# Science textbook solutions for Class 7 science Chapter 7 Motion, Force and Work

**Important points to remember**:

- The minimum distance traversed in a particular direction along a straight line is called displacement.
- The length of the route actually traversed by a moving body, irrespective of the direction, is called distance. Distance is a scalar quantity.
- Velocity is the distance traversed by a body in a specific direction in unit time.
- The unit of speed or velocity is written as metres/second (m/s).
- The velocity at a particular moment of time is called instantaneous velocity.
- The instantaneous velocity can be different at different times.
- The interaction that brings about the acceleration is called force.
- Force acts on a body. The scientist Sir Isaac Newton was the first to study force and the resulting acceleration.

**Question 1:**

**Fill 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**.

c) **Speed** is a scalar quantity.

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

**Question 2: Observe 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?

**Answer** : Actual distance travelled by Sachin and Sameer from A to E = AB + BC + CD + DE = 3 + 4 + 5 + 3 = 15 km

Displacement from A to E = AB + BD + DE = 3 + 3 + 3 = 9 km

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

**Answer** :

1. Work- Joule erg

2. Force – Newton – dyne

3. Displacement – Metre – cm

**Question 4: A 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.**

**Answer** : The total distance travelled by the bird during its flight = 2××(Distance between the point where the bird was sitting and the point from where it takes a turn)

The eventual displacement of the bird is zero as it returns to its initial point i.e. where it was sitting.

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

**Answer** :

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

2. **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.

3. **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.

4. **Acceleration** (a) is the change in velocity (Δv) over the change in time (Δt), represented by the equation a = Δv/Δt. This allows you to measure how fast velocity changes in meters per second squared (m/s^2). Acceleration is also a vector quantity, so it includes both magnitude and direction.

5. **Work** : In physics, work is defined as a force causing the movement—or displacement—of an object. In the case of a constant force, work is the scalar product of the force acting on an object and the displacement caused by that force.

6. **Force** : A force is said to do work if, when acting, there is a displacement of the point of application in the direction of the force.

**Question 6: A 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?**

**Answer** : The acceleration of the ball between A to B is zero as the speed and direction of the ball is constant. After point B, a force is applied. Thus, the ball will get accelerated.

Acceleration of the ball from B to C = Change in velocity from B to C / Time taken for this change

Acceleration of the ball from B to C = 4−22=1 m/s2

**Question 7: Solve 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?**

**Answer** : Work done by the force to stop the car=F×S=1000×10=10000 J

**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?**

**Answer** : Work done by the force =F×S=2×50=100 J

**# Find out**

**1. What is meant by speed ? **

**Answer** : Speed is the distance traveled per unit of time. It is how fast an object is moving. Speed is the scalar quantity that is the magnitude of the velocity vector. It doesn’t have a direction. Higher speed means an object is moving faster

**2. What is the formula for calculating speed ?**

**Answer** : To solve for speed or rate use the formula for speed, s = d/t which means speed equals distance divided by time. To solve for time use the formula for time, t = d/s which means time equals distance divided by speed.