Std 7 General Science · Chapter 7

7. Motion, Force and Work

Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.

Exercises

1Fill in the blanks with the proper words from the brackets. (stationary, zero, changing, constant, displacement, velocity, speed, acceleration, stationary but not zero, increases)

(a) If a body traverses a distance in direct proportion to the time, the speed of the body is constant.

(b) If a body is moving with a constant velocity its acceleration is zero.

(d) Velocity is the distance traversed by a body in a particular direction in unit time.

2Observe the figure and answer the questions. Sachin and Sameer started on a motorbike from place A, took the turn at B, did a task at C, travelled by the route CD to D and then went on to E. Altogether, they took one hour for this journey. Find out the actual distance traversed by them and the displacement from A to E. From this, deduce their speed. What was their velocity from A to E in the direction AE? Can this velocity be called average velocity?

The route actually travelled (solid) and the displacement from A to E (dashed).

Actual distance = AB + BC + CD + DE = 3 + 4 + 5 + 3 = 15 km
Displacement = AE = AB + BD + DE = 3 + 3 + 3 = 9 km (towards E)
Time = 1 hour
Speed = Distance ÷ Time = 15 km ÷ 1 hr = 15 km/hr
Velocity = Displacement ÷ Time = 9 km ÷ 1 hr = 9 km/hr in the direction AE

Yes, this velocity is the average velocity. Their velocity kept changing during the journey. The velocity of 9 km/hr is found for the whole journey from A to E. So it is the average velocity.

3From the groups B and C, choose the proper words, for each of the words in group A.

A (Quantity)	B (SI unit)	C (CGS unit)
Work	Joule	erg
Force	Newton	dyne
Displacement	Metre	cm

4A bird sitting on a wire, flies, circles around and comes back to its perch. Explain the total distance it traversed during its flight and its eventual displacement.

Distance: Distance is the length of the actual path travelled. The bird flew along a circular path. So the distance travelled is the length of its whole flight path.

Displacement: Displacement is the shortest distance between the starting point and the final point. The bird came back to the same place. So its displacement is zero.

5Explain the following concepts in your own words with everyday examples: force, work, displacement, velocity, acceleration, distance.

Force: A push or a pull is called force. It can change the speed, direction or shape of an object. Example: kicking a football. Unit: newton.
Work: Work is done when a force moves an object in its own direction. W = F × s. Example: lifting a bucket of water. Unit: joule.
Displacement: The shortest distance between the starting point and the final point in a particular direction. Example: school is 1 km east of home.
Velocity: Displacement per unit time in a particular direction. Example: a train moves at 60 km/hr towards Pune. Unit: m/s.
Acceleration: The rate of change of velocity. Example: a bus speeding up after starting. Unit: m/s².
Distance: The length of the actual path travelled. Example: the reading on a bicycle meter. Unit: metre.

6A ball is rolling from A to D on a flat and smooth surface. Its speed is 2 cm/s. On reaching B, it was pushed continuously up to C. On reaching D from C, its speed had become 4 cm/s. It took 2 seconds for it to go from B to C. What is the acceleration of the ball as it goes from B to C?

Initial velocity (at B) = 2 cm/s
Final velocity (at C) = 4 cm/s
Time = 2 s
Acceleration = Change in velocity ÷ Time
= (4 − 2) ÷ 2 = 2 ÷ 2 = 1 cm/s²

The acceleration of the ball from B to C is 1 cm/s².

7Solve the following problems.

(a) A force of 1000 N was applied to stop a car that was moving with a constant velocity. The car stopped after moving through 10 m. How much is the work done?

Force (F) = 1000 N, Displacement (s) = 10 m
Work (W) = F × s = 1000 × 10 = 10,000 J

The work done is 10,000 joule.

(b) A cart with mass 20 kg went 50 m in a straight line on a plain and smooth road when a force of 2 N was applied to it. How much work was done by the force?

Force (F) = 2 N, Displacement (s) = 50 m
Work (W) = F × s = 2 × 50 = 100 J

The work done is 100 joule.

★Project: Collect information about the study made by Sir Isaac Newton regarding force and acceleration and discuss it with your teacher.

Sir Isaac Newton was a great English scientist. He gave three laws of motion. The unit of force, newton, is named after him.

First law: An object remains at rest or keeps moving in a straight line unless a force acts on it.
Second law: The acceleration of an object depends on the force applied and its mass. (Force = mass × acceleration)
Third law: Every action has an equal and opposite reaction.

Newton also explained the force of gravitation.

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