Newton’s Laws of Motion
PSC 1121
Chapters 2-4
Newton’s Laws of Motion
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Chap 2 – First Law – Law of Inertia
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If you want to move something, you apply a force to it; you either
push or pull it
Aristotle – 4th century BC – most famous Greek scientist classified
motion into 2 kinds:
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Natural – went up or down;heavy objects fall faster than light objects
(ex.: boulder falls down, smoke goes up)
Unnatural – required forces – like pushing a cart
Ideas taken as facts for approx. 2000 yrs.
Galileo – 1500
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Expt at the Leaning Tower of Pisa – he dropped a heavy object and a
light object and showed that they fell to the ground at the same time
He said a force is required to start an object moving, but once moving
no force is required to keep it moving
Rolling balls expt. – reasoned that a ball moving horizontally would
move forever if friction were entirely absent
Newton’s Laws of Motion
 Tendency of things is to remain as they are: if moving,
they tend to remain moving, if at rest, they tend to remain
at rest
 This property of objects is called inertia
 Speed = distance covered per unit time = distance/time
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if you run 6 meters in 1 second your speed is 6m/s
 Velocity – vector quantity; tells you how fast and in
what direction
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A car travels at 60 km/h = speed
If it travels at 60 km/h to the north = velocity
Motion is relative
 When we say a space shuttle moves at 30,000 km/h we
mean relative to the Earth below, a racing car reaches a
speed of 300 km/h, we mean relative to the track.
Newton’s Laws of Motion
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The Law of Inertia
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Isaac Newton was born on Christmas day in the year Galileo died.
His laws of motion were built on Galileo’s findings
First Law – Every object continues in a state of rest, or in a
state of motion in a straight line at constant speed, unless it
is compelled to change that state by forces exerted upon it.
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Objects at rest tend to remain at rest unless a force is applied to set
them in motion
When an object is moving its tendency is to remain moving along a
straight line path if no friction acts against it.
Net Force
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Combination of all forces that act on an object
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Ex: when you throw a basketball, you have 3 forces acting on it: force of
gravity, air friction, pushing force you apply with your muscles (Fig. 210)
Newton = N = scientific unit of force
Forces are vector quantities
Newton’s Laws of Motion
 Equilibrium for objects at rest
 Net force = 0
 Forces acting upward on something at rest must be
balanced by forces acting downward (Fig. 2-11)
 The support force
 A book lies at rest on a desk – it is in equilibrium,
therefore net force = 0
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Force of gravity = weight of the book
Support force = the other force acting on the book to
cancel out force of gravity; it is coming from the desk
that supports the book
 ex: bathroom scale; force 1 = your
weight,downward pull of gravity, force 2 = upward
support force of the floor
Newton’s Laws of Motion
 Equilibrium for Moving objects
 An object moving at constant speed in a straight-line
path is also in equilibrium because two opposing
forces are acting on it
 It is known as dynamic equilibrium
 A crate being pushed horizontally across a factory
floor is in equilibrium if it moves at constant
velocity
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2 forces acting on the crate – force of friction between
crate and floor, and our pushing force
 Force 1 = force 2, net force = 0
 Static equilibrium refers to objects at rest
Newton’s Laws of Motion
 Chap. 3 – second Law – Force and Acceleration
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Acceleration- developed by Galileo;experimented with
inclined planes
 Acceleration = change in velocity/time interval
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Involves a change in velocity whether it is to speed up or slow
down (negative acceleration)
It is a change in speed, change in direction, or change in both
speed and direction
Ex: you are driving, in 1 sec you increase your velocity from 30
km/h to 35 km/h; in the next sec, you go from 35 km/h to 40
km/h; net result is you change velocity by 5 km/h each sec
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Accel = 5 km/h/1 sec = 5 km/h-s
 Force causes acceleration
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Acceleration is directly proportional to net force
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Acceleration ~ net force
Newton’s Laws of Motion
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If you push with 25 N on an object, and somebody else pushes
in the opposite direction with 15 N, the net force applied to the
object is 10 N
The object will accelerate as if a single 10-N force acts on it
Direction of the object is always in the direction of the net force
Acceleration also depends on mass of object being pushed or
pulled
Mass – measure of inertia
 The greater the mass of an object, the greater the
inertia; it takes a lot more force to get an elephant on
wheels to accelerate than it would take to get a
skateboard to move
 Mass is a measure of how much material an object
contains; measured in kilograms
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Different from volume which is a measure of space, reported
in cubic centimeters, cubic meters, or liters
Newton’s Laws of Motion
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Ex: equal size bags of cotton and rocks may have equal
volumes but unequal masses
Different from weight; weight depends on gravity
 you would weigh less on the moon than on Earth
because the moon’s gravity is weaker than Earth’s
Mass does not change if gravity varies
Mass and weight are directly proportional to each
other; objects with large mass have large weight,
objects with little mass have little weight
 1kg = 10 N (rounded off to 10 from 9.8)
 1 kg= 2.2 lb
 Mass resists acceleration
 For a given force acceleration is inversely
proportional to the mass
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Acceleration = 1/mass
Newton’s Laws of Motion
 Newton’s second Law
 The acceleration produced by a net force on an
object is directly proportional to the net force,
is in the same direction as the net force, and is
inversely proportional to the mass of the object
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Acceleration = net force/mass
a= F/m
F= net force in N, m = mass in kg, a =
acceleration in m/s2
Ex: 1000 kg car pulled by a cable with 2000N of force.
What will be the acceleration of the car?
 A = F/m = 2000N/1000 kg =
 2000kg-m/s2/1000kg= 2 m/s2
Newton’s Laws of Motion
 Friction is a force that affects motion
 Friction occurs when one object rubs against
something else
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Always acts in a direction to oppose motion
When an object falls downward through the air, force of
friction (air drag) acts upward
When you pull on a crate and it slides across a floor,
both your force and opposite force of friction affect the
motion – when you pull hard enough to match friction,
net force on the crate is Zero and it slides at constant
velocity
 Objects in free fall have equal acceleration
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Free fall is falling only under the influence of gravity;
inclined planes slow down acceleration
Acceleration of free fall doesn’t depend on mass
Newton’s Laws of Motion
 Why objects in free fall have equal acceleration
 a= F/m
 The ratio of weight to mass is the same for any
object
 All free falling objects have the same force/mass
ratio and undergo the same acceleration at the same
location
 Effect of air drag on acceleration
 Air drag depends on speed and surface area
 For free fall, downward net force = weight – air drag
 If air drag = weight, net force = 0, acceleration = 0
so object no longer picks up speed
 Object has reached terminal speed
Newton’s Laws of Motion
 Or if concerned with direction, like a sky diver falling,
we say he has reached terminal velocity
 Increasing surface area reduces terminal speed: a
parachute greatly increases air drag and terminal
speed can be reduced to 15-25 km/h
Newton’s Laws of Motion
 Chap. 4 – Newton’s third Law: Action and
Reaction
 Force – interaction between one thing and another
 force always occurs in pairs
 Non-living things can exert forces
 Ex: a truck crashes into a car; there is an interaction
between the truck and the car – forces are equal in
strength and opposite in direction and they occur at
exactly the same time
 When you push on a wall, there is an equal force
pushing back on your hand
Newton’s Laws of Motion
 Third Law- whenever one object exerts a force on a
second object, the second object exerts an equal and
opposite force on the first
 “To every action, there is always an opposed
equal reaction”
 Neither force exists without the other
 Ex: car travelling on a road
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Action: tire pushes on the road
Reaction: road pushes on tire
 Objects of different masses
 a = F/m
Newton’s Laws of Motion
 Action and Reaction forces on different objects
 Forces cancel only when they act on the same body
 If you kick a football the action is force your foot
exerts on the football- so the football accelerates;
the reaction is the football exerting a force on your
foot
 You can’t cancel the force on the football with a force
on your foot