the e tutor - laws of motion

8/2/2019 The E Tutor - Laws of Motion

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Sir Isaac NewtonDiscovered

Laws Of Motion


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NewtONs first Law (Law of Inertia): An object at rest tends to stay at rest and

an object in motion tends to stay in motionunless acted upon by an unbalanced force.

Inertia is the tendency of an object to resist

changes in its velocity: whether in motion ormotionless.


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Conceptual Example : Newtons first law.

Example- A school bus comes to a sudden stop, and all of the backpacks onthe floor start to slide forward. What force causes them to do that?


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Newtons First LawAlternative Statement

In the absence of external forces, when viewed

from an inertial reference frame, an object at restremains at rest & an object in motion continues inmotion with a constant velocity

Newtons 1stLaw describes what happens in the

absence of a net force Also tells us that when no force acts on an object, the

acceleration of the object is zero


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Inertia & MassInertia: Thetendency of a body to maintain its state of

rest or motion.

Mass:A measure of the inertia of a body,Quantity of matter in a body

Quantify mass by having a standard

mass = Standard Kilogram (kg) SI Unit of Mass = Kilogram (kg)

Weight:The force of gravity on an object


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NewtONs secONd LawNewtons second law gives the relationship ofacceleration to force and inertia.

Newtons second law:

The acceleration of an object is directlyproportionalto the net force acting on the object, is inthe direction of the net force, and is inversely

proportionalto the massof the object.

The acceleration of an object is always in thedirection of the net force.


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When objects are changing direction or

velocity:If any object is changing direction or velocity (the type of motion),

we say the object is accelerating (the state of motion). This unit

will show the concepts behind accelerations, what causes

acceleration (net force),

Newton's 2nd Law of Motion.


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Experiment : The net force F on a body and theacceleration a of that body are related.Thousands of experiments over hundreds of years find

(object of mass m):

a F/m (Proportionality) We choose the units of force so that this is not just a

proportionality but an equation:a F/m

or F = maF = the net force acting on mass mm = the mass (inertia) of the object.a = acceleration of object. Description of the

effect ofF. F is the cause ofa.

Newtons Laws of Motion


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Example :Force to stop a car.

What average net force is required to bring a 1500-kg car to restfrom a speed of 100 km/h with in a distance of 55 m?



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NewtONs third Law Of MOtiON The law of action and reaction. For every action there is an equal and

opposite reaction.

Whenever one object exerts a force on a secondobject, the second object exerts an equal (inmagnitude) and opposite (in direction) force on thefirst object

action = opposite reactionF1 = - F2


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Example :Rocket propulsion can also be explained using Newtons

third law: hot gases from combustion spew out of the tail of

the rocket at high speeds. The reaction force is what propels

the rocket.

Note that the rocket does not need anything

to push against.

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the e tutor - laws of motion

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