Chapter 7 Motion, Force and Work
Textbook Questions and Answers
1. Fill ¡n the blanks with the proper words from the brackets.
(stationary, zero, changing, constant, displacement, velocity, speed. acceleration, stationary but not zero. inc reuses)
Question a.
If a body traverses a distance in direct proportion to the time, the speed of the body is ……………… .
Answer:
constant
Question b.
If a body is moving with a constant velocity, its acceleration is ……………… .
Answer:
zero
Question c.
……………. is a scalar quantity.
Answer:
Speed
Question d.
…………….. is the distance traversed by a body in a particular direction in unit time.
Answer:
Velocity
2. Observe the figure and answer the questions.
Sachin and Sarneer started on a motorbike from place A, took the turn at 13, 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?
Question a.
Observe the figure and answer the questions
Sachin and Sarneer started on a motorbike from place A, took the turn at 13, 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:
1. Actual distance = + + + = 3 + 4 + 5 + 3
Actual distance = 15 km
2. Displacement =
= 3 + 3 + 3
Displacement = 9 km
3.
Distance = 15 km = 15 × 1000 = 15000 m
Time = 1 hr = 1 × 60 × 60 = 3600 sec.
s = or s = = 15km/hour
= 4.16 m/sec. or 15 km/hour
4. Velocity =
Displacement = 9 km = 9 × 1000 = 9000 m
Time = 1 hr = 1 × 60 × 60 = 3600 sec
V = or V = = 9 km/hour
= 2.5 m/sec. or 9 km/hour
5. Yes, this velocity can be called as average velocity.
3. From the groups B and C, choose the proper words, for each of the words in group A.
Question a.
From the groups B and C, choose the proper words, for each of the words in group A.
Answer:
Group ‘A’  Group B’  Group ‘C’ 
Work  Joule  erg 
Force  Newton  dyne 
Displacement  Metre  cm 
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.
Question a.
Answer:
The total distance the bird has traversed is the length of the distance covered by circling, but the eventual displacement are the bird is zero as its initial and final position are one and the same.
5. Explain the following concepts in your own words with everyday examples: force, work, displacement, velocity, acceleration, distance.
Question a.
Explain the following concepts in your own words with everyday examples: force, work, displacement, velocity, acceleration, distance.
Answer:
1. Force: The interaction that brings about the acceleration is called force.
e.g: An ox is pulling a cart, applying brakes to a bicycle, lifting heavy iron object with a crane.
2. Work: When an object is displaced by applying a force on it, work is said to be done.
e.g: A bucketful of water is to be drawn from a well and taken to the home by walking from well to home.
3. Displacement: The minimum distance
traversed by a moving body in one direction from the original point to reach the final point is called displacement.
e.g: A rolling of a ball from point A to point B in the same direction.
4. Velocity: Velocity is the distance traversed by a body in a specific direction in unit time.
e.g: A truck is covering a distance of 40km from A to D in a straight line in 1 hour.
5. Acceleration: It is change in velocity per second. It can be deduced.
e.g:
(i) In the above example a truck covered the distance AB at velocity of 60 km/hr, BC at 30 km/hr and CD at 40 km/hr. (ii) It means that the velocity for the distance CD is greater than the velocity for the distance BC. (iii) 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 (iv) Distance: The length of the route actually traversed by a moving body irrespective of the direction is called distance.
e.g: Ranjit travelled 1km. from his home to school.
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.
Question a.
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:
As its initial and final positions are one and the same.
Initial Velocity = 2 cm/s.
Final Velocity = 4 cm/s
Time taken for the change in velocity from B to
D = 4 cm/s – 2 cm/s = 2 cm/s
7. Solve the following problems.
Question 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 10m. How much is the work done?
Answer:
Force (F) = 1000 N
displacement (s) = 10m
work done (W) = ?
W = Fs
= 1000 × 10
W = 10,000 Joule
Question 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:
Force (F) = 2 N
Displacement (s) = 50 m
Work done (W) = ?
W = Fs
= 2 × 50
W = 100 Joule
Project:
Question a.
Collect information about the study made by Sir Isaac Newton regarding force and acceleration and discuss it with your teacher.
Important Questions and Answers
Fill in blanks:
Question 1.
Displacement is a …………. quantity.
Answer:
vector
Question 2.
The …………. of an object can change even while it is moving along a straight line.
Answer:
velocity
Question 3.
The …………. velocity can be different at different times.
Answer:
instantaneous
Question 4.
Change in velocity per second is called …………. .
Answer:
acceleration
Question 5.
The interaction that brings about the acceleration is called …………. .
Answer:
force
Question 6.
The scientist …………. was the first to study force and the resulting acceleration.
Answer:
Sir Isaac Nezvton
Question 7.
Ability to do work is called …………. .
Answer:
Energy
Question 8.
W = …………. × S.
Answer:
F
Question 9.
Unit of work is …………. and …………. .
Answer:
Joule, erg
Question 10.
Unit of force is …………. and …………. .
Answer:
Newton, dyne
Question 11.
Force is a …………. quantity.
Answer:
vector
Question 12.
The velocity at a particular time is called …………. velocity.
Answer:
instantaneous
Question 13.
The …………. of a body is the distance traversed per unit time.
Answer:
speed
Question 14.
Unit of acceleration is …………. and …………. .
Answer:
m/s^{2} and cm/s^{2}
Question 15.
Force is measured by the …………. that it produces.
Answer:
acceleration
Question 16.
Work done by a body with no displacement will be …………. .
Answer:
zero
Say whether True or False, correct the false 1 statements:
Question 1.
Velocity is distance travelled per unit of time.
Answer:
False. Speed is distance travelled per unit of time
Question 2.
In displacement, both distance and direction are taken into account.
Answer:
True
Question 3.
Speed = Distance/time.
Answer:
True
Question 4.
Change in speed per second is acceleration.
Answer:
False. Change in velocity per second is acceleration
Question 5.
Work done depends on the force and the displacement.
Answer:
True
Question 6.
C.G.S. unit of acceleration is m/s^{2}.
Answer:
False. C.G.S. unit of acceleration is cm/s^{2}.
Question 7.
M.K.S. unit of force is dyne.
Answer:
False. M.K.S. unit of force is Newton
Question 8.
Force is measured by the acceleration that it produces.
Answer:
True
Write the difference between the following:
Question 1.
Speed and Velocity
Answer:
Speed  Velocity 
1. Speed is distance travelled per unit of time.  1. Velocity is the distance traversed by a body in a specific direction in unit time. 
2. It is a scalar quantity.  2. It is a vector quantity. 
3. Formula: 
3. Formula:

Question 2.
Distance and Displacement
Answer:
Distance  Displacement 
1. The length of the route actually traversed by a moving body, irrespective of the direction is called distance.  1. The minimum distance traversed by a moving body in one direction from the original point to reach the final point is called displacement. 
2. It is a scalar quantity.  2. It is a vector quantity. 
Solve the following problems!
Question 1.
A bus travelled 200 km in the first 3 hours and then 100 kms for the next one and a half hours and then 120 kms for the next one and a half hours. What is the average velocity of the bus if it has moved in a straight line for the whole journey.
Answer:
Question 2.
See the diagram and calculate the Distance and Displacement travelled by the body from A to I.
Answer:
Distance travelled =
A → B → C → D → E → F → G → H + I
= 5 + 7 + 6 + 3 + 5 + 4 + 6 + 5
= 41 m
Displacement = A → I in a straight line shortest distance
= 1m
Use your brainpower:
Question 1.
The unit of acceleration is m/s^{2}, verify this.
Answer:
Question 2.
Acceleration is a vector quantity. Is force a vector quantity too?
Answer:
Yes, acceleration and force both are vector quantities, because both can be expressed completely only when magnitude and direction are given and the quantity which needs direction and magnitude both is called a vector quantity.