## Force and Laws of Motion

When we want to open a
door, we have to push the door handle. And when we want to close the door, we
have to pull the door handle with our hand. This means that to move a body, it
has either to be pushed or pulled. ** A push or pull on a body is called force. **The direction
in which a body is pushed or pulled is called the direction of the force.

➤ **Force is that cause that produces acceleration in the body on which it acts.**

**Effects of force**

**A force cannot be seen. A force can be judged
only by the effects which it can produce in various bodies (or objects) around
us**. A force can produce
the following effects.

1. A force can ** move**
a stationary body.

2. A force can** stop**
a moving body.

3. A force can ** change
the direction** of a moving body.

4. A force can ** change
the speed** of a moving body.

5. A force can ** change
the shape** (and size) of a body.

**A force is an influence that tends to
set a stationary body in motion or stop a moving body; or which tends to change
the speed and direction of a moving body; or which tends to change the shape
(and size) of a body.**

**Balanced and Unbalanced
Forces.**

**➤ If a set of forces acting on a body produces no acceleration in it, the forces are said to be balanced. If it produces a nonzero acceleration, the forces are said to be unbalanced.**

**If two forces balance each other, they must be in opposite directions and have equal magnitudes.**

**Some common forces**

**a) Contact forces**

**b) Normal forces**

**c) Friction**

**d) Forces exerted by spring**

**e) Forces exerted by a string**

**f) Weight**

**a) Contact forces**

#### b) Normal forces

**normal force.**

**Newton
has given three laws to describe the motion of bodies. These laws are known as
Newton's laws of motion.** Newton's laws of motion give the precise definition of force and established a relationship between the force
applied to a body and the state of motion acquired by it.

**Newton's First Law of
Motion**

According to Newton's
first law of motion: *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 an
external force to change its state of rest or of uniform motion. *

** The inability of a body
to change on its own its state of rest or of uniform motion is known as
inertia. **Inertia is that property of a body due
to which it resists a change in its state of rest or of uniform motion.

**Inertia may be thought of as having two
forms**: the ** inertia of rest**,
due to which a body at rest remains at rest, and

**, due to which a moving body keeps moving without any change in its velocity.**

*inertia of motion*Mass is a measure of the inertia of a body. If a body has
more mass, it has more inertia. That is heavier objects have more inertia than
lighter objects.

** ****Examples of Newton's
First Law**

**a)**
**Inertia of rest**
If we keep a body at rest at a place, it remains there for any length of time
if no force is applied to it.

**b) Jerks while
traveling** When we stand on a bus and the bus starts suddenly,
we tend to fall backward. This is because our feet are in contact with the
floor of the bus and the friction at the contact is high. This force does not
allow the feet to sleep on the floor. The feet, therefore move forward with the
floor. The upper part of the body does not feel the forward force immediately
and remains at rest for a while. So, the upper part of our body gets jerked
backward.

Similarly, when the bus
stops suddenly, the feet come to rest immediately, but the upper part of the body
continues to move in the forward direction. So, we tend to fall forward. We also tend to fall sideways when the bus
turns sharply. This is because

Initially, there are two forces on the coin. The earth pulls the coin downwards (weight) and the card pushes it upwards (normal force). The forces balance each other and the coin remains at rest.

When we apply a
horizontal force on the card, it is accelerated and it moves away. Since the
friction between the card and the coin is negligible, there is no force on the
coin in the horizontal direction. It
remains in its original position due to the inertia of rest.

**Newton's Second Law of
Motion**

`F_1` produces an acceleration of `5ms^-2` and `F_2` produces an acceleration of `10ms^-2`

in the same body then,

`F_2 = 2F_1` (mass fixed)

Suppose we apply a force `F_1` on a body of `2kg` which produces an acceleration of of `5ms^-2`. To produce same acceleration in a `4kg` body, we have to apply a force twice as strong as `F_1`.

`F_2 = 2F_1` (a is fixed)

*π The magnitude of the
net force acting on a body is proportional to the product of the mass of the
body and its acceleration. The direction of the force is the same as that of
the acceleration. *

If we denote the magnitude of the force by F and acceleration by a,

**`F\propto a`**(i)

Mass
*m* is fixed.

Producing
the same acceleration in two objects of unequal masses. Then,

**`F\propto m`**(ii)

**`F\propto ma`**

Where
*k* is a constant.

**Linear Momentum**

*πThe
product of the mass of a body and its velocity is called the linear momentum
of the body. Quite often we only use the
word momentum
for linear momentum. *

If *m* be mass and* v* be the velocity, the linear momentum* p* is

A bullet fired at a
wooden board can destroy it. But a small stone of the mass as the bullet when
thrown at the board will hardly cause any damage. Although their masses are the
same, the high velocity of the bullet gives it a large momentum, which causes damage.

**Newton's Second Law in
terms of Momentum**

*πThe rate of change of momentum
of an object is proportional to the net force applied to the object. The
direction of the change of momentum is the same as the direction of the net
force. *

Suppose the linear momentum at
time* `t_1` *is `p_1`* = `mv_1`* and that
at time *`t_2`* is `p_2`* = `mv_2`*

_{}

The rate of change of momentum is `\frac{p_2-p_1}{t_2-t_1}` or `\frac{\triangle p}{\triangle t}`

According to the second law `\frac{p_2-p_1}{t_2-t_1}\propto F`

The second law of motion is often seen in action in our everyday life.

Catching a fast-moving cricket ball, a fielder in the ground gradually pulls his hands backward with the moving ball.

If the ball is stopped suddenly then its high velocity decreases to zero in a very short interval of time. Thus, the rate of change of momentum of the ball will be large. Therefore, a large force would have to be applied for holding the catch that may hurt the palm of the fielder.

### Third Law of Motion

π**To every action, there is an equal and
opposite reaction and they act on two different bodies.**

These two forces are always equal in magnitude but opposite in direction.

The two opposing forces are also known as action and reaction forces.

It is important to note that even though the action and reaction forces are always equal in magnitude, these forces may not produce accelerations of equal magnitudes. This is because each force acts on a different object that may have a different mass.

### Conservation of Momentum

**πIn an isolated system (where there is no external force), the total momentum remains conserved.**

**Before collision**

**After collision**

**References**

- S. Chand Class IX physics
- Foundation Science Physics Class IX
- NCERT Science Class IX

Very excellent ☺️πππππ notes

ReplyDeleteThank you so much

Deletenice notes

ReplyDeleteBahut khoob sirπππππ

ReplyDeleteHm aapka saath hai ππππππππππππππππ

Infinite πππππππππ

π π π π π π π π π π π

Gourav Kumar

ReplyDeleteVery helpful notesππ Sir

You are very good Teacherπ©π«

πExcellent jobπ

ReplyDeleteπππ€π€

Super notes

ReplyDelete