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Summary of the Chapter-8-Force and Laws of Motion with Key-Points:
- First Law of Motion: An object will remain at rest or continue to move in a straight line at constant speed unless acted upon by an external unbalanced force.
- The resistance of objects to any change in their state of motion or rest is referred to as inertia.
- The amount of inertia an object possesses is directly related to its mass, which is measured in kilograms (kg), the SI unit.
- Friction is a force that always acts in opposition to the motion of an object.
- Second Law of Motion: The rate at which an object’s momentum changes is directly proportional to the unbalanced force applied in the direction of the force.
- The SI unit of force is the newton (N), which is equivalent to kg·m/s².
- A force of one newton imparts an acceleration of 1 m/s² to an object with a mass of 1 kg.
- Momentum is calculated by multiplying the object’s mass by its velocity, and it shares the same direction as the velocity. Its SI unit is kg·m/s.
- Third Law of Motion: For every action, there is an equal and opposite reaction, and these forces act on two different objects.
1. Which of the following quantities is required to change the state of motion of an object?
(a) Inertia
(b) Momentum
(c) Force
(d) Speed
Explanation: To change the state of motion of an object, an external force must be applied, as per Newton’s First Law of Motion.
2. Which of the following objects has the highest inertia?
(a) A small rubber ball
(b) A bicycle
(c) A train
(d) A five-rupee coin
Explanation: Inertia is the resistance of an object to change its state of motion. Heavier objects, like a train, have more inertia than lighter objects.
3. If no external force is acting on an object, which of the following will happen?
(a) It will stop moving
(b) It will accelerate
(c) It will continue moving with the same velocity
(d) It will move in a circular path
Explanation: According to Newton's First Law of Motion, an object will maintain its state of motion unless acted upon by an external force.
4. What causes a moving bicycle to slow down when the rider stops pedaling?
(a) Balanced forces
(b) Inertia
(c) Frictional force
(d) Gravitational force
Explanation: Friction between the bicycle's tires and the road opposes the motion, causing it to slow down.
5. Which of the following statements describes Newton's First Law of Motion?
(a) For every action, there is an equal and opposite reaction
(b) The rate of change of momentum is proportional to the applied force
(c) An object at rest stays at rest, and an object in motion stays in motion unless acted on by an unbalanced force
(d) Force is equal to mass times acceleration
Explanation: This is a direct statement of Newton’s First Law of Motion, also known as the Law of Inertia.
6. The mass of an object is a measure of its:
(a) Weight
(b) Momentum
(c) Inertia
(d) Velocity
Explanation: Inertia is the property of an object that resists changes to its state of motion. The more massive an object, the more inertia it has.
7. A person in a moving bus falls forward when the bus suddenly stops. This is due to:
(a) Inertia
(b) Force applied by the bus
(c) Gravity
(d) Balanced forces
Explanation: The person’s body tends to maintain its state of motion due to inertia, causing them to lurch forward when the bus stops.
8. What happens to the velocity of a marble rolling up an inclined plane?
(a) It remains constant
(b) It increases
(c) It decreases
(d) It depends on the angle of inclination
Explanation: As the marble rolls up an inclined plane, it decelerates due to the opposing force of gravity.
9. A rubber ball and a stone of the same size are dropped from the same height. Which will hit the ground first?
(a) The rubber ball
(b) The stone
(c) Both will hit at the same time
(d) It depends on their masses
Explanation: In the absence of air resistance, objects fall at the same rate regardless of their mass, according to Galileo's observations and Newton’s laws of motion.
10. According to Newton's Second Law of Motion, force is the product of:
(a) Mass and acceleration
(b) Mass and inertia
(c) Momentum and velocity
(d) Inertia and velocity
Explanation: Newton's Second Law of Motion states that force equals mass times acceleration (F = ma).
11. When a force is applied on a stationary object and it starts moving, which of the following changes?
(a) Its direction
(b) Its velocity
(c) Its shape
(d) Its size
Explanation: The application of force on a stationary object causes it to move, which means its velocity changes from zero to some value.
12. What is the SI unit of force?
(a) Joule
(b) Newton
(c) Watt
(d) Kilogram
Explanation: The SI unit of force is the Newton (N), which is equivalent to kg m/s².
13. What does Newton's Second Law of Motion describe?
(a) The relationship between action and reaction
(b) The force required to keep an object at rest
(c) The relationship between force, mass, and acceleration
(d) The nature of friction
Explanation: Newton’s Second Law states that force is the product of mass and acceleration (F = ma).
14. Which of the following quantities can be changed by applying force?
(a) Mass
(b) Time
(c) Velocity
(d) Distance
Explanation: Force can change the velocity of an object, leading to acceleration or deceleration.
15. An object of mass 2 kg is moving with a velocity of 3 m/s. What is its momentum?
(a) 3 kg m/s
(b) 6 kg m/s
(c) 12 kg m/s
(d) 5 kg m/s
Explanation: Momentum is calculated by multiplying mass and velocity:p = mv = 2 * 3 = 6kg m/s.
16. According to Newton’s Third Law of Motion, what happens when you push against a wall?
(a) The wall exerts no force
(b) The wall pushes you back with less force
(c) The wall pushes you back with equal force
(d) The wall pushes you back with more force
Explanation: Newton’s Third Law states that for every action, there is an equal and opposite reaction.
17. Which of the following statements is correct regarding balanced forces?
(a) They change the velocity of an object
(b) They change the shape of an object
(c) They cause an object to accelerate
(d) They do not change the state of motion of an object
Explanation: Balanced forces act on an object without changing its state of motion (i.e., they don’t cause acceleration).
18. A car moves with constant velocity. What can be inferred about the forces acting on it?
(a) The forces are balanced
(b) There is no friction
(c) The forces are unbalanced
(d) The car is accelerating
Explanation: When a car moves with constant velocity, the forces acting on it, including friction and applied force, are balanced.
19. What type of force opposes the relative motion between two surfaces in contact?
(a) Gravitational force
(b) Tension
(c) Friction
(d) Normal force
Explanation: Friction is the force that resists the motion of objects or surfaces sliding against each other.
20. What happens to the momentum of an object when its velocity doubles?
(a) The momentum remains the same
(b) The momentum halves
(c) The momentum doubles
(d) The momentum becomes zero
Explanation: Momentum is directly proportional to velocity. If velocity doubles, momentum also doubles.
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