# Force And Laws Of Motion - Class 9 - Science

Renews every month. Cancel anytime

### Your personal doubt-solving assistant

Chatterbot AI gives you 100% accurate answers to your questions in an instant.

## Back Questions - Force And Laws Of Motion | NCERT | Science | Class 9

An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on.

Yes, an object with net zero external unbalanced force can travel with non-zero velocity. This scenario occurs when an object is moving with constant velocity in a straight line; in other words, it is in a state of uniform motion according to Newton's first law of motion, also known as the law of inertia. The object must not be experiencing any acceleration or deceleration for its velocity to remain constant.

When a carpet is beaten with a stick, dust comes out of it, Explain.

### Improve your grades!

Join English Chatterbox to access detailed and curated answers, and score higher than you ever have in your exams.

Sign up nowWhy is it advised to tie any luggage kept on the roof of a bus with a rope?

### Improve your grades!

Join English Chatterbox to access detailed and curated answers, and score higher than you ever have in your exams.

Sign up nowA batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because

(a) the batsman did not hit the ball hard enough.

(b) velocity is proportional to the force exerted on the ball.

(c) there is a force on the ball opposing the motion.

(d) there is no unbalanced force on the ball, so the ball would want to come to rest.

### Improve your grades!

Join English Chatterbox to access detailed and curated answers, and score higher than you ever have in your exams.

Sign up nowA truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of $400 \mathrm{~m}$ in $20 \mathrm{~s}$. Find its acceleration. Find the force acting on it if its mass is 7 tonnes (Hint: 1 tonne $=1000 \mathrm{~kg}$.)

### Improve your grades!

A stone of $1 \mathrm{~kg}$ is thrown with a velocity of $20 \mathrm{~m} \mathrm{~s}^{-1}$ across the frozen surface of a lake and comes to rest after travelling a distance of $50 \mathrm{~m}$. What is the force of friction between the stone and the ice?

### Improve your grades!

A $8000 \mathrm{~kg}$ engine pulls a train of 5 wagons, each of $2000 \mathrm{~kg}$, along a horizontal track. If the engine exerts a force of 40000 $\mathrm{N}$ and the track offers a friction force of $5000 \mathrm{~N}$, then calculate:

(a) the net accelerating force and

(b) the acceleration of the train.

### Improve your grades!

An automobile vehicle has a mass of $1500 \mathrm{~kg}$. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of $1.7 \mathrm{~m} \mathrm{~s}^{-2}$ ?

### Improve your grades!

What is the momentum of an object of mass $m$, moving with a velocity $v$ ?

(a) $(m v)^{2}$

(b) $m v^{2}$

(c) $1 / 2 m v^{2}$

(d) $m v$

### Improve your grades!

Using a horizontal force of $200 \mathrm{~N}$, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?

### Improve your grades!

According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move. 100 words

### Improve your grades!

A hockey ball of mass $200 \mathrm{~g}$ travelling at $10 \mathrm{~m} \mathrm{~s}^{-1}$ is struck by a hockey stick so as to return it along its original path with a velocity at $5 \mathrm{~m} \mathrm{~s}^{-1}$. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.

### Improve your grades!

A bullet of mass $10 \mathrm{~g}$ travelling horizontally with a velocity of $150 \mathrm{~m} \mathrm{~s}^{-1}$ strikes a stationary wooden block and comes to rest in $0.03 \mathrm{~s}$. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.

### Improve your grades!

An object of mass $1 \mathrm{~kg}$ travelling in a straight line with a velocity of $10 \mathrm{~m} \mathrm{~s}^{-1}$ collides with, and sticks to, a stationary wooden block of mass $5 \mathrm{~kg}$. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.

### Improve your grades!

An object of mass $100 \mathrm{~kg}$ is accelerated uniformly from a velocity of $5 \mathrm{~m} \mathrm{~s}^{-1}$ to $8 \mathrm{~m} \mathrm{~s}^{-1}$ in $6 \mathrm{~s}$. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.

### Improve your grades!

Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation.

### Improve your grades!

The following is the distance-time table of an object in motion:

Time in seconds | Distance in metres |
---|---|

1 | 1 |

2 | 8 |

3 | 27 |

4 | 64 |

5 | 125 |

6 | 216 |

7 | 343 |

(a) What conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?

(b) What do you infer about the forces acting on the object?

### Improve your grades!

Two persons manage to push a motorcar of mass $1200 \mathrm{~kg}$ at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of $0.2 \mathrm{~m} \mathrm{~s}^{-2}$. With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort.)

### Improve your grades!

A hammer of mass $500 \mathrm{~g}$, moving at $50 \mathrm{~m} \mathrm{~s}^{-1}$, strikes a nail. The nail stops the hammer in a very short time of $0.01 \mathrm{~s}$. What is the force of the nail on the hammer?

### Improve your grades!

A motorcar of mass $1200 \mathrm{~kg}$ is moving along a straight line with a uniform velocity of $90 \mathrm{~km} / \mathrm{h}$. Its velocity is slowed down to $18 \mathrm{~km} / \mathrm{h}$ in $4 \mathrm{~s}$ by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.

### Improve your grades!

## Extra Questions - Force And Laws Of Motion | NCERT | Science | Class 9

Two blocks of equal mass $m$ are tied to each other through a light string. One of the blocks is pulled along the line joining them with a constant force $F$. Find the tension in the string joining the blocks.

Consider two blocks each of mass $ m $ linked by a light string. When a force $ F $ is applied to one of the blocks, both blocks experience acceleration due to the force transmitted through the string. Let's denote the tension in the string as $ T $ and the acceleration of each block as $ a $.

**From Newton's Second Law** applied to the first block where the force $ F $ is applied:
$$
F - T = ma \quad \text{(1)}
$$
Here, $ F $ is pulling the block and $ T $ is the tension in the string resisting the pull, resulting in the net force being the mass of the block times its acceleration.

**Now, applying Newton’s Second Law to the second block**, where no external force is applied other than the string tension:
$$
T = ma \quad \text{(2)}
$$
The only force acting on it is the tension pulling it which equals its mass times acceleration.

From equation (2), substituting $ a = \frac{T}{m} $ into equation (1), we have: $$ F - T = m \left(\frac{T}{m}\right) $$ Simplifying, we get: $$ F - T = T $$ $$ F = 2T $$ Hence, solving for $ T $, the tension in the string, yields: $$ T = \frac{F}{2} $$

Thus, the **tension in the string** joining the blocks is **$\frac{F}{2}$.**

Why does a person fall when he jumps out from a moving train?

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

A car moving at $40 \mathrm{~km/h}$ is to be stopped by applying brakes in the next $4.0 \mathrm{~m}$. If the car weighs $2000 \mathrm{~kg}$, what average force must be applied on it?

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

### Improve your grades!

A block of mass $2 \mathrm{~kg}$ rests on a rough inclined plane making an angle of $30^\circ$ with the horizontal. The coefficient of friction is 0.8 ($G=10 \mathrm{~m/s}^2$). The magnitude of the resultant of friction and normal reaction on the block will be:

A) $\sqrt{292} \mathrm{~N}$

B) $\sqrt{364} \mathrm{~N}$

C) $20 \mathrm{~N}$

D) $18 \mathrm{~N}$