Important Questions For All Chapters – Science Class 9
Short Questions
1. What is force?
Answer: A force is a push or pull on an object that can change its state of motion, direction, or shape.
2. What are balanced forces?
Answer: Forces that are equal in magnitude but opposite in direction and do not change the state of motion of an object are called balanced forces.
3. Define unbalanced force.
Answer: When two opposite forces acting on an object are unequal, they form an unbalanced force that changes the state of motion of the object.
4. State Newton’s First Law of Motion.
Answer: An object remains at rest or in uniform motion in a straight line unless acted upon by an unbalanced external force.
5. What is inertia?
Answer: Inertia is the tendency of an object to resist a change in its state of motion or rest.
6. On what factor does inertia depend?
Answer: Inertia depends on the mass of an object; heavier objects have more inertia.
7. Define momentum.
Answer: Momentum is the product of mass and velocity of an object.
Formula: Formula:
8. State Newton’s Second Law of Motion.
Answer: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
9. What is the SI unit of force?
Answer: The SI unit of force is newton (N).
10. State Newton’s Third Law of Motion.
Answer: For every action, there is an equal and opposite reaction acting on different bodies.
Long Questions
1. What do you mean by balanced and unbalanced forces? Explain with suitable examples.
Answer: When equal and opposite forces act on an object, and the object does not change its state of rest or motion, these are called balanced forces. For example, a book lying on a table remains at rest because the downward gravitational force is balanced by the upward force of the table.
If two opposite forces acting on a body are not equal, then they form unbalanced forces. These cause a change in the state of motion of the object. For example, when children push a box on a rough floor and the box moves, it happens because the applied force becomes greater than the frictional force. Hence, unbalanced force produces motion.
2. State and explain Newton’s First Law of Motion. Give examples to illustrate it.
Answer:
Newton’s First Law of Motion states that an object remains at rest or continues to move with a uniform velocity in a straight line unless acted upon by an unbalanced external force.
This law means that all objects resist a change in their state of motion. The tendency of objects to resist a change is called inertia, so this law is also known as the Law of Inertia.
Examples:
- When a bus suddenly starts, passengers fall backward due to the inertia of rest.
- When a moving bus stops suddenly, passengers fall forward because their bodies tend to continue moving.
- A coin placed on a card over a glass falls straight into the glass when the card is flicked because of the coin’s inertia.
3. Describe Galileo’s observations about motion and his conclusion.
Answer: Galileo observed the motion of marbles on inclined planes. He found that a marble rolling down an incline gained speed, while one rolling up lost speed. When both inclines were made equal, the marble reached the same height from which it was released.
He reasoned that if friction were completely removed and the second incline made horizontal, the marble would continue to move forever with the same speed. Hence, Galileo concluded that no force is required to keep an object in uniform motion; force is needed only to change its state of motion.
4. What is inertia? Explain its types with suitable examples.
Answer: Inertia is the property of an object to resist any change in its state of rest or motion. It depends on the mass of the object; heavier objects have more inertia.
Types of Inertia:
- Inertia of Rest: The tendency of a body to remain at rest.
Example: Dust falls off a carpet when beaten because dust particles tend to stay at rest. - Inertia of Motion: The tendency of a body to remain in motion.
Example: Passengers fall forward when a moving bus stops suddenly. - Inertia of Direction: The tendency of a body to resist change in direction.
Example: Passengers move sideways when a car takes a sharp turn.
5. Explain the relationship between mass and inertia with examples.
Answer: The mass of an object is a measure of its inertia. Larger the mass, greater is its inertia.
For example, a train requires a much larger force to start or stop compared to a bicycle. Similarly, it is easier to push an empty box than a full one because the full box has more mass and hence more inertia.
6. Define momentum and derive its mathematical expression and SI unit.
Answer: Definition: Momentum is the product of the mass and velocity of an object. It is a vector quantity and has the same direction as velocity.
Mathematically,
p = mv
where,
p = momentum, m = mass, v = velocity.
SI Unit: kg·m/s.
For example, a fast-moving heavy truck has large momentum and causes more damage in a collision than a slow-moving light vehicle.
7. State and explain Newton’s Second Law of Motion.
Answer: Newton’s Second Law states that the rate of change of momentum of an object is proportional to the applied unbalanced force and takes place in the direction of the force.
If a constant force acts on an object of mass m, changing its velocity from u to v in time t, then
This means the applied force depends on both the mass of the object and its acceleration.
Example: A cricket player moves his hands backward while catching a fast ball to increase the time and reduce the impact force.
8. Derive the mathematical formula for Newton’s Second Law of Motion.
Answer: Let mass of an object = m,
initial velocity = u, final velocity = v, time = t.
Initial momentum = mu, Final momentum = mv.
Change in momentum = mv – mu.
Rate of change of momentum = \(\frac{m(v-u)}{t}\)
According to the law,
or F = ma.
This is the mathematical form of Newton’s Second Law. Its SI unit is newton (N).
9. Explain how Newton’s Second Law applies in daily life situations.
Answer:
- While catching a cricket ball, a fielder moves his hands backward to reduce the impact by increasing the time of contact.
- Athletes fall on a cushioned or sand bed during high jumps to increase the stopping time and reduce force.
- In car collisions, airbags inflate to increase the time of impact, reducing injury.
These examples show how increasing the time of momentum change reduces force.
10. Explain Newton’s Third Law of Motion with examples.
Answer:
Newton’s Third Law states: For every action, there is an equal and opposite reaction acting on different bodies.
This means that forces always occur in pairs — when one body exerts a force on another, the second body exerts an equal and opposite force back.
Examples:
- When a gun is fired, the bullet moves forward and the gun recoils backward.
- When we walk, our foot pushes the ground backward, and the ground pushes us forward.
- A swimmer pushes water backward, and water pushes the swimmer forward.
11. Explain the concept of action and reaction forces using the example of two spring balances.
Answer: When two spring balances are connected face to face and one is pulled, both show equal readings. This shows that the force exerted by one balance on the other is equal and opposite to the force exerted by the second on the first.
These equal and opposite forces are called action and reaction. They act on different bodies but are equal in magnitude and opposite in direction.
12. What is meant by the recoil of a gun? Explain using the law of conservation of momentum.
Answer:
When a bullet is fired, the gun exerts a forward force on the bullet. The bullet exerts an equal and opposite force on the gun, causing it to move backward.
Before firing, total momentum = 0.
After firing, momentum of bullet (forward) + momentum of gun (backward) = 0.
Thus, momentum is conserved, and the gun recoils backward with smaller velocity because it has greater mass.
13. State the law of conservation of momentum and prove it mathematically.
Answer: The law of conservation of momentum states that when no external force acts on a system of interacting bodies, the total momentum remains constant.
14. Why do passengers fall forward when a moving bus suddenly stops?
Answer: When a moving bus stops suddenly, the lower part of the passenger’s body in contact with the seat comes to rest, but the upper body tends to continue in motion due to inertia of motion. Hence, passengers fall forward.
15. What have you learned from this chapter “Force and Laws of Motion”?
Answer:
From this chapter we learn:
- Force is a push or pull that changes motion.
- Balanced forces do not cause motion; unbalanced forces do.
- Newton’s First Law defines inertia.
- Newton’s Second Law gives the relation F=ma.
- Newton’s Third Law explains action and reaction forces.
- Momentum depends on mass and velocity and is conserved in collisions.
These laws form the foundation of classical mechanics and help explain almost all types of motion around us.
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