Motion Class 9 Notes Science Chapter 8

Motion Class 9 Notes

Motion Class 9, What is Motion Class 9, Motion Chapter Class 9 Notes pdf, Physics notes for motion 9th

Topics in this chapter 

1. Movement of a body 
2. Distance and Displacement
3. Uniform and Non-uniform motion
4. Speed and velocity
5. Acceleration
6. Uniform circular motion

Movement of a Body

A common characteristic of all the moving bodies is that they change their position with time. 

A body is said to be in motion (or moving) when its position changes continuously with respect to a stationary object taken as a reference point.

For example, when the position of the car changes continuously with respect to stationary objects like houses and trees. We say that the car is moving or that the car is in motion.

Distance 

The distance travelled by a body is the actual length of the path covered by a moving body irrespective of the direction in which the body travels.

For example, in this case, the actual length of the path covered by the man is 5 km + 3 km = 8 km , so the distance travelled by the man is 8 km.

Displacement

When a body moves from one point to another , the distance travelled refers to the actual length of the indirect path whereas displacement refers to the straight line path between the initial and the final positions.

In simple words, when a body moves from one position to another, the shortest (straight line) distance between the initial and final position of the body , along with direction is known as its displacement.

The quantities like distance, displacement etc. are known as physical quantities. The magnitude of a physical quantity means size of the physical quantity. A physical quantity having only magnitude is known as scalar quantity. A scalar quantity has no direction. On the other hand, a physical quantity having magnitude as well as direction is known as vector quantity.

1. Distance is a scalar quantity (only magnitude)
2. Displacement is a vector quantity (have magnitude and direction)

The distance travelled by a moving body can not be zero but the final displacement of a moving body can be zero.

Sample problem : a man travels a distance of 1.5 m toward east , then 2.0 m towards south and finally 4.5 m towards east.

1. What is the total distance travelled? 
2. What is his resultant displacement? 

Solution :  1. Total distance travelled = 1.5 + 2.0 + 4.5 = 8.0m

2. By the help of graph, 

Thus, the final displacement as represented by AD is 6.3m

Uniform and Non-uniform motion

A body has a uniform motion if it travels equal distances in equal intervals of time.

For example, a car running at a constant speed of 10 m per second, will cover equal distances of 10 m, every second, so its motion will be uniform. 

The distance- time graph for uniform motion is a straight line.

A body has a non-uniform motion if it travels unequal distances in equal intervals of time.

For example, a falling body is an example of non-uniform motion.

The distance- time graph for a body having non-uniform is a curved line.

Speed 

The speed of a body gives us an idea of how slow or fast that body is moving.

In other words, the speed of a body is the distance travelled by it per unit time.

If a body travels a distance s in time t, then its speed v is given by : 

For example, a car travels a distance of 100 km in 4 hours, then the speed of this cr is given by  : 

Speed = 100 km / 4 hours

Speed = 25 km per hours (25 km/hr)

The SI unit of speed is metres per second (m/s)

For small values, the SI unit is cm/s.

For large values, the SI unit is km/hr.

Speed has magnitude only, it has no specified direction , therefore speed is scalar quantity .

Average speed

The average speed of a body is the total distance travelled divided by the total time taken to cover the distance.

Average speed = total distance travelled / total time taken

The SI unit of average speed is the same as speed m/s.

Uniform speed

A body has a uniform speed if it travels equal distance in equal intervals of time, no matter how small these time intervals may be.

For example, a car is said to have a uniform speed of say, 60 km per hour, if it travels 30 km every half hour, 15 km every quarter of hour, 1 km every minute and 1/60 km every second. As we have already discussed above, in actual practice the speed of the body rarely remains uniform (or constant) for a long time. 

Question : a scooterist covers a distance of 3 km in 5 min. Calculate his speed in : 

Solution : (1) in order to calculate the speed in cm per second , we should convert the given distance of 3 km into cm and the given time of 5 min into seconds. 

Please note that 1 km is 1000 m and 1 metre is 100 cm.

Now, 

Thus, the speed of a scooterist is 1000 cm per second.

Solution : (2) in order to express the speed in m per second we should convert the given distance of 3 km into m and then the given time of 5 min into seconds . 

Thus in this case,

Distance travelled = 3 km

                              = 3 × 1000 m 

                              = 3000 m 

So the speed of a scooterist is 10 m per second.

Solution : (3) in order to calculate the speed in kilometres per hour, we should express the given distance in km and the given time in hours. 

So in this case :

Thus the speed of a scooterist is 36 km per hour. 

Velocity 

Velocity of a body is the distance travelled by it per unit time in a given direction.

If a body travels a distance s in time t in a given direction, then its velocity v is given by : 

v = s / t

We know that the distance travelled in a given direction is known as displacement.

So in other words, velocity of a body is the displacement produced per unit time. 

Velocity = displacement / time taken 

v = s / t 

The SI unit of velocity is the same as that of speed, metres per second (m/s).

For bigger values, the SI unit is km per hour (km/hr)

For smaller values, the SI unit is cm per second (cm/s)

The difference between speed and velocity is that speed has only magnitude, it has no specific direction, but velocity has magnitude as well as direction. 

The direction of the velocity is the same as the direction of displacement of the body.

Uniform velocity 

A body has a uniform velocity if it travels in a specified direction in a straight line and moves over equal distance in equal intervals of time, no matter how small these time intervals may be.

The velocity of a body can be changed in two ways : 

1. By changing the speed of the body and
2. By keeping the speed constant but by changing the direction.

Acceleration 

Acceleration of a body is defined as the rate of change of its velocity with time 

Acceleration = change in velocity /  time taken for change 

Now, the change in velocity is the difference between the final velocity and the initial velocity. that is 

Change in velocity =  final velocity – initial velocity 

So , acceleration = final velocity – initial velocity / time taken 

  And t = time taken for the change in velocity

 The SI unit of acceleration is metres per second per second or metres per second square (m / s²)

Acceleration is a vector quantity. 

When a body is moving with uniform velocity, its acceleration will be zero.

Uniform acceleration 

A body has a uniform acceleration if it travels in a straight line and its velocity increases by equal amount in equal intervals of time.  

In simple words, a body has a uniform acceleration if its velocity changes at a uniform rate.

The velocity – time graph of a body having uniformly accelerated motion is a straight line.

Non-uniform acceleration 

A body has a non-uniform acceleration if its velocity increases by unequal amounts in equal intervals of time.

In simple words, a body has a non-uniform acceleration if its velocity changes at a non-uniform rate.  

The velocity – time graph for a body having non uniform acceleration is a curved line.

Retardation 

If the velocity of a body increases, the acceleration is positive and if the velocity of a body decreases, the acceleration is negative. 

A body is said to be retarded if its velocity is decreasing.

Retardation is measured in the same way a acceleration, retardation is equal to change in velocity / time taken and has the same unit of metres per second square (m/s²)

Retardation is actually acceleration with negative sign.

Average velocity 

The average velocity is given by the arithmetic mean of the initial and final velocity for a given period of time.

Average velocity = initial velocity + final velocity / 2

Uniform circular motion 

When a body moves in a circle, it is called circular motion.

In other words, motion in a circle is circular motion.

When a body moves along a circular path, then its direction of motion keeps changing continuously.

Since the velocity changes (due to change in direction), the motion along a circular path is said to be accelerated.

When a body moves in a circular path with uniform speed, its motion is called uniform circular motion.

A force needed to make an object travel in a circular path is called centripetal force.

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Examples of Uniform Circular Motion 

• Artificial satellites move in uniform circular motion around the earth. the motion of a satellite around the earth is accelerated.
• The moon moves in uniform circular motion around the earth.
• The earth moves around the sun in uniform circular motion.
• An athlete moving on a circular track with a constant speed exhibits uniform circular motion.
• The tip of a second hand of a watch exhibits uniform circular motion on the circular dial of the watch.

Some Important Questions

Q1. What is motion?

Answer- A body is said to be in motion when its position changes continuously with respect to a stationary object taken as reference point.

For example, we see a car at position A in front of a house and a tree at a particular time. Now, after 5 seconds, we see the car at position B which is quite far away from the house and the tree.

This mean that the position of this car is changing continuously with respect to a stationary object, house or tree. So we say that this car is moving or that this car is in motion.

Q2. What is uniform motion?

Answer- A body has a uniform motion if it travels equal distance in equal intervals of time ,no matter how small these time intervals.

Q3. What is non-uniform motion?

Answer- A body has a non-uniform motion if it travels an unequal distance in equal intervals of time.

Q4.  is speed can be zero?

Answer- We know that speed is a scalar quantity, so it can be either positive or zero.

Q5. Can velocity be negative?

Answer- We know that velocity is a vector quantity so it can be either positive, negative or zero.