Chapter 4 Forces and Newton’s 3 Laws of Motion
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Chapter 4
Forces and Newton’s3 Laws of Motion
Isaac Newton (1642-1727)
Isaac Newton (1642-1727)
Isaac Newton is without a doubt one of the most influential
men in history.
Just a few of his accomplishments:
•Built the first practical reflecting telescope
•Developed a theory of color including the idea that white
light is composed of all colors of the rainbow
•Studied the speed of sound
•Developed calculus from scratch!
•Defined the 3 Laws of Motion that govern all objects
•Studied the effects of gravity (story about the apple)
Force
A force is a push or a pull.
Force is not a thing in itself, but rather an interaction between two objects.
Force is a vector quantity… direction matters in the answer!
Common Forces
Common Forces
Newton’s First Law
“The Law of Inertia”
A body remains at rest or moves in a straight line at a constantspeed unless acted upon by a netforce. Objects do not accelerate unless a net force is
applied.Speed up, Slow down,
or change direction
Newton’s First Law
Inertia is a property of an object most closely related to it’s mass(not to be confused with momentum) that explains why objects with greater mass resist a change in motion more than those with a lesser mass.
Net Force
Net force is the vector sum of ALLforces acting on an object. If there is zero net force, then there is zero
acceleration (constant velocity), this is a special case called equilibrium.
If there is a net force, there will be an acceleration. That means that the object will be speeding up, slowing down, or changing direction.
Free Body Diagrams A Free Body Diagram is a simple drawing that
shows the magnitude and direction of all of
the force vectors acting on an object.
The length of the arrows in relation to each
other is VERY important.
Each arrow must point away from the “free
body” and be labeled appropriately.
The system, the object the force is applied to,
is drawn as a shaded circle.
Free Body Diagrams
Here is an example of a
FBD of a book at rest on
a table top.
Fg is acting downward
but is “balanced” by FN
acting upward.
Results is no net force
and zero acceleration.
Fg
FN
The
book is
drawn
as a ball
Free Body Diagrams Here is an example of a
FBD of a box being
pulled by a rope at a
constant speed on a flat
surface.
Fg and FN are still
opposite and equal.
FT and Ff are also
opposite and equal.
Fg
FN
FTFf
Object is in motion,
but not accelerating.
Free Body Diagrams Here is an example of a
FBD of a ball under free
fall conditions.
Fg is the only force acting
on this object.
The net force is down
and the object is
accelerating.
Fg
Object is in motion
and accelerating.
Review Questions: p. 89 #1-5Draw a FBD for the following situations:
1. A flowerpot falls freely from a windowsill. (Ignore any
forces due to air resistance.)
2. A sky diver falls downward through the air at constant
velocity. (The air exerts an upward force on the person.)
3. A cable pulls a crate at a constant speed across a
horizontal surface. The surface provides a force that
resists the crate’s motion.
4. A rope lifts a bucket at a constant speed. (Ignore air
resistance.)
5. A rope lowers a bucket at a constant speed. (Ignore air
resistance.)
Review Questions Answers
#1 #2
Fg
Fg
Ff
Review Questions Answers
#3 #4
Fg
FT
Fg
FN
FTFf
Review Questions Answers
#5
Fg
FT
Newtons’ Second Law
The acceleration of an object is directly
proportional to the net force acting on
the object, and inversely proportional to
the mass of the object.
a Fnet
m a m
Fnet
Newton’s Second law of Motion …mathematically
Net Force = (mass)(acceleration)
Fnet = ma
NEWTON'S 2nd LAW
Fa
or amF
aFm
F am
m
F a
m
m
m
ma
1
F a
F a
F aM
M
M
Newton’s 2nd Law Practice
Two horses are pulling a 100-kg cart in the same direction, applying a force of 50 N each. What is the acceleration of the cart?
A. 2 m/s2
B. 1 m/s2
C. 0.5 m/s2
D. 0 m/s2
Answer B
Reason: If we consider positive direction to be the direction of pull
then, according to Newton’s second law,
netnet= ,Since 50 N 50 N 100 N,
Fa F
m
2100 N= =1m/s
100 kga
Newton’s Third Law
Action-Reaction LawTwo forces that make up an interaction pair of forces are equalin magnitude, but opposite in direction and act on differentobjects.
Newton’s Third Law
For every action, there is
always an equal (magnitude)
and opposite (direction)
reaction.
By “action” or “reaction”, we
mean a force.
Action/reaction forces do not
act on the same object.
Action: rocket pushes on gasesReaction: gases push on rocket
Action: tire pushes on road
Reaction: road pushes on tire
Identify at least six pairs of action-reaction
force pairs in the following diagram:
WeightLocation Mass
Earth
Moon
Orbiting
Earth
18.4 kg
18.4 kg
18.4 kg
180 N
30 N
0 N
1/6 of
Earth’s
Mass
Mass is the amount of matter in an
object (not to be confused with weight)
Also considered a measure of the
inertia of an object
measured in SI unit of kilograms
(kg)… if mass is given in grams you
must convert!
Weight
Weight is the downward force
upon an object due to
acceleration caused by gravity
weight = mass acceleration due to gravity
Fg = mg
measured in Newtons (N)
The weight of a 10 kg brick is...
A) 98 N
B) 10 kg
C) 9.8 kg
D) 10 N
E) 98 kg
Mass and Weight
On the Moon, the force of gravity is only 1/6 as strong as on the Earth. (approx. 1.63m/s2)
While orbiting, you are practically weightless but your mass remains unchanged.
Your mass does not depend on where you are. e.g. Earth, Moon, or space
Falling with Air Resistance
Air resistance (drag
force) increases with
speed and increased
cross-sectional area
and can be affected
by the size and shape
of an object.
Terminal VelocityAcceleration = g
Acceleration < g
Acceleration << g
Acceleration = 0
v = 0but motion is about to
begin
v increasing
downward
v still increasing
downwardjust not as rapidly as
before
terminal
velocity
Net Force
ZERO
Terminal Velocity
Terminal velocity occurs when the drag force of air
resistance becomes large enough to balance the force of
gravity.
At this instant in time, there is no net force — the object
stops accelerating (see D below); terminal velocity has been
reached.
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