Force and laws of motion

Force and laws of motion

We need to put some effort to make a stationary object move, 

For Example, a push, a hit or a pull.

What is a force?

Whenever we push or pull an object a force acts upon them and makes them move from one place to another. Hence, force can –

--> initiate motion in a motionless object

--> change (increase or decrease) the velocity of the moving object

--> alter the direction of a moving object

--> change the shape and size of an object

Balanced Forces:

=> When equal amount of forces are applied on an object from different directions such that they cancel out each other

=> They do not change the state of rest or motion of an object

=> They may change the shape and size of an object

Unbalanced Force:

=> When forces applied to an object are of different magnitude(or not in opposite directions so as to cancel)

=> They can alter state of rest or motion of an object

=> They can cause acceleration in an object

=> They can change the shape and size of an object

What is the force of friction?

=> It is a force extended when two surfaces are in contact with each other.

=> It always acts in a direction opposite to the direction of motion of the object.

Galileo’s Observation:

He observed the motion of objects on an inclined plane.

When a marble is rolled down an inclined plane its velocity increases.

Galileo’s Arguments:

When a marble is rolled down from the left – It will go up on the opposite side up to the same height at which it is dropped down.

If the inclination of planes is equal – The marble would travel equal distances while climbing up as travelled while rolling down.

If we decrease the angle of inclination of the right plane – The marble would travel further until it reaches its original height.

If the right side plane is made flat – Marble would travel forever to achieve the same height.

Galileo's Inference:

We need an unbalanced force to change the motion of the marble but no force is required when the marble is moving uniformly. In other words, objects move at a constant speed if no force acts upon them.

Based on Galileo’s ideas Newton presented the three Laws of Motion

First law of motion or The law of Inertia

Second law of motion or The law of momentum

Third law of motion or The law of conservation of momentum

First law of motion or The Law of Inertia:

Whether an object is moving uniformly on a straight path or is at rest, its state would not change until and unless an external force is applied on to it.

Hence, we can say that objects oppose a change in their state of motion or rest. This tendency of objects to remain in the state of rest or to keep moving uniformly is called Inertia.

Inertia:

Inertia, property of a body by virtue of which it opposes any agency that attempts to put it in motion or, if it is moving, to change the magnitude or direction of its velocity. Inertia is a passive property and does not enable a body to do anything except oppose such active agents as forces and torques. A moving body keeps moving not because of its inertia but only because of the absence of a force to slow it down, change its course, or speed it up.

Types of Inertia:

Inertia of rest - An object stays where it is placed, and it will stay there until you or something else moves it. (i.e. Dust particles stay at rest until you shake a carpet.)

Inertia of motion - An object will continue at the same speed until a force acts on it. (i.e. Body going forward when a car stops.)

Inertia of direction - An object will stay moving in the same direction unless a force acts on it. (i.e. One's body movement to the side when a car makes a sharp turn.)

Inertia of Rest Examples:

If pulled quickly, a tablecloth can be removed from underneath the dishes. The dishes have the tendency to remain still as long as the friction from the movement of the tablecloth is not too great.

When a car is abruptly accelerated, drivers and passengers may feel as though their bodies are moving backward. In reality, inertia is making the body want to stay in place as the car moves forward.

Inertia of Motion Examples:

Seat belts tighten in a car when it stops quickly.

Men in space find it more difficult to stop moving because of a lack of gravity acting against them.

If you are on a train and the train is moving at a constant speed, a toy tossed into the air will go straight up and then come down. This is because the toy has inertia like the train and you.

If a car is moving forward it will continue to move forward unless friction or the brakes interfere with its movement.

 Examples of inertia of direction:

When you stir coffee or tea and stop, the swirling motion continues due to inertia.

Objects that establish orbit around the earth, such as satellites, continue on their trajectory due to inertia.

If you throw a rock straight up, it will not vary from its direction.

Inertia enables ice skaters to glide on the ice in a straight line.

Inertia and Mass:

The inertia of an object is dependent upon its mass.

Lighter objects have less inertia, that is, they can easily change their state of rest or motion.

Heavier objects have large inertia and therefore they show more resistance.

Hence ‘Mass’ is called a measure of the inertia of an object.