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A tight rope-walker carries a long stick in his hand to maintain the CG such that the vertical line passes through the rope. 

Mercury will flow downwards when a hole is made at the top part of the tube. The atmospheric pressure exerted on the mercury in the vessels supports the mercury level in the tube. When a hole is made at the top, air enters the tube and the pressure is distributed throughout uniformly and hence the mercury level comes down.

fig 2. When density of liquid increases the pressure increases, water is denser than kerosene.

(d) act on different bodies

Correct answer is (b) newton

(c) the object will have zero acceleration

The effect of force will be observed when unbalanced force act on an object. Unbalanced forced acting on an object, change its speed or direction of motion.

SI unit of mass is kg SI unit of acceleration is ms^-2.

The SI unit of momentum is kg ms^-1.

1. Balanced forces : 

• The effect of the force cannot be observed. 
• No change in the speed or direction of the motion or both. 
• Equilibrium position is maintained, 

2. Unbalanced force : 

• The effect of the force can be observed. 
• Change in the speed or direction of the motion or both. 
• Equilibrium position is not maintained.

A passenger travelling in a bus/train share the motion of the bus/train. Thus the passenger’s body is moving in the direction of the bus with the same speed. When the passenger jumps out of a fast-moving bus/train, his feet will come to rest on touching the ground, whereas the upper part of his body continues to move forward, due to the inertia of motion. As a result, if he does not run forward he will fall with his face in downwards direction.

1. An iron nail sinks in water because its density is more than that of water. 

2. A ship made from iron, due to the particular shape given to it, displaces a large amount of water so that the buoyant force acting on the ship due to water balances the weight of the ship. Hence, the ship floats on water.

initial velocity = 2 ms^-1 

velocity at rest = 0 ms^-1 

time interval = 1 second

mass m = 10 kg 

change in velocity in one second or acceleration

 \(=\frac{0-2}{1} \) = 2 ms^-1

F = ma 

F = 10 × 2 

= 20 kg ms^-2 or -20 newton or-20 N. 

The negative sign tells us that the force exerted is opposite to the direction of motion.

Momentum of the rifle = momentum of the bullet 

Mass of the rifle × velocity of recoil 

= mass of the bullet × Velocity of the bullet 

2 × velocity of recoil = \(\frac{25}{1000} \times 2\)

2 × velocity of recoil =\(\frac{1\times2}{40} = \frac{1}{20}\)

velocity of recoil =\(\frac{1}{2} \times \frac{1}{20}= \frac{1}{40}\)

= 0.0025 ms^-1.

F = ma 

m = 4 kg 

a = 2 ms^-2 

F = ? 

F = 4 kg x acceleration 

F = 4 x 2 

= 8 kg ms^-1 or 8 newton or 8 N.

The structure of a ship is much bigger than that of iron sheet. When a ship enters in water, it displaces a large volume of water, whereas, the weight of water displaced by an iron sheet is much higher when it enters in water than the ship. As a result, iron sheet sinks, while a ship floats on water.

Conservation of momentum states that the total momentum of a system conserved when the net external force acting on the system is zero. 

mv + MV = 0 or mv = – MV.

The acceleration given to a body by a force applied to it is directly proportional to the force and in the same direction of force and inversely proportional to the mass of the body. F = ma.

The person getting down from a moving bus runs for a certain distance in the direction of the bus due to inertia which may lead to accident. Therefore it is dangerous to get down from a moving bus.

When a tanker full of any liquid is moved, due to the force pulling it (the action), an equal and opposite reaction acts on the liquid. So, the liquid rolls and rises up and down, for which some space is left in the tanker. Otherwise, the liquid would split out.

We are given that Force (F) = 525 N 

Acceleration (a) = 3.5 m/s² . 

Using, F = ma

\(\frac{F}{a}=\frac{525}{3.5}\) = 150 m/s

This is because the acceleration, given to a body by a force applied to it is directly proportional to the force when a body moves, it will continue to move with the some velocity when no force act on it. According to this principle, If an athlete runs from a distance he can jump a long distance with greater force.

A body is said to be in uniform motion, if it travels an equal distance in equal interval of time, no matter how small these time interval may be. 

Example: The movement of the earth around the sun.

This is due to the inertia of rest. When the branch is shaken swiftly, the branch suddenly moves a little distance, but the fruits attached to it by a slender stalk (petiole) tend to remain where they are (due to inertia). So, the stalk feels a jerk and breaks away and the fruit falls down.

(a) Uniform circular motion. 

(b) Uniformly accelerated linear motion.

When a man jumps from a boat to the shore, he pushes the boat behind with his leg (action) and the boat pushes (reaction) the man in the opposite direction. Because of this, the man moves towards the shore and the boat moves away in the opposite direction. Therefore the boats are tied to the bank with a rope.

Action and reaction do not cancel each other, because they act on different bodies.

Law of conservation of momentum.

When a horse pulls a cart with a certain force, the cart also pulls the horse with the same force. But, in an effort to pull the cart, the horse pushes the ground backwards with his feet. To this action of the horse, the ground offers reaction. A part of this reaction from the ground balances the weight of the cart, while the other part makes the cart move in the forward direction.

Athlete acquired the inertia of motion by running and then continues in this state so that he can jump to a longer distance.

An odometer measures the distance covered by the automobile in a kilometre.

The cart will move with a constant velocity if and only if no external force acts on it. When the horse applies the force, the frictional force also acts between the wheels of the cart and road to oppose the motion. The cart will move with constant velocity, only if the force applied by the horse is equal to the frictional force.

When forces acting on the body are balanced and no unbalanced force acts on it.

When an object is immersed wholly or partly in a fluid (liquid or gas), it experiences an upward force, called buoyant force which is equal to the weight of the fluid displaced by it.

The answer is (A) are always balanced forces

(a) The two forces must be equal in magnitude and in opposite direction. So, they balance each other. As there is no unbalanced force, the body does not move. 

(b) The net resultant force on the body is zero.

The answer is (C) 9.8 N

The answer is (B) there is always a decrease in its speed

The answer is (D) all above

The answer is (B) velocity remain constant

The answer is (A) 50 N

As we know that it is very difficult to change the state of rest or of uniform motion of a heavier body than that of a lighter one. So, mass is a measure of inertia.

During rainy days, the wet ground becomes very smooth. The friction between our feet and the ground is reduced to the maximum. Thus, we slip.

The answer is (B) mass of an object

Newton’s first law of motion defines inertia as the inherent property of a body by virtue of which it resists any change in its state of rest or of uniform motion in a straight line on its own. This law is also known as the law of inertia.

Inertia can be categorised into three types:

(a) Inertia of rest

(b) Inertia of motion

(c) Inertia of direction

Examples:

(a) The carpet is beaten with a stick to remove the dust particles. Initially, both the carpet and the dust in it are at rest. When the carpet is shaken or beaten with a stick, the carpet is set into motion. Due to the inertia of rest, the dust particle tends to remain at rest. As a result, the dust particles fall off.

(b) The passengers in a bus tend to fall back when it starts suddenly. When a passenger is sitting in a stationary bus, both the bus and the passenger are at rest. When the bus starts suddenly, the lower part of the passenger’s body starts moving forward with the bus. However, the upper part of the passenger’s body tends to remain in the position of rest, due to the inertia of rest. As a result, the passengers fall or lean backwards.

First Law of Motion: 

Its states that a body at rest will remain at rest and a body in motion will continue in motion in a straight line with uniform speed unless it is compelled by some external force to change its state of rest or of uniform motion. 

Example: 

When a bus starts moving all of a sudden, the head of the passenger sitting in the bus strikes the back of his seat. This can be explained on the basis of the first law of motion. The lower part of the body of the passenger remains in contact with the body of the bus, but the upper part is free. When the bus takes a sudden start, the bus comes in motion and so the lower part of the body, but the head and upper part of the passenger’s body not in contact with the bus, tend to remain at rest and thus, jerked backwards. Similarly, we can explain the jerking of passengers in the forward direction of a bus, when the moving bus stops all of a sudden.

Third law of motion: It states that to every action, there is an equal and opposite reaction. Action and reaction always operate on different bodies. 

Example: 

When a man jumps from a boat on to the bank of a river, then he pushes the boat behind with his leg (action) and the boat pushes (reaction) the man in the opposite direction. Because of this, the man move towards the bank of the river and the boat moves away from the bank in the opposite direction.

Newton’s first law of motion-According to this law, a body at rest or in uniform motion will continue to be in the state of rest or uniform motion unless it is compelled by an external force to change its state of rest or uniform motion.

Examples:

(i) An athlete runs for a certain distance before taking a long jump.

(ii) A person sitting/standing on a bus fall back when the bus suddenly starts moving forward.

(iii) The carpet is beaten with a stick to remove the dust particles.

The second law of motion: The acceleration produced in the body by the action of a force acting on it is directly proportional to the force and inversely proportional to the mass of the body. Mathematically, F = ma, where, F = force, m = mass of the body. In other words,
Newton’s second law of motion can be stated as:

The rate of change of momentum is equal to the force applied to the body and the change in momentum always takes place in the direction of the force.

Examples: 

(i) A cricket player lowers his hands while catching the ball.

(ii) A person falling on a cemented floor gets injured, but a person falling on a heap of sand does not get injured.

Third law of Motion: According to this law:

“For every action, there are an equal and opposite reaction” (Law of action and reaction) 

Examples:

(i) Gun gives a jerk to shoulder of the gunman on firing a bullet from it.

(ii) Boatman pushes the river bank with a bamboo pole to take his boat into the river.

Scalar and vector quantities: The physical quantities which have only magnitude and need no idea of direction are called scalar quantities.

For example: Speed, mass, time, length, area, volume, density, work, energy etc.

Vector quantities: There are certain quantities which require magnitude as well as direction, to express them clearly, i.e., the physical quantities which have magnitude, as well as direction, are called vector quantities.

For example: Velocity, acceleration, force, pressure etc.

Vector representation: A vector quantity is represented by a single arrow. It is written as \(\bar { AB } \). Here, length of AB is the magnitude of |\(\bar { AB } \)|.

Unit Vector

A unit vector is a vector whose magnitude is unit and direction is the direction of the vector, say \(\bar { AB } \). A unit vector is denoted by \(\hat { A } \) and it is read as A cap.
\(\bar{A} =|\bar{A}|\hat {A} =\hat{A} =\frac{\bar{A}}{|\bar{A}|} \)

The answer is (D) it’s inertia