Forces

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Forces

• What are forces?
• Inertia
• Gravity
• Newtons Laws
• Projectile motion

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FORCES
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What is a force?

• A push or a pull
• May be unbalanced or balanced

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What does a force do?

• Gives energy to an object
• Can set an object in motion, make it stop, or
  change its motion

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Unbalanced forces change motion
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Unbalanced forces change motion
The truck motion is changed, but the ladder keeps
going
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Friction

• A force which opposes motion, bringing objects to
  rest
• Types

sliding
fluid
rolling
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Inertia
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Inertia
I have more inertia

• The tendency of an object to stay at rest or in
  motion.
• Objects with greater mass have greater inertia

but we fall at the same rate
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Inertia keeps astronauts in motion
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Seat belts protect against inertia
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Gravity
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Gravity
The attractive force between all objects in
the universe
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• More mass means greater gravity
• More distance means less gravity

My gravitational attraction is small no matter
what the distance!
Massive objects have lots of gravity
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The earth exerts a force on the student and the
student exerts a force on the earth because of
GRAVITY!
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Gravity warps the fabric of space-time
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Gravity
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gravity pulls it in inertia keeps it going
What forces are acting on the moon?
Inertia
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Newtons Laws of Motion

• 1. Inertia
• 2. Fma
• 3. Action-reaction

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Newtons First Law of Motion

• The law of inertia
• An object at rest stays at rest
• an object in motion stays in motion
• unless acted upon by an outside force.

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Inertia keeps astronauts in motion
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Inertia keeps satellites in orbit
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Inertia keeps you at rest
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Newtons Second Law of Motion

• Force Mass x acceleration
• A force is needed for acceleration
• Acceleration will occur in the direction of the
  force.

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Which cars are accelerating?

• If you watch carefully, the green and blue cars
  are accelerating. The red car is traveling at
  constant speed.

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A force is needed for acceleration

• Will these accelerate?
• A skier going downhill?
• What force is acting on
• the skier?
• An astronaut on a
• spacewalk?
• Are any forces acting
• on the astronaut?

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Which of the following will
accelerate?

• A bullet shot from
• a gun?
• What forces are
• acting on the
• bullet?
• A hammer and
• feather dropped on
• the moon?
• What force is acting?

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Newtons 2nd Law
Force mass x acceleration

• F_____
• M A
• Force is measured in newtons. A newton is a
  kg m
• s2
• Example A car with mass 1000kg hits a tree and
  decelerates at 50m/s/s. What force does it exert
  on the tree?

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Answer

• F_____
• M A
• 1000kg 50m/s/s
• 50,000 kgm or 50,000N
• s2

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F ma

• A car is traveling at constant speed. Is a force
  acting on it?
• Answer although there are forces acting on the
  car (gravity, friction, air resistance,etc)
  since there is no acceleration, the net force in
  the direction of motion must be zero.

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More 2nd Law Problems

• 1. What is the acceleration on a mass of 50kg if
  a force of 10N is applied?
• 2. An object accelerates due to gravity at a
  rate of 10m/s/s. If its mass is 15kg, what force
  is acting on the mass?

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Answers

• _F_
• M A
• 1. _10N_ 0.2 n/kg 0.2 kg m 0.2 m/s2
• 50kg A s2/kg
• 2. F______
• 15kg 10m/s2
• 150 kgm/s2 or
  150N
  

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Gravity is a force that causes an acceleration

• On earth, ALL objects accelerate at 9.8m/s2
  (ignoring air resistance) because of gravity.
• No matter what the mass, ALL objects on earth
  accelerate at 9.8 m/s2

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Free fall without air resistance
Both the elephant and the feather land at the
same time!
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Free fall with air resistance
The feather takes lots longer to hit the ground!
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Newtons Third Law

• For every action, there is an equal and opposite
  reaction

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For every action, there is an equal and opposite
reaction

• In rocketry

The gases being expelled is the action. The
rocket propelling forward is the reaction.
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How do Newtons laws describe the motorcycle
rider?

• 1st Inertia keeps the rider moving forward.
• 2nd The force with which the rider hits the
  ground will be equal to his mass times his
  deceleration.
• 3rd The action of hitting the ground will equal
  the reaction of the dirt.

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Projectile motion
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Projectile motion

• An object moves forward due to inertia and
  accelerates downward due to gravity.

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Projectile motion of a ball fired vertically
The motion of the truck causes the ball to
project forward.
What would happen if the truck stopped?
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In this video, the ball is dropped straight down
and moves forward due to inertia
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Package dropped from plane
The forward motion of the plane causes the
package to land in front of where its dropped.
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Monkey in gravity-free environment
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With gravity- monkey lets go when banana is fired
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with gravity- cannon shoots at slow speed
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Satellites
Projectile with no gravity
with gravity- launched at lt 8000m/s
with gravity- launched at gt 8000 m/s
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The End