Download PowerPoint-Force and Motion

Physical Science
Forces and
Motion
Gravity and Motion
Moving on to Something
New!!
• In science a force is simply a push or a
pull.
• Scientist express force using a unit called
the newton (N).
Forces, Forces Everywhere!!
Take a moment to consider the word force.
A force is always exerted by one object on
another object.
Where do you see a force happening in the
room right now?
Which object is exerting the force, and which
is receiving it?
Determining Net Force
• Usually more than one force is acting on
an object.
Net Force is the combination of all the
forces acting on an object.
Let’s Practice


What would be the net force
being exerted on this box if the person on the right
pushed with a force of 25 N and the person on the
left pushed with a force of 17N? What direction
would the box move?
Friction is a Hater on Motion
Friction is a force that opposes motion
between two surfaces that are in contact.
Friction can cause a moving object, such as
a ball, to slow down and eventually stop.
Where does Friction come
from?
• Friction occurs because the surface of any
object is rough (even if it feels smooth).
• Rougher surfaces have more friction
because there are more microscopic hills
and valleys compared to an object with a
smoother surface.
Two Types of Friction
Kinetic friction: the word “kinetic” means
moving, and kinetic friction depends on the
surface where the friction is taking place.
Sliding kinetic friction
Rolling kinetic friction
The other type of Friction
When a force is applied to an object but
does not cause the object to move, static
friction occurs.
The word “static” means not moving.
The object does not move because the force
of static friction balances the force applied.
Friction is Harmful and
Helpful
What if we had a life without friction?
Write a short paragraph to describe what life
would be like if friction did not exist.
Be prepare to share
Gravity
• Gravity is a force of attraction between
objects that is due to their masses.
The force of gravity can change the motion
of an object by changing its speed, direction,
or both.
The Size of Earth’s
Gravitational Force
• Compared with all objects around you,
Earth has a huge mass.
• You must apply forces to overcome
Earth’s gravitational force any time you lift
an object or even move your body.
The Law of Universal
Gravitation
• This law describes
the relationships
between gravitational
force, mass, and
distance.
• It applies to all objects
in the universe
Part 1: Gravitational Force
Increases as Mass Increases
• Why can’t you lift an
elephant?
The amount of gravity
between an elephant
and Earth is greater
than the amount of
gravity between you
and Earth.
Part 2: Gravitational Force
Decreases as Distance Increases
The Sun is larger than
the Earth, so why
doesn’t the Sun have a
larger gravitational pull
on us than Earth?
Why is there such a
large gravitational force
between the planets
and the Sun?
Weight as a Measure of
Gravitational Force
• Weight is a measure
of the gravitational
force on an object.
• Mass is the amount
of matter in an object
What’s volume again??
Acceleration and Velocity
• Acceleration is the rate velocity changes
over time. The acceleration of an object
affected by gravity is 9.8 m/s.
• Change in velocity of falling objects can be
measured by the following equation:
∆v = g x t
OR 9.8 m/s times the number of seconds
an object falls…
Gravity and Falling Objects
• Objects fall to ground at the same rate because
acceleration due to gravity is the same for ALL
objects
• Why? Acceleration depends on
both force and mass.
– A heavier object experiences a
greater gravitational force, BUT it
is also harder to accelerate.
– Galileo was a genius.
Air Resistance
• Air resistance – the force that
opposes the motion of objects
through air
– Amount of air resistance depends
on the size, shape, and speed of
the object
Discussion
• Why would a feather dropped from the
same height as an acorn fall to Earth more
slowly than the acorn?
Discussion
• How can a sky diver benefit from air
resistance?
Terminal Velocity
• Terminal Velocity – when air resistance =
gravity
– 0 N or a balanced force!
– It’s the fastest an object will fall
– It’s also why rain drops don’t kill
you… think about it…
Free Fall
• Free fall - when gravity is the only force acting
upon an object
– Can only happen where there is NO air resistance:
SPACE (or a vacuum)
•
- Orbiting objects are in
free fall. Orbit is caused
by two motions:
- Orbiting objects
move forward, but are
also in free fall – so are
the astronauts
Orbiting and Centripetal Force
• Orbit is caused by two
things that create an
unbalanced force
– Gravity provides
centripetal force to
the orbiting object
– Centripetal force – the
unbalanced force that
causes an object to
move in a circular path
– Ex: planets around the
sun or the moon around
the Earth
Projectile Motion
• The curved path an object follows when it is
thrown near the surface of the Earth
• Composed of horizontal motion and vertical
motion
– Horizontal motion (like throwing a ball) causes the
object to go forward
– Vertical motion (gravity) causes the object to fall
• Together they create a curved path: BUT still hits the
ground the same time as a dropped object
– This is why you always have to aim above a target
when trying to hit it…
An example of projectile motion
Discussion
1. If a baseball and a cannonball are
dropped from the same height at the same
time, and there is no air resistance, which
ball will hit the ground first?
a. The cannonball lands first.
b. The baseball lands first.
c. The balls land at the same time.
d. The ball with the larger volume lands
first.
2. What feature of an object does not affect
air resistance?
a. its size
b. its chemical properties
c. its shape
d. its speed
4. In what directions is a projectile
accelerated?
a. both vertically and horizontally
b. vertically downward
c. vertically upward
d. horizontally forward