# CBSE 9th Physics Tutorials | Motion-Very Short Answer Type Questions

## Motion-Very Short Answer Type Questions

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

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.

m/s2.

Zero.

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/s2 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.

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.

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.

# CBSE Physics Tutorials | Motion: Speed, Velocity and Acceleration

## Motion: Speed, Velocity and Acceleration

### Speed: –

Speed of a body is defined as the distance travelled by it per unit time.

i.e hence S.I Unit of speed is metre /sec. (m /s or ms – 1)

### Uniform speed:

If a moving body covers equal distances in equal interval of time, then the speed of the body said to be uniform speed

Example:

Suppose a car travels 5 m in every second throughout its Journey then the speed of this car is uniform.

### Non- Uniform Speed: –

If a moving body covers unequal distances in equal interval of time, then the speed of the body is said to be non uniform speed.

Example:

Suppose a car travels 5 m in first second, 10 m in second second, 7 m in third second, 9 m in fourth second, and so on. Then the speed of this car is changing with time and is said to be non- uniform.

### Average Speed: –

Average speed of a moving body is defined as the ratio of the total distance travelled by it to the total time taken by it to cover the distance.

Thus Vav

Example: –

When a bus starts its journey from a bus stand, it moves slowly and then its speed is increased Now if it enters a crowded road its speed decreases.

After crossing the crowded road, its speed increases. After traveling few kilometers, suppose the bus has to be stopped, then its speed again decreases. Thus we see that the bus is not moving with the same speed during the whole journey.

Now if we added total distance covered by the bus with different speeds and divide it by the total time taken, then we get the average speed of the bus.

### Velocity: –

Velocity of an object is defined as the displacement of the body per unit time

OR,

Velocity of an object is defined as the distance travelled by it per unit time in a particular direction.

i.e.

Hence S.I. unit of velocity is metre /sec (m/s)

### Uniform velocity: –

If a moving body covers equal displacement (i.e. equal distance in a particular direction) in equal intervals of time, then the velocity of the body is said to be uniform velocity.

Example: –

Suppose a car travels 5 m in every second in a particular direction then the velocity of car is uniform.

### Non- uniform velocity: –

If a moving body makes unequal displacement in equal interval of time, then the velocity of the body is said to be non-uniform velocity.

Example: –

Let a car moves over a horizontal road along the east direction. Let its cover unequal distances in each hour, then the velocity of this car is non uniform.

### Average Velocity: –

when the velocity of an object changes with time, then the average velocity is defined as the ratio of the total displacement made by it to the total time taken by it

Note:

If the velocity of an object changes at uniform rate in a particular direction, then the average velocity of the object is equal to the mean of the initial and final velocities of the object.

i.e.

where u = initial velocity of the object

v = final velocity of the object

Note: –

Velocity of a body can be changed in two ways.

(i) By changing the speed of the body while keeping the direction of motion of the body same.

(ii) By changing the direction of motion of the body, but keeping the speed of the body same.

e.g. The velocity of a ball dropped from the top of a building changes, because its speed changes although its direction of motion remains the same.

Also, the velocity of a car, moving in a circular path changes, because its direction of motion changes continuously although it is moving with a constant speed.

### Difference between speed and velocity

 Speed Velocity 1. Speed of a body is defined as the distance travelled by it per unit time i.e. 2. Direction is not associated with speed. 3. Speed is a scalar quantity. 4. Speed is always positive. 5. Its value can not be zero for a moving body. 1. Velocity of a body is defined as the displacement of the body per unit time i.e. . 2. Direction is associated with velocity 3. Velocity is a vector quantity 4. Velocity can be positive or negative 5. Its value is zero if initial and final position of the body is same.

### Acceleration: –

In non-uniform motion, the velocity of a body changes with time. The change in velocity of an object with time is expressed by a physical quantity known as acceleration.

Acceleration tells us, by how much amount the velocity of a body changes in unit interval of time.

In other words, the acceleration is a quantity which measures how quickly the velocity of a body or an object changes

Acceleration of a body is defined as the change in velocity per unit time

i.e.

acceleration

Acceleration is a vector quantity and its unit is m/s/s or m/s2 or ms – 2

### Uniform Acceleration:

If the velocity of an object changes by an equal amount in equal interval of time, then the acceleration of the object is known as uniform acceleration.

e.g. acceleration of freely falling body

Acceleration of a ball rolling down on a smooth inclined plane.

### Non- uniform acceleration:

If the velocity of an object changes by an unequal amount in equal interval of time, then the acceleration of the object is known as non-uniform acceleration.

e.g. acceleration of a bus on a crowed road

### Uniform circular motion:-

The circular motion of a body having constant or uniform speed is known as uniform circular motion.

### How uniform circular motion is an accelerated motion?

When a body moves in a circular path with constant speed, its direction of motion changes continuously.

It means that the body moving in a circular path, changes its velocity. And we know that the change of velocity of a body with time gives rise to an acceleration.

Thus the uniform circular motion is accelerated motion.