object will resist changes to its motion, whether it is at rest or moving at a constant speed.

Key points about inertia:

• Objects remain in their current state unless acted upon by an external force.
• Inertia explains why we continue moving forward in a car when it suddenly stops; our bodies want to maintain their state of motion.
• The more massive an object, the greater its inertia. For example, a train has more inertia than a small cart.
• Inertia is quantitatively measured by an object's mass, with the SI unit being kilograms (kg).

In summary, inertia is the natural tendency of objects to resist changes in their state of motion or rest, and it is a fundamental concept in understanding motion.

Q2. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because

(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion.
(d) there is no unbalanced force on the ball, so the ball would want to come to rest.

Ans: Correct Answer is Option (c)

The cricket ball comes to rest because:

• There is a force acting on it that opposes its motion.
• This opposing force is typically friction between the ball and the ground.
• As a result, the ball slows down and eventually stops.

Q3. An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.

Ans: No, it is not possible for an object to travel with a non-zero velocity if it experiences a net zero external unbalanced force. This is because:

• Unbalanced forces cannot equal zero.
• If the net force is zero, the object will either remain at rest or continue moving at a constant velocity.
• To change the object's speed or direction, an unbalanced force must be applied.

Q4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Ans: When a moving bus brakes to a stop, we fall forward due to our body's inertia. Here's how it works:

• We are initially moving at the same speed as the bus.
• When the bus suddenly stops, the lower half of our body comes to rest.
• The upper half, however, continues moving forward because it is not in close contact with the bus.

Conversely, when the bus accelerates from rest:

• We start off at rest, seated in the bus.
• As the bus moves forward, our feet are pushed along with it.
• The upper part of our body tends to remain still due to inertia, causing us to fall backwards.

Q5. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Ans: Some leaves may detach from a tree when its branch is vigorously shaken due to the principle of inertia. This occurs because:

• When the branch moves, the leaves tend to stay in their original position.
• This resistance to change causes some leaves to fall off as the branch shakes.

In summary, the shaking motion does not affect all leaves equally, leading to detachment.

Q6. Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Ans: It is important to tie luggage on the roof of a bus with a rope for safety reasons. Here are the key points:

• When the bus moves, the luggage moves with it at the same speed and direction.
• If the bus suddenly changes direction or slows down, the luggage continues moving due to its inertia.
• This inertia can cause the luggage to slide off the roof, potentially leading to accidents.

Therefore, securing luggage with a rope helps prevent it from being thrown off during travel.

Q7. Why are the passengers sitting in a moving bus pushed in the forward direction when the bus stops suddenly?

Ans: The passengers in a moving bus are pushed forward when the bus stops suddenly due to inertia. Here's how it works:

• As the bus comes to a stop, the passengers' feet also stop.
• However, the upper part of their bodies continues to move forward because of inertia.
• This difference in motion causes them to fall forward.

Q8. State Newton’s first law of Motion?

Ans: Newton's first law of motion states that:

• An object at rest will remain at rest.
• An object in motion will continue moving in a straight line at a constant speed.
• This continues until an external force acts on it.

This principle is known as inertia, which describes the tendency of objects to resist changes in their state of motion.

For example, when a car suddenly brakes, passengers feel a jolt forward because their bodies want to maintain their state of motion due to inertia.

In summary, the first law highlights that:

• Objects resist changes in motion.
• The mass of an object is a measure of its inertia.

Q9. Define force and what are the various types of forces?

Ans: Force is a push or pull that can change the state or shape of an object. It can:

• Make an object move faster or slower.
• Change the direction of an object's motion.
• Alter the shape and size of objects.

The main types of forces include:

• Gravitational force: Attracts objects towards each other.
• Electrostatic force: Acts between charged particles.
• Electromagnetic force: Influences charged particles in motion.
• Nuclear force: Holds protons and neutrons together in an atom's nucleus.

Q10. State Newton’s second law of motion?

Ans: Newton's second law of motion states that:

• The rate of change of momentum of an object is proportional to the unbalanced force acting on it.
• This change in momentum occurs in the same direction as the applied force.

The formula for this law can be expressed as:

• F = ma, where:
• F is the force applied,
• m is the mass of the object, and
• a is the acceleration produced.

The unit of force is the newton (N), which is defined as the force required to accelerate a 1 kg mass by 1 m/s².

Q11. When a carpet is beaten with a stick, dust comes out of it. Explain.

Ans: When a carpet is beaten with a stick, dust comes out because:

• The carpet fibres vibrate back and forth.
• Dust particles that are loosely bound remain at rest due to inertia.
• As the carpet moves, these particles are dislodged and come out.