Laws of Motion
1. Match the first column with appropriate entries in the second and third columns and remake the table.
Answer :
| S.no | Column 1 | Column 2 | Column 3 |
| 1 | Negative acceleration | The velocity of the object decreases. | A vehicle moving with the velocity of 10 m/s, stops after 5 seconds. |
| 2 | Positive acceleration | The velocity of the object increases. | A car initially at rest, reaches a velocity of 50 km/hr in 10 seconds. |
| 3 | Zero acceleration | The velocity of the object remains constant. | A vehicle is moving with a velocity of 25 m/s. |
2. Clarify the differences
A. Distance and displacement
Answer:
| Distance | Displacement |
| (i) Distance is the length of the actual path travelled by an object. (ii) It is a scalar quantity. (iii) It is either equal to or greater than displacement. (iv) Distance travelled is always positive. | (i) Displacement is the minimum distance between the starting and finishing points. (ii) It is a vector quantity. (iii) It is either equal to or less than distance. (iv) Displacement may be positive or negative or zero. |
B. Uniform and non-uniform motion.
Answer:
| Uniform motion | Non-uniform motion |
| (i) If an object covers equal distances in equal intervals of time it is said to be in uniform motion. (ii) Distance – time graph for uniform motion is a straight line. (iii) In uniform motion, acceleration is zero. | (i) If an object moves unequal distances in equal intervals of time, its motion is said to be nonuniform. (ii) Distance – time graph for non-uniform motion is not a straight line. (iii) In non-uniform motion acceleration is non-zero. |
3. Complete the following table.
Answer :
(1)
| u (m/s) | a (m/s2) | t (sec) | v = u + at (m/s) |
| 2 | 4 | 3 | 14 |
| 10 | 5 | 2 | 20 |
Explanation:
(i)
u = 2; a = 4; t = 3; v = u + at (m/s) = ?
v = u + at (m/s)
= 2 + 4 × 3
= 2 + 12
∴ v = 14
(ii)
u = ?; a = 5; t = 2; v = u + at (m/s) = 20
v = u + at (m/s)
⇒ 20 = u + 5 × 2
⇒ 20 = u + 10
⇒ 20 – 10 = u
∴ u = 10
(2)
| u (m/s) | a (m/s2) | t (sec) | s = ut + 1/2 at2 (m) |
| 5 | 12 | 3 | 69 |
| 7 | 8 | 4 | 92 |
Explanation:
(3)
| u (m/s) | a(m/s2) | s(m) | v2 = u2 + 2as (m/s)2 |
| 4 | 3 | 8 | 8 |
| 4 | 5 | 8.4 | 10 |
Explanation:
(i) u = 4, a = 3, v= 8, s = ?
v2 = u2 + 2as
⇒ (8)2 = (4)2 + 2 × 3 × s
⇒ 64 = 16 + 6s
⇒ 64 – 16 = 6s
⇒ 48 = 6s
⇒
= s
∴ s = 8
(ii) a = 5, s = 8.4, v = 10, u = ?
v2 = u2 + 2 as
⇒ (10)2 = u2 + 2 (5 × 8.4)
⇒ 100 = u2 + 84
⇒ 100 – 84 = u2
⇒ 16 = u2
∴ u = 4
4. Complete the sentences and explain them.
a. The minimum distance between the start and finish points of the motion of an object is called the displacement of the object.
b. Deceleration is negative acceleration
c. When an object is in uniform circular motion, its velocity changes at every point.
d. During collision momentum remains constant.
e. The working of a rocket depends on Newton’s third law of motion.
5. Give scientific reasons.
a. When an object falls freely to the ground, its acceleration is uniform.
Answer:
- When the body falls freely to the ground, there are equal changes in velocity of the body in equal intervals of time.
- Thus the acceleration of the body is constant, and it possesses uniform acceleration.
b. Even though the magnitudes of action force and reaction force are equal and their directions are opposite, their effects do not get cancelled.
Answer:
- Action and reaction forces act on different bodies.
- They don’t act on the same body, hence they cannot cancel each other’s effect.
- Hence, even though the magnitudes of action force and reaction force are equal, they do not cancel each other.
c. It is easier to stop a tennis ball as compared to a cricket ball, when both are traveling with the same velocity.
Answer:
- Momentum of an object depends on its mass as well as its velocity.
- Cricket ball is heavier than a tennis ball. Although they are thrown with the same velocity, cricket ball has more momentum than a tennis ball.
- The force required to stop a cricket ball is more than a tennis ball.
- Hence it is easier to stop a tennis ball than a cricket ball moving with same velocity.
d. The velocity of an object at rest is considered to be uniform.
Answer:
- When a body is at rest there is no change in velocity.
- A body with constant velocity is said to be in uniform motion.
- Hence, the state of rest is an example of uniform motion.
6. Take 5 examples from your surroundings and give an explanation based on Newton’s laws of motion.
Answer :
Example 1:
When we travel by bus, the stopped bus first feels backwards as it starts. This is an example of Newton’s first law of motion.
Explanation:
- When an object is at rest or in motion, the object does not change its state by its own inertia.
- When the bus is at rest, we are also at rest inside that bus.
- When the bus starts, sitting in the bus also gives us speed; But the upper part of the body tries to remain in a state of rest.
- As a result, inertia at rest tends to push us back. So, the bus starts in the next direction.
Example 2:
When the carpet is lifted and shaken, the dust in it falls down. This is an example of Newton’s first law of motion.
Explanation:
- When the carpet is lifted and cleaned, its speed increases.
- There is dust on the carpet and it is at rest due to inertia.
- Shaking removes the dust particles from the carpet, gravity causes the loose particles to fall, and the carpet is cleaned.
Example 3:
Players withdraw their hands while catching the ball. This is an example of Newton’s second law of motion.
Explanation:
- Players move their arms backwards to catch the ball, allowing more time for the hands to catch the ball.
- According to Newton’s second law of motion, the change in momentum is proportional to the magnitude of the force.
- Taking more time to catch the ball results in a very small change in momentum. Therefore, the player catches the ball with less force and his hand is not jerked.
Example 4:
The book placed on the table remains the same. This is an example of Newton’s third law.
Explanation:
- A book placed on a table has some weight because this weight is the force exerted on the table.
- According to Newton’s third law of motion, action and reaction forces act simultaneously.
- Hence, the force exerted on the book towards the table and the weight of the book balance.
- Also, the two forces are balanced, and there is no acceleration. Hence, the book placed on the table remains stationary.
Example 5:
A balloon filled with air goes forward when released from the hand. This is an example of Newton’s third law of motion.
Explanation:
As the air is released downward, it exerts an equal opposite force on the balloon, pushing the balloon forward.
7. Solve the following examples.
a) An object moves 18 m in the first 3 s, 22 m in the next 3 s and 14 m in the last 3 s. What is its average speed? (Ans: 6 m/s).
Answer:
Given:
Total distance (d) = 18 + 22 + 14 = 54 m
Total time taken (t) = 3 + 3 + 3 = 9 sec
To find:
Average speed = ?
The object moves with an average speed of 6 m/s.
b) An object of mass 16 kg is moving with an acceleration of 3 m/s2. Calculate the applied force. If the same force is applied on an object of mass 24 kg, how much will be the acceleration? (Ans: 48 N, 2 m/s2).
Answer:
The force acting on the 1 body is 48 N and the acceleration of the 2” body is 2 m/s2
c) A bullet having a mass of 10 g and moving with a speed of 1.5 m/s, penetrates a thick wooden plank of mass 90 g. The plank was initially at rest. The bullet gets embedded in the plank and both move together. Determine their velocity. (Ans: 0.15 m/s).
Answer:
The plank embedded with the bullet moves with a velocity of 0.15 m/s.
d) A person swims 100 m in the first 40 s, 80 m in the next 40 s and 45 m in the last 20 s. What is the average speed? (Ans: 2.25 m/s2).
Answer:
Given:
Total distance (d) = 100 + 80 + 45 = 225 m
Total time taken (t) = 40 + 40 + 20 = 100 sec
To find:
Average speed =?
The person swims with an average speed of 2.25 m/s.
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