Force
A force is a push or a pull.
Pushing on a stalled car is an example.
The force of friction between your feet
and the ground is yet another.
Force
Weight is the force of the earth's gravity
exerted downward upon your body.
It is true that the more mass you have, the more
you weigh.
Weight and mass are proportional but they are
not the same thing!
All forces have a direction.
Weight, being a force, always points down,
toward the center of the earth.
Mass has no direction.
Force vs. Mass
Mass is related to how much matter there
is in an object.
An anvil has a
great deal of mass,
and here on Earth
it weighs quite of
bit, too.
Force vs. Mass
But in space, where
there is no gravity,
the astronaut and the
anvil are both
weightless.
They both have the
same amount of
mass they did on
earth.
Mass and Weight
Weight is the force of gravity exerted upon a mass.
Weight and mass are related, but are not the same.
An object's weight depends upon two things:
1. its mass
2. the acceleration of gravity in the part of the
universe it occupies.
Weight is Proportional to Mass
Here is the equation for weight.
weight = mass x acceleration of gravity
Weight also Depends on Gravity
On the surface of the earth, the acceleration
of gravity is 9.8 m/s2.
It is quite a bit less on the moon: 1.6 m/s2.
It is too small to notice in interstellar space.
Consequently, an object from Earth would
weigh less on the Moon, and would weigh
almost nothing in space.
Metric Units of Weight
Many people mistakenly use the “kilogram”
as a unit of weight. This is wrong. The
kilogram is actually a unit of mass.
w = mg
w = (1 kg)(9.8 m/s2)
w = 9.8 Newtons
1 kilogram actually
weighs 9.8 newtons.
Isaac Newton
The metric unit of force is named after Isaac
Newton (1642-1727), one of the greatest
figures in all of science.
His mathematical
description of the
world went
unchallenged until
the twentieth
century.
Newton's First Law of Motion
An object at rest tends to remain at rest
and an object in motion tends to move in
a straight line at a constant speed, unless
acted upon by an unbalanced force.
Why Things Move
Force is a push or a pull.
An unbalanced force
causes an object to
accelerate.
The direction of a force is just as important
as its size.
Why Things Move
If the unbalanced force is in the same
direction that the object was already
moving, then the object speeds up.
Force
motion
Why Things Move
If the unbalanced force is in the opposite direction
as the object was moving, then it slows down.
motion
Force
Why Things Move
If the unbalanced force is applied sideways to
the object's motion, then the object will change
direction.
Why Things Move
When an unbalanced force is applied
to an object, it will either speed up,
slow down, or change direction.
All of these results are called
acceleration.
Why Things Move
When a single
force is applied
to a motionless
object, it will
cause the
object's motion
to change.
The single force was unbalanced.
The football will now accelerate.
Why Things Move
The direction of a force is important.
Two equal forces,
applied to the
same object but in
opposite
directions, cancel
each other out.
In this case, there is no unbalanced force, and the
object has no change in its motion.
Why Things Move
The term “net force” has the same meaning as
“unbalanced force”.
To find the net force means to sum up the
effects of all the forces acting upon an
object.
Only a net force can change the motion of
an object.
Why Things Move
When you push on a book,
the force of friction resists you.
What is the net force acting on this book?
Why Things Move
Two students try to push a stalled car.
What is the net force acting upon this car?
Why Things Move
The larger the net force, the greater the
acceleration becomes.
Why Things Move
If the net force is doubled, that causes the
acceleration to double.
We say that the acceleration is directly
proportional to the net force.
a ∝ Fnet
Why Things Move
In the previous example, we had to hold the
mass of the object constant while we changed
the applied force.
What do you predict would happen
if we used the same net force on a
larger amount of mass?
Why Things Move
The larger the mass, the smaller the amount of
acceleration.
Why Things Move
Note that the objects still speed up, but not as
quickly as before.
In fact, if you double the mass, the acceleration
of it becomes only half as great.
The acceleration is proportional to the
inverse of the mass.
1
a
m
∝
nd
Newton's 2 Law of Motion
Combining both proportionalities into one
equation:
Action-Reaction
Forces come in pairs!
Whenever an object exerts a force on another
object, the other object exerts the same amount
of force back on the first, but in the opposite
direction.
rd
This is known as Newton's 3 Law of Motion.
It is popularly known as the
law of action-reaction.
rd
Newton's 3 Law
Newton's 3rd Law is always true,
even when the objects are different sizes...
Hammer's force
drives the nail;
Nail's force
decelerates hammer.
Why Things Move
Newton's 3rd Law is always true, even when
one or more of the objects doesn't move.
You push on the
wall;
the wall pushes
back on you.
Why Things Move
rd
Newton's 3 Law of Motion, also known
as the Law of Force Pairs:
For every force, there is a reaction force equal
in magnitude but opposite in direction.
If object A exerts a force on object B, then
object B exerts the same amount of force back
on object A.
rd
Newton's 3 Law
Action-reaction force pairs:
Not-so-obvious Forces
Forces that are often overlooked:
The normal force is
the force the table
exerts on the cement
block in reaction to
the block pressing
down upon it.
Not-so-obvious Forces
More forces that are easily overlooked:
If the force of
friction is as large
as your “push”,
then there is no
unbalanced force
and the block will
not accelerate.
It will move forward at constant speed.
The Free-body Diagram
The five forces acting upon an automobile.
Normal
force
Friction with
the air
Drive force
Rolling
friction
Force of gravity
(weight of car)
Explaining Changes in Motion
How those five forces combine (sum up) will
determine how much, if any, net force is exerted
on the automobile.
The magnitude of that net force, along with its
direction will determine what will happen next to
the car.
In other words, whether it will speed up, slow
down, change direction, or just continue to go
ahead at constant speed.
Explaining Changes in Motion
Imagine you are taking a trip in your car.
See if you can use Newton's Laws of Motion to
explain what will happen when you...
Stop at a stop light.
Accelerate when the light turns green
Make a left turn
Jam on the brakes when a pedestrian steps in
front of your car