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# Forces

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by

## Jeff Berg

on 9 May 2018

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#### Transcript of Forces

Forces
May the FORCE be with you!
28
29
30
31
LT 28 - I can describe how
balanced
and
unbalanced

forces affect an object's motion.
Force
A push or a pull.
3 Types of Forces
Contact Force
Gravity
Friction
Contact Force
A push or pull through physical contact.
Gravity
The force of attraction between two objects.
The force of gravity depends on an objects mass.
Friction
A force that resists motion between
two surfaces that are touching.
high friction
low friction
A
vector
(arrow) shows
size
and
direction
of force.
30 N
size = 30 Newtons
direction = right
10 N
Size =

direction =
10 Newtons
Up
Net Force
The overall force acting on a object when all the forces are combined.
10 N
30 N
Net Force = 20 N to the right
How to solve this problem:

Subtract the two forces because they are acting in
opposite directions
.
30 N - 10 N = 20 N
(right)
(left)
(right)
If they were pointing in the same direction you would add them!
Balanced Force
If the net force equals
zero
, the forces are
balanced
.
Has the same effect as
no force
at all.
Motion of the object
does not change
.
5 N
5 N
no motion
Unbalanced Force
Can (and will)
change
the
motion
of an object.

Balanced Force on a Moving Object
Balanced forces
can't change
an objects
speed
or
direction
.
If balanced forces are acting on a moving object, the object will maintain it current velocity (speed and direction).
13 N
13 N
38 N
If a toy car travels across a table with a force of 38 N between two fans pointed at each other both with a force of 13 N what force will the car have after it passes the fans?
38 N (up)
Unbalanced Force on a Moving Object

An unbalanced
force
is needed to
change
an object's
motion
.
200 N
100 N
motion
I an
unbalanced force
acts on a moving object, the object's velocity will change causing the speed to increase, decrease, or change direction.
Newton's 1 Law of Motion
st
An object at rest will remain at rest, an object in motion will remain in motion with the same velocity, unless acted upon by an unbalanced force.
Inertia
The resistance of an object to change
its speed or direction of its motion.
LT 29 - I can describe and predict how
the
sum
of the
forces
affects
the
motion
of an object.
LT 30 - I can identify
forces
acting on an object as either a direct
contact
force or forces acting at a
distance
.
Gravity is a
force
exerted by
masses
.
Gravity is considered universal force because it acts between any two masses anywhere in the universe.
The Force of Gravity
As the
mass
of an object
increases
, the force
of gravity
increases
.
As the
distance
between two objects
increases
, the force of gravity decreases.
Two of the same objects one dropped and one thrown horizontally from the same height will fall at the
same rate
.
If a hunter shoots at a monkey right as a
monkey lets go of a branch, will the bullet
hit or miss the monkey?
Hit the monkey!
In a vacuum, where there is no air, all falling objects have the same
acceleration
.

Gravity is a
downward
force, therefore only
changes the downward speed.
LT 31 - I can distinguish between
mass
and
weight
.
Weight
The force of
gravity
on an object
Measured in pounds.
Measured with a scale (uses springs).
Will
change
depending on the
amount of
gravity
.
Mass
Mass is a measurement of the
amount
of
matter
an object contains.
Measured in grams.
Measured with a balance.
Will
not change
depending on your
distance from the surface of the Earth.
Mass 50 kg 50kg
Weight 110 lbs 17.6 lbs
Earth Moon
A spacecraft in orbit is in free fall towards the Earth. The astronauts inside the spacecraft are falling at the exact same rate behaving as if there were no gravity.
Friction
is a
force
that
opposes

motion
between two surfaces in contact.
Friction can make some tasks more difficult, but most activities would be impossible without friction.
There are several factors that determine the friction between two surfaces in contact.
1 - Types of surfaces: Different combinations of
surfaces
produce different frictional forces.

2 - Motion of surfaces: You need a
larger
force to start something moving than you do to
keep
it something moving.

3 - Force pressing the surfaces together: The
harder
two surfaces are pushed together, the more
difficult
it is for the surfaces to slide over each other.

Air Resistance
Friction due to air particles.
Factors that affect air resistance:
1 - Surface area - the greater the
surface
in contact with air, the
more air
needs to be pushed out of the way.

2 - Speed - the
faster
an object is moving the
greater
resistance of air to move out the way.

Name some forces that act at a distance.
Gravity
Magnets
Static
Electricity

Summary -
Gravity
is a universal force of
attraction
between any two objects with mass. Gravity acts on all
objects
everywhere.
Friction
is a force that
opposes
the movement between two surfaces.

Pushing a wheelchair
Horses pulling a wagon
More mass =
More inertia

Less mass =
Less inertia
Summary:
Forces act on all objects all of the time. Balanced forces
DO NOT
change the motion of objects. Unbalanced forces
WILL
change the motion of the object.
Friction is a force that
resists
the
motion
between two surfaces in contact.
Full transcript