MOTION AND FORCE Chapter # 04 Physics 10th - Question Answers

 Physics 10th - Question Answers

MOTION AND FORCE
Chapter # 04

Q.1: Define force. Write down its formula and units. Also write down the factors on which force depends?

Ans: FORCE:
Force is that agent which changes or tends to change the state of rest or of motion of a body. It can also distort or tends to distort the shape of a body to which it is applied. It is denoted by $F$. It is a vector quantity.

Formula:

$$F=ma$$

Where
$F$ stands for force
$m$ stands for mass
$a$ stands for acceleration

Unit:
In the S.I. system, the unit of force is Newton (N).

Newton:
It can be defined as:

Force acting on a body is said to be one Newton if it produces an acceleration of $1m\mathrm{/}{s}^2$ in the body of mass $1kg$ in the direction of the applied force.

Factors On Which Force Depends:
There are two factors on which force depends:

• Force is directly proportional to the mass of the body.
• It is directly proportional to the acceleration produced in the body.

Q.2: State the following laws?

• Newton’s first law of motion
• Newton’s second law of motion
• Newton’s third law of motion
• Law of inertia

Ans: NEWTON’S FIRST LAW OF MOTION:
Every object continues its state of rest or of uniform motion in a straight line unless it is acted upon by an external force which changes its state of rest or of uniform motion.

NEWTON’S SECOND LAW OF MOTION:
When a force acts on a body, it produces acceleration in the body in its own direction. This acceleration is directly proportional to the magnitude of the applied force.

NEWTON’S THIRD LAW OF MOTION:
To every action, there is an equal but opposite reaction. Action and reaction do not act on the same body but act on two different bodies.

LAW OF INERTIA:
The first law of motion is also called the law of inertia. Inertia is the property of a body due to which it resists against any change in its state of rest or of uniform motion.

Q.3: Give any two examples of inertia?

Ans: INERTIA:

Example No. 1:
If we put a coin on a card and place a card over a glass and flick away the card with the finger, the coin drops into the glass.

Example No. 2:
Suppose passengers are sitting in a bus. If it starts moving suddenly, the passengers will feel a jerk in the backward direction. It is because their bodies are in contact with the seat of the bus and come in motion with the motion of the bus while the upper parts of their bodies remain at rest due to inertia, and so the passengers feel a jerk in the backward direction.

Similarly, if the bus is moving and stops suddenly, the passengers will feel a jerk in the forward direction; it is also due to inertia.

Q.4: State Newton’s second law of motion?

Ans: STATEMENT:
The acceleration of a body is directly proportional to the force acting on it and inversely proportional to the mass of the body.

Derivation Of F = ma:
When a force is applied to a body, it produces acceleration in the body in the direction of force. This acceleration is directly proportional to the force. Mathematically,

$$\begin{array}{cccc}& a\propto F& & \text{(i)}\end{array}$$

It means the greater the force acting on the body, the greater will be the acceleration, provided that the mass of the body is constant.

The magnitude of acceleration is inversely proportional to the mass of the body. Mathematically:

$$\begin{array}{cccc}& a\propto \frac{1}{m}& & \text{(ii)}\end{array}$$

It means that the greater the mass of the body, the less will be the acceleration, provided that the force is constant.

Combining equation (i) and (ii), we have:

$$a\propto \frac{F}{m}$$$$a=K\cdot \frac{F}{m}$$

Put $K=1$, then $a=1$:

$$a=\frac{F}{m}$$$$F=ma$$

Hence, $F=ma$

Q.5: Give any two examples of Newton’s third law of motion?

Ans: NEWTON’S THIRD LAW OF MOTION:

Example No. 1:
While walking on the ground, we push the ground in the backward direction with our feet. This is our action on the ground. As a result, the ground pushes us with a force in the forward direction. This is the reaction of the ground due to which we move forward.

Example No. 2:
The gases formed in rocket engines due to the combustion of fuel rush out with great speed through a jet on the backside, and as a reaction, the rocket moves in the upward direction.

Q.6: Define mass and weight?

Ans: MASS:
Mass is the quantity of matter possessed by a body. It is denoted by $m$. Its S.I. unit is kilogram (kg). It is a scalar quantity. It can be found out by the following formulas:

$$m=\frac{F}{a}$$$$m=\frac{W}{g}$$

WEIGHT:
Weight is the force with which the Earth attracts a body towards its center. It is denoted by $W$. Its S.I. unit is Newton (N). It is a vector quantity. It can be found out using the formula:

$$W=mg$$

Q.7: Write down the difference between mass and weight?

Ans: DIFFERENCE BETWEEN MASS AND WEIGHT:

Mass	Weight
The quantity of matter present in a body is called its mass.	Weight is the force with which the Earth attracts a body towards its center.
Its S.I. unit is kilogram (kg).	Its S.I. unit is Newton (N).
It is a scalar quantity.	It is a vector quantity.
It can be measured by a physical balance.	It can be measured by a spring balance.
It is a universal constant.	It depends upon the height.

Q.8: Define tension?

Ans: TENSION:
Tension is a force which is directly exerted by a string on a body to which it is attached. It is denoted by $T$. Its S.I. unit is Newton (N). It is a type of force, so it is also a vector quantity.

Q.9: Find the equations for acceleration and tension in the string when two bodies having different masses $m_1$ and $m_2$ connected by a string pass over a frictionless pulley in such a way that the two bodies hang vertically?

Ans:
We consider two bodies $A$ and $B$ having unequal masses $m_1$ and $m_2$ respectively, connected by a string that passes over a frictionless pulley in such a way that the two bodies hang vertically.

Suppose $m_1$ is greater than $m_2$. Hence, body $A$ will move down with acceleration $a$ while body $B$ will move up with the same acceleration. Let $T$ be the tension in the string.

To calculate the acceleration of the bodies and tension in the string, let us consider the motion of body $A$ first.

Downward motion of the body $A$:
Two forces are acting on the body $A$:

• Force of gravity (or weight of the body), $W_1=m_{1}g$, acting in the downward direction.

Tension $T$ in the string acts in the upward direction. Since the body $A$ is moving downwards, then:

$$m_{1}g>T$$

Net force acting on the body $A$:
Downward Force - Upward Force

Force:

$$F_1=m_{1}g-T$$

According to Newton’s 2nd law of motion:

$$F_1=m_{1}a$$

Therefore:

$$\begin{array}{cccc}& m_{1}a=m_{1}g-T& & \text{(i)}\end{array}$$

Upward Motion Of The Body $B$:
Two forces are also acting on the body $B$:

• Force of gravity (or weight of the body), $W_2=m_{2}g$, acting in the downward direction.
• Tension $T$ in the string acting in the upward direction.

Since the body $B$ is moving upward, then:

$$T>m_{2}g$$

Net force acting on the body $B$:
Upward Force - Downward Force

Force:

$$F_2=T-m_{2}g$$

According to Newton’s 2nd law of motion:

$$F_2=m_{2}a$$

Therefore:

$$\begin{array}{cccc}& m_{2}a=T-m_{2}g& & \text{(ii)}\end{array}$$

1. 
   

To Find The Expression For Acceleration $a$:
Adding equations (i) and (ii), we get:

$$\begin{array}{cccc}& m_{1}a=m_{1}g-T& & \text{(i)}\end{array}$$$$\begin{array}{cccc}& m_{2}a=T-m_{2}g& & \text{(ii)}\end{array}$$$$m_{1}a+m_{2}a=m_{1}g-T+T-m_{2}g$$$$m_{1}a+m_{2}a=m_{1}g-m_{2}g$$