Download Motion and forces introduction PowerPoint

Getting an A in Science
 Taking
ownership or control of the information
learned in class and taking responsibility for
your work

This means:
• Practicing (reviewing, restating, preparing)
• Applying (creating, thinking, using)
• Planning (studying, completing projects and homework,
being prepared-pencils, books)
Intro to Physics
 What
is physics?

https://www.youtube.com/watch?v=2vYI2NcVsXY

https://www.youtube.com/watch?v=Nd4PlvzMsaQ
Motion & Forces
Chapter 2
Motion
 What
does motion mean?
 How can we tell if something is in motion?
 Is this marble in motion?
 How do you know?
 How can we provide evidence?
Motion
 Motion: a
change in position, measured by
distance and time.
Frame of Reference
•
•
Determining if an object changes position
requires a frame of reference.
Frame of reference is the object or point from
which movement is determined
What is the frame
of reference for
this video? How
do I know the
skater is in
motion?
Frame of Reference
 Movement
is relative to an object that
appears stationary
 Earth is the most common frame of
reference
Motion of the Marble
 We
can tell if the marble is in motion by
measuring its change in position along the
ruler (our frame of reference).
 Right now, the marble is NOT in motion. It
is not changing position. How can we get
it to change position?
How to make things move
 https://www.youtube.com/watch?v=LEs9J
2IQIZY
Force
 A force
is a push or a pull
 Forces can affect motion
in several ways.
 Forces can make objects:
• Start moving
• Move faster
• Move slower
• Stop moving
• Change direction
• Change shape
What forces are at work…
 During



a rocket launch?
Upwards force (thrust) from
engines
Downwards force from
gravity
Downwards force (drag)
from air resistance
What forces are at work…

pushing a truck?




Forwards force
from pushing
Backwards force
from friction
Downwards force
from gravity
Upwards force
from ground.
What forces are at work…
Kicking a football?
Upwards force from kick
Downwards force from gravity
Fair
Backwards force from air friction
Fkick
Fgrav
Friction
 Friction
force that opposes motion between 2 surfaces
It depends on the:
• types of surfaces

• force between the surfaces
Friction
 Friction is


greater...
between rough surfaces
when there’s a greater force between the
surfaces (e.g. more weight)
How strong is the force?
 Forces
are measured in Newtons
https://w
ww.youtu
be.com/
watch?v
=PCxP2
4qj2UQ
Wrong kind of force…
Measuring Forces that cause
motion
 Units
are Newtons
(N)
 Forces are
measured using a
spring scale or
force meter.
https://www.youtube.com/watch?v
=AFu1advEnhw
Forces occur in pairs
 The
force pairs can be balanced or
unbalanced
Balanced Forces
 Balanced
forces
cause NO change
in motion
 Balanced forces
are equal in size
and opposite in
direction
Balance forces = no motion
Unbalanced Forces
 Unbalanced
forces always cause a
change in motion, speed, or direction
 Which way will the boy in the middle be
pulled?
Unbalanced Forces
Causes Motion
Adding Forces
Subtracting Forces
Unbalanced Forces
Causes motion
Adding Forces
5N
Object
5N
Notice that all the forces are pointed in the
same direction. Hence they add together.
Subtracting Forces
10 N
=5N 5N
• Two forces can subtract to produce a net force in
the direction of the larger force.
– Negative motion occurs (Slow Down)
Unbalanced Forces
Causes Motion
Subtracting Forces
5N
Object
5N
Notice that all the forces are unequal and
pointed in the opposite direction. Hence they
are unbalanced and in opposition to each
other – or one partially cancels the other.
5N
Which game will result in a
winning team?
How do you know?
Balanced Forces
10N
10N
10N
Balanced
Push
i.e. Pushing a
Car
10N
Balanced
Pull
No motion
i.e. Tug-of-war
No motion
Un-Balanced Forces
Additive
10N
10N
Subtractive
10N
10N
10N
Un-Balanced
Same
Direction
i.e. sled
Faster
motion
Un-Balanced
Opposite
Direction
i.e. football
tackle
Slower
motion
Why do I care?
• Understanding an object’s motion can help
you understand how to interact with the
world around you.
Review of balanced and
unbalanced forces
https://www.youtube.com/watch?v=HEJOy
bRxclk
Stop here
Universal Law of Gravitation
 Mass
and gravity have a directly
proportional relationship.
 The
distance between the centers of
masses of the two objects and gravity
have an inversely proportional
relationship.
Gravity

Gravity


force of attraction between any two
objects in the universe
increases as...
• mass increases
• distance decreases
Gravity

Who experiences more gravity - the astronaut or
the politician?

Which exerts more gravity the Earth or the moon?
less
distance
more
mass
Gravity

Would you weigh more on Earth or
Jupiter?

Jupiter because...
greater mass
greater gravity
greater weight
Balanced Forces
(Balanced Forces = No Motion)
Ground pushes up
Gravity pulls down
Gravity pulls down on you…
The ground pushes back up…
THIS KEEPS YOU WHERE YOU ARE!
If these football players push
on each other equally as hard,
will either one move?
Newton’s First Law
 Newton’s

First Law of Motion
An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net or unbalanced
force.
Two Key Points
Newton’s First Law

Newton’s First Law of Motion


“Law of Inertia”
Inertia
 tendency of an object to resist any change
in its motion
 increases as mass increases
Can you explain how this trick
works, using newton’s first law of
motion?
Newton’s Second Law

Newton’s Second Law of Motion

The acceleration of an object is directly proportional
to the net force acting on it and inversely
proportional to its mass.
F = ma
Newton’s Second Law
F
a
m
F = ma
F
m a
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2
Calculations

What force would be required to accelerate a 40
kg mass by 4 m/s2?
GIVEN:
WORK:
F=?
m = 40 kg
a = 4 m/s2
F = ma
F
m a
F = (40 kg)(4 m/s2)
F = 160 N
Calculations

A 4.0 kg shotput is thrown with 30 N of force.
What is its acceleration?
GIVEN:
WORK:
m = 4.0 kg
F = 30 N
a=?
a=F÷m
F
m a
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
ConcepTest
 Is

the following statement true or false?
An astronaut has less mass on the moon
since the moon exerts a weaker gravitational
force.

False! Mass does not depend on gravity, weight
does. The astronaut has less weight on the
moon.
Newton’s Third Law
 Newton’s

Third Law of Motion
When one object exerts a force on a
second object, the second object exerts
an equal but opposite force on the first.
Newton’s Third Law

Action-Reaction Pairs

The hammer exerts a
force on the nail to the
right.

The nail exerts an equal
but opposite force on
the hammer to the left.
Newton’s Third Law

Action-Reaction Pairs
 The rocket exerts a
downward force on the
exhaust gases.
 The gases exert an equal
but opposite upward force
on the rocket.
FG
FR
Newton’s Third Law

Action-Reaction Pairs

Both objects accelerate.

The amount of acceleration depends on the
mass of the object and the force applied.

Force = mass x acceleration F=ma

Small mass  more acceleration

Large mass  less acceleration