Maharashtra Board Class 7 Science Chapter 7 Motion Force and Work PDF Download

7. Motion, Force And Work

7.1 Work

What is meant by motion?

What causes a change in motion?

We have seen that a change occurs in the motion or the shape of a body when a force acts on it. Now let us see how work is done when a force acts.

Distance And Displacement

Ranjit's house is at place A. The figure alongside shows the distance traversed by Ranjit to reach his school at D. If we do not take direction into account, we find that Ranjit traversed a distance equal to AB+BC+CD. However, on doing this, his displacement was equal to AD. Ranjit's displacement from his house to the school is shown in the picture with the broken line AD. AD is the minimum distance along a straight line from Ranjit's house to the school.

The minimum distance traversed in a particular direction along a straight line is called displacement.

Distance: The length of the route actually traversed by a moving body, irrespective of the direction, is called distance. Distance is a scalar quantity.

Displacement: The minimum distance traversed by a moving body in one direction from the original point to reach the final point, is called displacement.

In displacement, both distance and direction are taken into account. Therefore, displacement is a vector quantity.

The unit of measurement of distance and displacement is the metre, in the SI as well as in the MKS system of measurement.

Velocity: Velocity is the distance traversed by a body in a specific direction in unit time. The velocity of a body can be calculated by the following formula:

Velocity = $\frac{\text{Displacement}}{\text{Period of time required for the displacement}}$

Teacher's Note

Distance is like the total path you walk from home to school. Displacement is the shortest straight line from home to school. Like going from Mumbai to Delhi - the flight distance is less than the road distance.

Exam Trick

Remember: Distance = total path. Displacement = straight line. A ball thrown up and caught = distance is more, but displacement is zero!

Points To Remember

Distance is the total path travelled by a body.
Displacement is the straight line distance from start to end point.
Distance is a scalar quantity (no direction needed).
Displacement is a vector quantity (direction is needed).
Both are measured in metres.

Speed And Velocity

1. What is meant by speed?

2. What is the formula for calculating speed?

When we say that the speed of a car is 40 km per hour, there is no need to specify the direction, but, to predict whether a storm will reach a particular place or not, a mention of its direction is essential.

Teacher's Note

Speed tells you how fast something moves, like saying a car goes 60 km/hour. Velocity tells you speed plus direction, like saying a car goes 60 km/hour towards Delhi.

Exam Trick

Speed = distance divided by time. Velocity = displacement divided by time. Remember: Velocity is faster than speed? No! But velocity needs direction!

Points To Remember

Speed is distance divided by time.
Velocity is displacement divided by time.
Speed does not need direction.
Velocity needs direction to be complete.
Both are measured in metres per second (m/s).

Let's find the unit.

The unit of speed or velocity is written as metres/second (m/s)

Let us now use the above formulae to find out Ranjit's velocity and speed when he goes to school as shown in the figure 7.2.

The actual distance traversed by Ranjit from home to school

= AB + BC + CD

= 500 m + 700 m + 300 m = 1500 m.

Total time from home to school = 8 minutes + 11 minutes + 6 minutes = 25 minutes

Ranjit's displacement from home to school, AD = 1000 metres

Thus, Ranjit's velocity when going from home to school

Velocity = $\frac{\text{Displacement}}{\text{Total time}}$ = $\frac{\text{1000 metres}}{\text{25 minutes}}$ = $\frac{\text{40 metres}}{\text{60 seconds}}$ = 0.66 metres/second

Ranjit's speed while going to school

Speed = $\frac{\text{Distance traversed}}{\text{Total time}}$ = $\frac{\text{1500 metres}}{\text{25 minute}}$ = $\frac{\text{60 metres}}{\text{60 seconds}}$ = 1 metre/second

Ranjit did not take the straight route of minimum distance while going to the school. Therefore, the magnitudes of his velocity and speed came out to be different. Had Ranjit actually gone by the straight route AD, then the magnitude of his velocity and speed would have been the same.

Average Velocity And Instantaneous Velocity

The velocity of an object can change even while it is moving along a straight line. Suppose that a truck is covering a distance of 40 km from A to D in a straight line. That is, its displacement will be 40 km.

If it requires altogether 1 hour for this, its average velocity is 40 km/hour. However, if the truck traverses the 10 km distance AB in 10 minutes, BC in 20 minutes and CD in 30 minutes, then

Velocity for the distance AB in km/hr = $\frac{\text{10 km}}{\text{10 minutes}}$ = $\frac{\text{60 km}}{\text{60 minutes}}$ = 60 km /hr

Now deduce the velocities for the distances BC and CD. We see that the velocity of the truck is different in the different segments of the road AB, BC and CD. However, the average velocity for the entire route AD is 40 km/hour. The velocity at a particular moment of time is called instantaneous velocity. The instantaneous velocity can be different at different times.

Teacher's Note

Average velocity is like the average speed of a long car journey. Instantaneous velocity is like what the speedometer shows right now. In a traffic jam, average speed might be 20 km/hour but right now the car is stopped!

Exam Trick

Average velocity = total displacement by total time. Instantaneous velocity = speed at that exact moment. A car's speedometer shows instantaneous velocity!

Points To Remember

Average velocity is the total displacement divided by total time.
Instantaneous velocity is the velocity at one exact moment.
Average velocity can be the same even if instantaneous velocity keeps changing.
Speedometer in a car shows instantaneous velocity.
Both are measured in metres per second.

Acceleration

In the previous example, the truck covered the distance AB at the velocity of 60 km/hour, BC at 30 km/hour and CD at 40 km/hour. It means that the velocity for the distance CD is greater than the velocity for the distance BC. From the number of seconds required for this change in velocity to take place, the change in velocity per second can be deduced. This is called acceleration. What is the cause of acceleration?

You know that the truck driver increases or decreases the velocity of the truck by means of the accelerator. You might have seen a toy car that runs on a clockwork spring. When it is released on a flat floor, after winding up the spring, it goes in a straight line. However, when it is hit on one side, it changes direction and keeps going. If it collides into a wall, it stops. It means that its velocity changes. How did this change take place? It happens because the car comes into contact with something external to it. On a football ground, how does the direction of the ball moving in a straight line, change? We see some player changing its direction by kicking it. When its direction changes, the velocity of the ball changes, that is to say, an acceleration takes place. The interaction that brings about the acceleration is called force. Force acts on a body.

Acceleration = $\frac{\text{Change in velocity}}{\text{Time taken for change}}$

Use your brain power! The unit of acceleration is m/s². Verify this.

Force And Acceleration

Try this. Take a glass marble and let it roll on a big smooth tabletop. After some time its velocity will decrease and it will stop. On a carrom board, too, the carrom coin, pushed by a striker will move forward some distance, and then come to a stop. If the coin is pushed after applying talcum powder to the carrom board, it will keep moving for a longer time and then come to a stop.

What can we infer from this?

The velocity of the coin decreases due to the force of friction, and the coin stops. If the friction between the carrom board and the coin is reduced, the coin keeps moving for a longer time. It would mean that, if no force of friction is acting on a moving body, it will keep on moving with a constant velocity.

The scientist Sir Isaac Newton was the first to study force and the resulting acceleration.

Newton's First Law Of Motion: If no force is acting on a body, its velocity does not change, i.e. the body does not accelerate. In other words, if a body is stationary, it will remain stationary. If it is in motion, it

Teacher's Note

Acceleration happens when a vehicle speeds up, slows down, or changes direction. When you sit in a bus and it suddenly brakes, you feel a jerk. That jerk is acceleration!

Exam Trick

Acceleration = change in velocity divided by time. If velocity changes from 0 to 10 m/s in 2 seconds, acceleration is 5 m/s². Remember the formula and you can solve any problem!

Points To Remember

Acceleration is the change in velocity per unit time.
When a vehicle speeds up or slows down, acceleration happens.
Acceleration can also happen when direction changes.
The unit of acceleration is metres per second square (m/s²).
Force causes acceleration in a body.

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