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Forces and Motion
Chapter 23
What is motion?
Motion- an object changing position
Types of Motion
There are 2 types of motion:
Distance
Displacement
Distance
Distance is the total path traveled. Look at the
map- Is the dotted line or the straight line
distance?
Displacement
This is the distance and direction between the
starting and ending point.
It is the straight line distance between start
and finish.
Relative Motion
How do you know what direction something
is?
Is Florida north or south?
Is Alaska north or south?
Relative Motion
Motion must have a reference point. This
means you must compare the motion of an
object to something else.
Describing Motion
There are 3 main ways to describe the motion
of an object.
Speed
Velocity
Acceleration
Speed
Speed is distance traveled divided by the time
taken to travel the distance.
speed (in meters/second)= distance/time
s= d/t
Speed
Constant Speed= the speed at any given time
will be the same.
When can we achieve this?
Changing Speed- When your speed is
changing, not constant. We call this
instantaneous speed.
What measures this for us?
Calculating Speed
Here is a simple way to solve speed
equations!
Practice Problems!!!
Velocity
Velocity= speed with a direction
15 mph NE
It is calculated just like speed except with a
direction!
Acceleration
Acceleration is the change in velocity.
Acceleration can occur 3 ways:
Speed Up
Slow Down
Change Directions
Calculating Acceleration
To calculate acceleration you must know:
Final Speed
Starting or Initial Speed
Time
Acceleration Equation
The equation looks like this:
a= sf - si
t
Forces
An object motion changes because of forces.
A force is a push or a pull
A force has a size and a direction.
Why must we have both?
Forces
Contact Forces- a force that is exerted when
two objects are touching each other.
Long range forces- a force exerted when the
objects are NOT in contact.
Can you think of any examples.
Examples- Gravity, Magnetism, Electricity
Unit of Forces
Forces are measured in newton (N).
It is named after Isaac Newton.
One Newton is equal to the force it takes to lift
half a cup of water.
Newton’s First Law of
Motion
This law states: “An object remain at rest or
move in a straight line with constant speed
unless it is acted upon by a force”
Inertia and Mass
The first law is often called the law of inertia.
Inertia= an object’s tendency to resist a
change in motion. It wants to keep moving or
sit still.
What causes inertia?
inertia=mass
Adding Forces
Motion depends upon the size and direction of
all forces acting on an object.
Balanced Forces- Forces that are equal but in
opposite directions.
No motion will occur
Unbalanced Forces
Unbalanced Forces- Forces of an object are
not equal and opposite.
Motion will occur in the direction of the
largest force!
Newton’s Second Law
This is used to predict where and how motion
will occur!
Newton’s Second Law
This law states: “An object acted on by an
unbalanced force will accelerate in the
direction of the force.”
Newton’s Second Law
Newton’s Second Law says that the
acceleration of an object can be calculated by
the following:
Acceleration= force (N)
mass (kg)
How can you use the
Second Law?
You can determine the acceleration of an
object if you know the mass and the force.
OR
You can determine the force that is needed to
make an object accelerate if you know the
acceleration and mass!
Gravity
Every object exerts an attractive force on
every other object!
Gravitational Force- the attractive force
between two objects, it depends on two things:
Mass (More Mass=More Gravity)
Distance (Closer Together=More
Gravity)
Gravity
The acceleration due to gravity is ALWAYS 9.8
m/s2
Mass vs. Weight
What is the difference between mass and
weight?
Mass
The amount of matter in an object
This cannot change!
Weight
This is the force of gravity on an object!
This can change...
On Earth, 150 lbs.
On other planets
Mercury- 56.7
Jupiter- 394
Venus- 136
Saturn- 179
Moon- 24
Uranus 133
Mars- 56.5
Neptune- 168.7
Friction
Friction- a force that resists sliding motion
between surfaces that are touching.
There are 4 types of friction.
Static Friction
Static friction is keeps an object at rest.
Sliding Friction
This is the friction that occurs while an object
is in motion.
It typically slows down objects.
When would you want to increase or
decrease sliding friction?
How would you do this?
Rolling Friction
Rolling Friction allows a wheel to roll on a
surface.
Walking is an example of rolling friction.
Air Resistance
This is also called fluid friction.
Molecules in the air collide with forward
moving objects. This will slow down motion.
This is called air resistance.
It acts as a force in the opposite direction of
motion.
Newton’s 3rd Law
This law states- “Forces always act in equal
but opposite pairs”
You probably know it better, “For every action
there is an equal and opposite reaction!”
Action and Reaction
Forces
Any time there is motion there are 2 forces!
When you jump, what are the forces?
You push on the ground, it pushes up
on you.
Using the 3rd Law
These are not balanced forces.
These forces act on different objects!
When you kick a ball a force is put on
your foot and an equal one on the ball.
That is why the harder you kick the
more it hurts.
Think about this...
If Newton’s 3rd Law is true, then why don’t we
slide backwards when we push a door closed?
Because of friction. When the door pushes
you it is also pushing on the Earth!
What about Gravity?
Do we exert the same force on the Earth as
the Earth’s gravity exerts on us?
So why don’t we move when we jump into a
swimming pool?
Because we have so little mass!