Newton’s Three Laws of

Motionare laws that relate force and motion

Newton’s First Law of Motion

(Law of Inertia) – “The motion of

an object will not change unless an

unbalanced force will act on it.”

Newton’s Second Law of Motion

(Law of Acceleration) – “The

object will accelerate in the

direction of the net force that will

act upon it.”

FR = ma

Newton’s Third Law of Motion

(Law of Action-Reaction) – “If an

object exerts a force on another

object, the second will react

exerting an equal but opposite

force on the first.”

First and Third Laws

are used in the study of equilibrium

conditions

Second Law

will be used to find the resultant

force acting on object and the

acceleration which is the result of this

force

If the acceleration is due to

gravity

FR = mg

this force is called Weight, W

UNITS OF FORCE

N = kg m/s2

dyne = g cm/s2

lb = slug ft/s2

1N = 1 x 105 dynes = 0.2248 lb

EXERCISES

1. A 600-N object is to be given an

acceleration of 0.70 m/s2. How large

an unbalanced force must act upon

it?

2. A force gives a 0.32 –slug object an

acceleration of 79 ft/s2. If the same

force is applied, what is the

acceleration of 1.4-slug object?

3. Find the weight of a body which is

accelerated 5 m/s2 by a net force of

30 newtons.

4. A body starting from rest acquires a

velocity of 18 cm/s in 3 seconds. If

90 dynes of force acts upon the

body, what is its weight?

5. An object of mass 30 g is subjected

to a force of 5 dynes. If the body

starts from rest, how far will it travel

after 8 seconds?