# CBSE 9 Physics Motion Solved Assignment

Question 1. when graph is parallel to time axis, what can you say about the motion?Answer.

The body remains at rest.

Question 2. What is the value of acceleration, if v-t graph is a straight line parallel to the time axis?

Zero, since slope is zero.

Question 3. Under what condition, the distance covered by a particle is equal to the magnitude of displacement of particle?

This happens when the body moves with a constant velocity.

Question 4. Is it possible that the average velocity of a particle is zero when its average speed is not zero?

This occurs when the particle returns to the starting point.

Question 5. What is meant by a point object? Give some examples.

Whenever the size of the object is much smaller than the distance it moves in a given time interval, the size of the object can be neglected. Then the object may be regarded as the point object. Examples: (i) A car covering a distance of 10 km can be treated as a point object, (ii) Earth can be regarded as a point object for studying its motion around the sun.

Question 6. Can an object be at rest as well as in motion at the same time?

Yes. An object may be at rest relative to one object and at the same time it may be in motion relative to another object.

Question 7. Can the average speed of a moving body ever be zero?

No, speed, being a scalar quantity, is always positive. So its average speed can never be zero.

Question 8. A body is moving with a velocity of 10 m/s. If the motion is uniform, what will be the velocity after10 s?

As the motion is uniform, the velocity remains equal to 10 m/s even after 10 s.

Question 9. Define velocity. What is its SI unit?

The distance traveled by a body per unit time in given direction is called its velocity. The SI unit of velocity is m/s.

Question 10. Can the speed of a body vary if its velocity is constant?

No. When the velocity of a body is constant, its speed is also constant.

Question 11. Can a body have constant speed but variable velocity?

Yes. For example, a body in uniform circular motion has constant speed but its velocity changes at every point due to the change in the. direction of motion.

Question 12. What is the SI unit of retardation?

m/s^2.

Question 13. What is acceleration of a body moving with uniform velocity?

Zero.

Question 14. If the acceleration of the particle is constant in magnitude but not in direction, what type of path does the body follow?

Circular path.

Question 15. A ball is thrown up in air. What is the acceleration and velocity at the instant it reaches its highest point?

Acceleration is 9.8 m/s^2 acting downwards and velocity is zero.

Question 16. A car travels 1 km and returns back to the same point in a different path. What is the average velocity.

Zero. Since total displacement is zero.

Question 17. Give an example of a motion in which the acceleration is in the direction of motion.

When a body falls from the roof of a house, then the acceleration of the body is in the direction of motion.

Question 18. Give an example of a motion in which the acceleration is against the direction of motion.

When a body is thrown upward, then the acceleration is against the direction of motion.

Question 19. What does the slope of a speed-time graph indicate?

Acceleration

Question 20. What do we get from the area between the speed- time graph of a body and the time-axis?

Distance traveled by the body.

Question 21. If the velocity-time graph of a body is a line parallel to the time-axis, what would be the acceleration of the body?

Zero, because the body possesses uniform velocity.

Question 22. A satellite goes round the earth in a circular orbit with constant speed. Is the motion accelerated?

Yes.

Question 23. If the displacement of a body is zero, is the distance covered by it necessarily zero?

No, it is not necessary that the distance covered by a body is zero when its displacement is zero.

Question 24. Can the displacement be greater than the distance traveled by an object?

No, the displacement of an object can be either equal to or less than the distance traveled by the objects.

Question 25. Under what condition will the distance and displacement of a moving object have the same magnitude?

When the object moves along the same straight line in the same fixed direction.

Q. What do you mean by motion and rest?

A body is said to be in motion if its position changes in time with respect to a fix point.

A body is said to be in rest if its position does not change i time with respect to a fix point. Therefore, to describe the position of an object we need to specify a reference point called the origin.

Q. How can you say that motion and rest are relative?

An object may be in motion for one person and in rest for some other. So, motion and rest are relative. For example, a passenger sitting in a moving bus is in rest with respect to other passengers sitting in bus. Same time he is in motion with respect to outside surrounding.

Q. What are distance and displacement?
Ans:

The length of the path covered by a moving body is called distance.

The shortest distance measured from the initial to the final position of an object is known as the displacement.

Automobiles are fitted with a device that shows the distance travelled. Such device is known as an odometer.

The displacement during motion may be zero but the distance cannot be zero.

If the final position coincides with the initial position the displacement will be zero.

The magnitude of the displacement may be equal or less than that of distance travelled by an object.
Distance is always positive whereas displacement may be positive as well as negative and even zero.

Q. What do you mean by magnitude?
Ans: The numerical value of a physical quantity is called its magnitude.

Q. What do you mean by Scalar and Vector Quantity?

The physical quantities which have both direction and magnitude are known as a vector Quantity.

E.g. Displacement, velocity, acceleration, Force, momentum, weight and electric field etc.

The physical quantities which have only magnitude are known scalar quantity or a scalar.

E.g. Charge, distance, area, speed, time temperature, density, volume, work, power, energy, pressure, potential etc.

Q. An object has moved through a distance. Can it have zero displacement? If yes, support your answer with an example.
Ans:

Yes, An object can have displacement zero if the final position coincides with the initial position .For example during circular motion on completing one round displacement will be zero.

Q.Which of the following is true for displacement?

(a) It cannot be zero.

(b) Its magnitude is greater than the distance travelled by the object.

Ans: Both the statements are not true for displacement.

Q. A runner running along a circle runs the circle completely. What is his displacement? What distance has he run?

Displacement = zero and Distance = 2πr

Q. When a body is said to be travel with uniform motion or non-uniform motion?
Ans:

If an object covers equal distance in equal interval of time, it is said to be in uniform motion where as if an objects cover unequal distance in taken to cover that distance.
Average speed = total time taken / total distance travelled

Q. What do you mean by velocity and Average velocity?

Velocity is rate of the change of position of an object in specific direction. It is a vector quantity.
Velocity = displacement / time

Average Velocity: If an object moves with variable velocity we need to find average Velocity.

Case: 01- It is defined as the ratio of its total displacement to the total time interval.
Average velocity = Total displacement/Total time
Case: 02 – If initial velocity of body is u and final velocity is v then the arithmetic means of velocity is called average velocity.

Average velocity =(u+v)/2

Note:

(a) The velocity is positive if the object is moving towards the right of the origin and negative if the object is moving towards the left of the origin.

(b) In uniform motion, the instantaneous velocity is equal to the average velocity at all time because velocity remains constant at each instant, at each point of the path.

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