Unit 01

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Unit 01 Forces and the Laws of
MotionIntroduction to Forces
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Newtons 3 Laws of Motion

• 1st Law Inertia
• 2nd Law Fma
• 3rd Law Action/Reaction

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Newtons 1st Law of Motion

• An object at rest will stay at rest
• and
• An object in constant velocity motion will stay
  in constant velocity motion
• Unless acted on by an unbalanced force.

• Demonstrations
• Tablecloth Trick
• Car Accidents
• Skateboarder

1st Law can also be written as An object in
equilibrium will stay in equilibrium unless
acted on by an outside force. Or More mass,
more inertia Harder to change an objects
motion. Less mass, less inertia Easier to change
an objects motion.
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Skateboarding

• In object in constant velocity motion
• will stay in constant velocity motion
• unless acted on by an outside force!

Before, Donald and the skateboard are in
motion. After, the skateboard has a force on it
and stops. Donald has no force and he
continues moving.
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Skateboarding
Before, Donald and the skateboard are in
motion. After, Donald has a force on him and
stops. The skateboard has no force and it
continues moving.
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Answer Before What was moving?After
What stopped? What continued moving?Why?
What had a force on it? What didnt have a
force on it?What Law is it?
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Example Tablecloth Trick
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Newtons 2nd Law Fma

• Force is equal to the mass of an object
  multiplied by its acceleration.
• Force mass x acceleration

• Demonstrations
• Skateboarders
• Basketball and Tennis Ball
• Earth and People

2nd Law can also be written as The acceleration
of an object is directly proportional to the
force on the object and inversely proportional to
the objects mass.
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F and a are directly proportional
m and a are indirectly proportional
F a F a m a m a

• Basketball and Tennis Ball Trick

The basketball has more mass, so it accelerates
less. The tennis ball has less mass, so it
accelerates more.
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Example Two Ice Skaters
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A cool calculation

• When you fall to the ground, you pull the Earth
  with an equal and opposite force as Earth pulls
  you with! But we dont see Earth move! This is
  because Earth is so massive its acceleration is
  too small to see! The Earth does move, but we
  cant feel it!
• Question What if EVERY person on the Earth jump
  up and hit the Earth at the same time in the same
  place would the Earth move enough to notice

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A cool calculation

• Mass 70kg 7,000,000,000 490,000,000,000kg
• Acceleration -9.8m/s2
• FEarth on People ?
• FPeople on Earth
• Mass 6,000,000,000,000,000,000,000,000kg
• Acceleration ?

F ma
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Newtons 3rd Law Action/Reaction

• For every force applied to an object,
• the object applies
• an equal and opposite force back.

• Demonstrations
• Rollerblader
• Force sensors
• Punch you teacher

3rd Law can also be written as For every action
there is an equal and opposite reaction.
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Action-Reaction Pairs
Action Reaction
I push the chair
The wind (air molecules) push the flag.
The Earth pulls a ball.
The chair pushes me
The flag pushes the air molecules
The ball pulls the Earth.
With equal and opposite force!!!!
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Equal and Opposite Forces
Action Your hand push Ms Buccis
Face. Reaction Ms Buccis face pushes your
hand.
If you punch Ms Buccis face, her face hits your
hand back with an equal and opposite force.
When you get in a fight, it is easy to get the
last hit as soon as they hit you, you hit
them back, according to Newtons 3rd Law!
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Equal and Opposite Forces
The BIG truck and the SMALL truck hit each other
with equal and opposite forces.
Action The big truck hits the small
truck. Reaction The small truck hits the big
truck with an equal and opposite force!
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Equal and Opposite Forces
Action The bug hits the windshield. Reaction Th
e windshield hits the bug.
The bug hits the windshield with the SAME force
that the windshield hits the bug with!
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Example Pushing a Table Why is it wrong to say
I pushed myself off the table?