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Solution of Force and Law of Motion (Page No - 57) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 51:
A plastic ball and a clay ball of equal masses, travelling in the same direction with equal speeds, strike against a vertical wall. From which ball does the wall receive a greater amount of momentum ?
Solution :
The wall will receive equal momentum from both the balls because both balls have equal mass and velocity.

Question 52:
A moving bicycle comes to rest after sometime if we stop pedalling it. But Newton’s first law of motion says that a moving body should continue to move for ever, unless some external force acts on it. How do you explain the bicycle case ?
Solution :
In this case, the bicycle has been compelled to change its state of motion by the external force of air resistance and friction. If there were no air resistance and no friction to oppose the motion of the bicycle, then according to the first law of motion, the bicycle would go on moving forever.

Question 53:
A man throws a ball weighing 500 g vertically upwards with a speed of 10 m/s.

  1. What will be its initial momentum ?
  2. What would be its momentum at the highest point of its flight ?

Solution :
Mass of ball = 500 g = 0.5 kg
Initial velocity = 10 m/s

  1. Initial momentum = 0.5 x 10 = 5 kg.m/s
  2. Velocity at the highest point = 0 m/s

Momentum at the highest point = 0.5 x 0 = 0 kg.m/s

Question 54:
A car is moving on a level road. If the driver turns off the engine of the car, the car’s speed decreases gradually and ultimately it comes to a stop. A student says that two forces act on the car which bring it to a stop. What could these forces be ? Which of these two forces contributes more to slow down and stop the car ?
Solution :
The two forces acting on the car are force of friction and air resistance. Force of friction contributes more to slow down and stop the car.

Question 55:
There are two types of forces X and Y. The forces belonging to type X can produce motion in a stationary object but cannot change the shape of the object. On the other hand, forces belonging to type Y cannot produce motion in a stationary object but can change the shape of the object. What is the general name of the forces such as (a) X, and (b) Y ?
Solution :
a) X are unbalanced force.
b) Y are balanced forces.

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FAQs on Solution of Force and Law of Motion (Page No - 57) - Physics by Lakhmir Singh, Class 9 - Extra Documents & Tests for Class 9

1. What is the definition of force according to Newton's laws of motion?
Ans. According to Newton's laws of motion, force is defined as any push or pull on an object that can cause it to accelerate or change its state of motion.
2. What are the three laws of motion formulated by Sir Isaac Newton?
Ans. Sir Isaac Newton formulated three laws of motion: - The first law, also known as the law of inertia, states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an external force. - The second law states that the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. - The third law states that for every action, there is an equal and opposite reaction.
3. How does Newton's second law of motion relate force, mass, and acceleration?
Ans. According to Newton's second law of motion, the force acting on an object is directly proportional to its mass and acceleration. Mathematically, it can be expressed as force = mass × acceleration (F = ma). This means that the greater the force applied to an object, the more it will accelerate, and the greater the mass of an object, the more force is required to accelerate it.
4. Can an object be at rest if a force is acting on it?
Ans. No, according to Newton's first law of motion, if a force is acting on an object, it will either start moving or change its state of motion. An object can only remain at rest if the net force acting on it is zero.
5. How are the concepts of force and motion related?
Ans. Force and motion are closely related concepts. Force is required to cause motion or change the state of motion of an object. Without any force acting on an object, it will remain at rest or continue moving with a constant speed and direction. The magnitude and direction of the force determine the resulting motion of an object, as described by Newton's laws of motion.
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