Maharashtra Board Class 9 Science Chapter 1 Laws of Motion PDF Download

1. Laws of Motion

In this chapter, we will learn about:

Motion

Distance and displacement

Acceleration

Newton's laws of motion and related equations

Motion Of An Object

In which of the following examples can you sense motion?

How will you explain presence and absence of motion?

The flight of a bird

A stationary train

Leaves flying through air

A stone lying on a hill

We see the motion of several objects every day. Sometimes we cannot see the motion of an object directly, as in the case of a breeze. Can you list other examples of motion, besides those given here?

Teacher's Note

Motion is when something changes position. For example, a bus moving on the road shows motion, but a parked bus does not move.

Exam Trick

Remember: Motion means the position of an object is changing. If position stays the same, it is at rest.

Points to Remember

Motion happens when position changes with respect to surroundings.

An object at rest does not change its position.

Motion is a relative concept - it depends on what we compare it to.

You have learnt that motion is a relative concept. If the position of an object is changing with respect to its surroundings, then it is said to be in motion. Otherwise, it is said to be at rest.

Think About It

You are travelling in a bus. Is the person sitting next to you in motion?

What do you take into consideration to decide if an object is moving or not?

Think About It

Sheetal first went to her friend Sangeeta's house on her way to school. Prashant went straight from home to school. Both are walking with the same speed. Who will take less time to reach the school and why?

In the above example, is there a difference between the actual distance and the distance travelled? What is it?

Displacement And Distance

Let's Try This

Measure the distance between points A and B in different ways as shown in the figure.

Now measure the distance along the dotted line. Which distance is correct according to you and why?

Distance is the length of the actual path travelled by an object in motion while going from one point to another, whereas displacement is the minimum distance between the starting and finishing points.

Teacher's Note

Distance is how far you walk. Displacement is how far you are from your starting point. If you go to a shop and come back home, your distance is long, but your displacement is zero.

Exam Trick

Remember: Distance = path taken. Displacement = straight line from start to end. Distance is always more than or equal to displacement.

Points to Remember

Distance is the actual path length travelled.

Displacement is the shortest distance between start and end points.

Distance can never be less than displacement.

Displacement can be zero even when distance is not zero.

Use Your Brain Power

Every morning, Swaralee walks round the edge of a circular field having a radius of 100 m. If she starts from the point A and takes one round, how much distance has she walked and what is her displacement?

If a car, starting from point P, goes to point Q and then returns to point P, how much distance has it travelled and what is its displacement?

Even if the displacement of an object is zero, the actual distance traversed by it may not be zero.

Speed And Velocity

Can You Recall

What are vectors and scalars?

Which of the quantities distance, speed, velocity, time and displacement are scalars and which are vectors?

Speed is Total distance travelled divided by Time required

The distance travelled in a particular direction by an object in unit time is called its velocity. Here, unit time can be one second, one minute, one hour, etc. If large units are used, one year can also be used as a unit of time.

The displacement that occurs in unit time is called velocity.

Velocity is Displacement divided by Time

Always Remember

The units of speed and velocity are the same. In the SI system, the unit is m/s while in the CGS system, it is cm/s.

Speed is related to distance while velocity is related to the displacement.

If the motion is along a straight line, the values of speed and velocity are the same, otherwise they can be different.

Velocity is the displacement that occurs in unit time.

Teacher's Note

Speed tells you how fast you are moving. Velocity tells you how fast and in which direction. A car moving at 60 km/hr has speed. A car moving at 60 km/hr toward Delhi has velocity.

Exam Trick

Remember: Speed has no direction. Velocity has direction. If a question asks for direction, use velocity. If it asks only "how fast," use speed.

Points to Remember

Speed = distance / time

Velocity = displacement / time

Speed is always positive.

Velocity can be positive or negative based on direction.

Speed and velocity are equal only for straight line motion.

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