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Forces
Part 1: What are Forces?
What Is A Force?
A force is either a push or a pull.
Forces are examples of vectors.
They have a magnitude and a direction.
What Do Forces Do?
Forces can have 4 effects on an object.
Make it start moving.
Make it stop moving.
Make it change direction.
Make it change shape.
Names of Forces
Contact Forces
Distance Forces
Normal
Gravity
Buoyancy
Electrical
Friction (+ Air Resistance)
Magnetic
Tension
Spring
Applied
Measuring Forces
Forces are measured
using a spring scale.
The strength of a force is
measured in newtons (n).
One newton (1 n) is the
roughly amount of force
used to pick up an apple
off the floor.
Interacting Forces
Forces usually act together.
When 2 forces act in the same direction,
the forces add together.
80
40NN
Interacting Forces
Forces can be balanced or unbalanced.
When
2 forcesforce,
act inor
equal
and opposite
The resulting
net force,
is zero.
they are
balanced.
The directions,
object’s motion
does
not change.
15 N
no net force
Interacting Forces
Forces can be balanced or unbalanced.
Butthe
when
forces
unequal,
Since
net two
force
is notare
zero,
the object
thein
forces
are unbalanced.
will move
the direction
of the net force.
20 N
5N
net force of 5 N to the right
Drawing Forces
Forces are shown with arrows.
The length of the arrow is the size of the force
NORMAL
The direction of the arrow is the direction of the force
APPLIED
FRICTION
GRAVITY
Free-Body Diagrams
Forces are drawn using free-body diagrams.
Free-body diagrams show an object as a
empty box + the forces acting on it.
What forces are
acting on this crate?
FNormal
FGravity
Free-Body Diagrams
The skydiver is accelerating downward.
F Air resistance
FGravity
Free-Body Diagrams
The car is moving at a constant speed left.
FNormal
FApplied
FFriction
FGravity
Forces
Part 2: Friction
Introduction
Friction is caused when 2 surfaces rub together.
Friction is a force that acts in the opposite
direction of a moving object.
Friction will cause an
object to slow down or
stop moving completely.
How Friction Works
Friction is caused by rough surfaces.
No surface is perfectly smooth.
Under a microscope, most
surfaces have rough edges
When surfaces rub together,
these edges catch on each
other, creating friction!
Showing Friction
Friction can be shown on a force diagram:
Showing Friction
Friction can be shown on a force diagram:
FNormal
FApplied
FFriction
FGravity
Types of Friction
The ear is divided
into
3 major sections:
1. Static
Friction
Occurs between objects that aren’t moving.
Types of Friction
The ear is divided
intoFriction
3 major sections:
2. Sliding
When solid objects slide over a surface.
Types of Friction
The ear is divided
intoFriction
3 major sections:
3. Rolling
When objects roll over a surface.
Types of Friction
The ear is divided
into
3 major sections:
4. Fluid
Friction
When an object moves through a fluid.
Fluid Friction = Air Resistance
The fluid friction created when an object
moves through the air is called air resistance.
friction
FApplied
FAir Resistance
Fluid Friction = Air Resistance
friction
The fluid friction created when an object
moves through the air is called air resistance.
FAir Resistance
FGravity
Good & Bad Friction
Friction can be helpful or unhelpful.
“Good” Friction
“Bad” Friction
Brakes
Engine damage
Shoes
Tire wear
Pencils
Running on sand
Sand paper
Reducing Friction
Friction always occurs, but it can be reduced.
There are 2 ways to reduce
friction between surfaces.
Make the surfaces
smoother, such as
using sand paper.
Reducing Friction
Friction always occurs, but it can be reduced.
There are 2 ways to reduce
friction between surfaces.
Add a liquid lubricant to
make the surfaces
smoother and slide easier.
Example: oil
Forces
Part 3: Mass, Weight, and Gravity
Weight
If you wanted to know how much you weigh,
you would just step on a scale.
Metric scales give
measurements in
several units:
Milligrams (mg)
Grams (g)
Kilograms (g)
A Problem With Mass
There is a problem with our everyday wording.
When we describe how much an object
weighs, we actually are measuring its mass.
Mass is a measure of how
much matter something is
made out of.
A Problem With Mass
We say the apple “weighs” 100 g.
But we should really say the
apple has a mass of 100 g.
“Weight” A Minute!
What does weight mean, then?
Weight is a force.
Weight is measured in newtons (n).
Weight is a measurement
of the force of gravity
pulling on down on
an object.
“Weight” A Minute!
This apple has a mass of 100 g.
And it has a weight of 1 n!
1n
Another Example
Take this soda can
and put it on a scale:
The scale shows a mass of 350g.
Take the same soda
can and put it on a
spring scale:
The meter shows a weight of 3.5 n.
Remember!
Mass is a measure of how much
matter something is made out of.
It is measured in mg, g, kg and t.
Weight is a measure of the force of
gravity acting on something.
It is measured in newtons (n).
Losing Weight!
The reason why we have weight on Earth
is because Earth’s gravity pulls on our mass.
Einstein has a
mass of 50 kg
on Earth.
Which equals a
weight of 500 n.
But this situation is different on Mars…
Losing Weight!
Instant weight
loss. Cool!
Einstein still has
a mass of 50 kg:
that doesn’t
change!
But his weight
will be less,
since the force
of gravity is less.
His weight is closer to 400 n on Mars!
That Soda Can, Again!
Our soda can has a
weight of 3.5 n on Earth.
But it will weigh different on other planets!
Earth
3.5 n
Mars
2.5 n
Jupiter
7.0 n
Pluto
1.7 n
The space shuttle has a mass of 4,500 kg!
But in space, it is weightless. Why?
Any Questions?