Newtons First Law

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Newtons First Law

• The law of inertia

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Daily Thinker

• What is a force? What unit is force measured in?
  Give 2 examples of forces (not the two given
  below).
• What is net force?
• What is normal force?

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Example Problem

• If I push on a crate with a force of 50 N to the
  north, and you push with 20 N to the south, what
  is the net force on the crate? What kind of
  motion will the crate have, assuming these are
  the only noticeable forces on it?

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What do these forces look like?
Gravity (weight)
50 N
20 N
Normal force
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What about an object at rest?Do any forces act
on this sweet baby?What kind? What about net
force?
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Daily thinker 2

• Draw a free body diagram for the baby in the
  previous slide. Label the force arrows with names
  and make them appropriate sizes. Do not assign
  them number values.
• Define equilibrium.
• Contrast static and dynamic equilibrium

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Daily thinker 3

• What is the motion of an object that is in
  equilibrium?
• Define velocity and explain how it is different
  from speed.
• What is acceleration?

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Daily thinker 5

• Give one example of an object in static
  equilibrium and one in dynamic equilibrium.
• Describe what happens to an objects velocity if
  its forces are not in equilibrium
• Imagine spinning your tires on the ice so your
  speedometer reads 40 mph even through you are not
  moving. Predict your motion once your tires hit
  dry pavement and gain traction.

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Daily thinker 3

• You are riding in a car traveling 60 miles per
  hour. You are eating a taco. Some of the lettuce
  and cheese fall out of the taco. Describe the
  path you would see the lettuce and cheese take.
• The Earth is hurtling through space at 30 km/s.
  If you stand next to a wall and jump straight up
  into the air, will the wall smack into you as the
  earth it is connected to moves under you while
  you are airborne? Explain.
• c. You give a hockey puck a push on
  frictionless ice. Once you have released it,
  describe the net force on the puck.Why does the
  puck continue to slide at a constant speed in a
  straight line?

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Inertia the tendency of an object at rest to
remain at rest unless unbalanced external forces
act upon it.

• If a net force acts upon an object at rest the
  object will
• Accelerate! What exactly does that mean?
• Acceleration can be a change in either
• Speed or direction

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An object with a lot of inertia is lazy

• It resists any change in motion starting from
  rest, coming to a stop, speeding up, slowing
  down, or changing directions.
• Ex Which is easier to set into motion, turn a
  corner, or stop if already moving an empty
  shopping cart, or a full one?
• Why?
• The more mass an object has, the greater its
  inertia.
• Even if something is weightless it can still
  have inertia

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The bigger your mass, the lazier you must be.

• The more mass (not weight) something has, the
  greater its inertia.
• Click cartoon for a neat demo.

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Why do you think this demo works better with a
heavy lead ball than it would with a tennis ball?

• The heavy ball has more inertia.

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What is the one word answer to the question
What is Newtons first law all about?

• Inertia!

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Daily thinker 5

• Describe your velocity if you have a constant
  acceleration
• Describe your acceleration if you have a constant
  velocity.
• An object starts at rest and reaches a velocity
  of 15 m/s in 3s. What is the objects average
  acceleration?