LAWS OF MOTION

What causes the motion ?Why does the speed of an object change with time ?

Do all motion require a cause ?

If so what is the nature of this cause ?

Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalism Principia Mathematica (mathematic principles of natural philosophy) in 1687. Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects on the scale we experience in our everyday lives.

Sir Isaac Newton

WHAT IS THE LAW OF MOTION ?

Newton's laws of motion are three physical laws which provide relationships between the forces acting on a body and the motion of the body.

Force Force is an action that can change

motion . A force is what we call a push or a pull, or any action that

has the ability to change an object’s motion. Forces can be used to increase the speed of an object,

decrease the speed of an object, or change the direction in which an

object is moving.

BALANCED & UNBALANCED FORCES

If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion.

If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

FIRST LAW OF MOTIONAn object at rest tends to stay at rest and object in motion tends to stay in motion unless acted upon by an unbalanced force.

What does this mean ?Basically, an object will keep doing what it was doing, unless acted on by an unbalanced force.

If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving.

It takes force to change the motion of an object.

NEWTON’S FIRST LAW OF MOTION ALSO CALLED LAW OF INERTIA

INERTIA: the tendency of an object to resist changes in its state of motion

The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

More Examples from Real Life

A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

Newton’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes in their motion. When a car going on 80 km/hour is stopped by the brick wall, your body still keeps on moving at 80 km/hour.

NEWTON’S SECOND LAW OF MOTION

FORCE EQUALS MASS TIME ACCLERATION

F= ma

Acceleration: a measurement of how quickly an object is changing speed.

Force is directly proportional to mass and acceleration.

WHAT DOES F=ma MEAN ?F = ma basically means that the force of an object comes

from its mass and its acceleration.

Something very massive (high mass) that’s changing speed very slowly (low acceleration),like a glacier, can still have great force.

Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force.

If We increase the amount of net force than the acceleration also increases in the same amount. If we decrease the net force than acceleration also decreases.

Let’s see it from the given picture below.

In the Picture given we double the force than the magnitude of acceleration also becomes double. On the contrary we decrease the force then, acceleration also decreases. We understand the relation between force and acceleration. Well, do you think mass affects the acceleration? Suppose that you push a box that is empty, you can easily push it. If the box is full, then can you push it easily with the same force? The answer is of course “NO”. Bigger mass results in bigger force. Thus, we find another relation of force which is:Net force ~ mass

Force is directly proportional to mass and acceleration. If the mass is constant, when we increase the force the object gains acceleration with the same amount or, if the force is constant, when we decrease the mass acceleration increases with the same amount. Therefore:Net Force ~ acceleration Net Force ~ mass

F=ma

Net Force ~ mass x acceleration

NEWTON’S THIRD LAW OF MOTION

For every action there is an equal and opposite reaction

What does this mean?For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

EXAMPLESThe astronauts working on the space station have a serious problem when they need to move around in space: There is nothing to push on. The solution is to throw something opposite the direction you want to move. This works because all forces always come in pairs.

The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases.

The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

Helicopters – blades push air down. Air pushes chopper up.

Lift – upward reaction force

When lift equals weight of chopper, it can hover

When lift is greater. Thus the chopper rises.

Newton’s 3rd Law in NatureConsider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

Measuring WeightWhen you stand on

a scale, your weight pushes down on the scale. This causes the scale pointer to point to your weight.

At the same time, by Newton’s third law the scale pushes up on you with a force equal to your weight.

This force balances the downward pull of gravity on you.

Free Fall and WeightlessnessNow suppose you

were standing on a scale in an elevator that is falling.

A falling object is in free fall when the only force acting on the force is gravity. You and the scale are both in free fall.

Because the only force acting on you is gravity, the scale no longer is pushing up on you.

According to Newton’s third law, you no longer push down on the scale. So the scale pointer stays at zero and you seem to be weightless.

Weightlessness is the condition that occurs in free fall when the weight of an object seems to be zero.

AGNES RICHARDIX –B

ROLL NO. 29

THANK YOU