Download Chapter One Notes

FORCE
and
Newton’s
Laws of Motion
Definitions of Force
Force is a push or a pull acting
on an object.
Force is a vector.
(magnitude & direction)
Force can be contact or long
range (aka non-contact or
field).
Types of Forces
Contact Forces
Long Range
(Field) Forces
Friction (Ff)
Gravity or weight
Tension (FT)
(FW or Fg)
Normal (FN)
Electrical (FE)
Spring (FS)
Magnetic (B)
Air Resistance (FR)
Applied (FA)
More on Force…
Forces can change the motion of
an object
SI unit = newton (N)
Net force is the vector sum of all
forces acting on ONE object.
Since force is a vector, the net
force will have magnitude and
direction.
More on Net Force
Forces in same direction: add
Net force in same direction as forces.
See picture 2 below
Forces in opposite directions:
subtract.
Net force in direction
of larger force.
See picture 3 below
Equal forces in opposite
directions :
subtract.
Net force is zero
See picture 1
What is the net force acting on
each object below?
http://www.slideshare.net/meenng/forces
Balanced forces
Net force = 0
Balanced forces do NOT cause any change in an object’s
motion.
Balanced forces
Net force = 0
Unbalanced force
Net force ≠ zero
Causes an object to change its motion
Many unbalanced forces causing motion: video
Balanced and unbalanced
forces, cont.
Two tugboats are moving a barge. Tugboat
A exerts a force of 3000 Newtons on the
barge. Tugboat B exerts a force of 5000
Newtons in the same direction. What is the
combined force on the barge?
Draw arrows showing the individual and
combined forces of the tugboats.
Balanced and unbalanced
forces, cont.
Now suppose that Tugboat A exerts a force
of 2000 Newtons on the barge and Tugboat
B exerts a force of 4000 Newtons in the
opposite direction. What is the combined
force on the barge?
Draw arrows showing the individual and
combined forces of the tugboats.
Balanced and unbalanced
forces, cont.
Could there ever be a case when Tugboat A
and Tugboat B are both exerting a force on
the barge but the barge doesn't move? Draw
arrows showing the individual and
combined forces in such a situation.
Newton’s
st
1
Law
Galileo’s Law of Inertia: An object

at rest will stay at rest, and an object in
motion will stay in motion at constant
velocity, unless acted upon by an
unbalanced force.
Concerned with the force on ONE
object
The motion of an object changes
only when there are unbalanced
forces (Net force is NOT zero)
Newton’s 1st Law, cont.
What two kinds of motion can an
object have if the forces are
balanced?
1)
2)
An object at rest stays at rest
An object in motion stays in
straight line motion at a
constant speed (constant
velocity)
Balanced Forces said
another way…
Equilibrium: Net Force = Zero
1) Static Equilibrium
oNet force = zero
oNo motion
2) Kinetic Equilibrium
oNet force = zero
oConstant speed in a straight line
(constant velocity)
st
1
Newton’s
Law
Summary
- Also known as the Law of
Inertia
- Inertia is an object’s resistance
to change in motion
- More mass = more inertia!
Newton’s
st
1
Law & You
Don’t let this be you. Wear seat belts!
Because of inertia, objects (including you)
resist changes in their motion. When the car
going 80 km/hour is stopped by the brick
wall, your body keeps moving at 80 m/hour.
Newton’s
nd
2
Law
Net Force equals mass times
acceleration.

Fnet = ma
Fnet = net force (N)
m = mass (kg)
a = acceleration (m/s2)
Newton’s
nd
2
Law and
Acceleration due to gravity (g)
On Earth’s surface:
g = 9.8 m/s2
On the moon’s surface:
g = 1.6 m/s2
Newton’s
nd
2
Law and
Weight
Symbol = FW or Fg
Weight is the force of gravity on
an object.
Unit – Newton (N)
Formula
FW= mg
Examples




1. What acceleration will result when a 12 N
net force applied to a 3 kg object?
2. A net force of 16 N causes a mass to
accelerate at a rate of 5 m/s2. Determine the
mass.
3. How much force is needed to accelerate a
66 kg skier 1 m/sec2?
4. What is the force on a 1000 kg elevator
that is falling freely at 9.8 m/sec2?
Check Your Understanding

1. What acceleration will result when a 12 N net
force applied to a 3 kg object?
Given
FNet = 12 N
m = 3 kg



Unknown
a=?
Equation
FNet = ma
a = FNet/m
Substitute
a = 12 N /3 kg
Equation
FNet = ma
m = FNet /a
Substitute
m = 16 N /5 m/s2
Equation
FNet = ma
Substitute
FNet = 66 kg(1 m/s2)
Solve
4 m/s2
2. A net force of 16 N causes a mass to accelerate at a
rate of 5 m/s2. Determine the mass.
Given
FNet = 16 N
a = 5 m/s2
Unknown
m=?
Given
m = 66 kg
a = 1 m/s2
Unknown
FNet = ?
Given
m = 1000 kg
a = 9.8 m/s2
Unknown
FNet = ?
Solve
3.2 kg
3. How much force is needed to accelerate a 66 kg
skier 1 m/s2?
Solve
66 N
4. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/s2?
Equation
FNet = ma
Substitute
FNet = 1000 kg(9.8 m/s2)
Solve
9800 N
Newton’s
rd
3
Law
For every action force there is an equal
and opposite reaction force.

◦Forces are always in balanced
pairs
◦TWO objects acting on each
other
https://www.youtube.com/watch?v=8bTdMmNZm2M&index=4&list=PLZHDrmpjw4MxSWeF9fklm_AgvMm
zkL2I7
Newton’s
rd
3
Law, cont.
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.
Newton’s 3rd Law, more examples
The finger pushes on
the wall and the wall
pushes on the finger.
https://karlamamporte.wordpress.com/
http://web.eng.fiu.edu/mathmatters/m2ed/newtons3rdlaw/index.htm
The rocket pushes on
the gases and the gases
push on the rocket.
http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/space_for_reflection/action_reaction/revision/1/