What are Forces?

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What are Forces?

• Module 3

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What are forces?

• In module 2 we learned that a force is a push or
  a pull.
• We learned that Newtons 2nd Law states that a
  Force is equal to the mass of a moving object
  times its acceleration.
• We learned that Newtons 3rd Law states that for
  every force there is an equal and opposite
  reaction force.

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Newtons Second Law of Motion

• Force mass X acceleration
• F ma
• Force and acceleration are directly proportional.
  The greater the force, the greater the
  acceleration.
• Mass and acceleration are inversely related. The
  greater the mass of an object the less the
  acceleration if the same force is applied.

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What is friction?

• Friction is a force which opposes motion.
• Friction is an important force in our lives. We
  rely on friction in many ways. An athlete usually
  wears shoes which provide him or her with a
  greater friction between the shoe and the
  surface. We rely on friction as an important
  aspect of our motion. In what other ways does
  friction play a role in our everyday lives?

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What causes friction?

• There are two factors which affect friction
  between two surfaces
• Kind of surfaces in contact (rough or smooth)
• Amount of force pressing the surfaces together.
• The rougher the surface and the stronger the
  force between the surfaces, the greater the
  amount of friction.

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What is static friction?

• If you were trying to move a box like pictured
  below and the box did not move, this would be
  static friction.
• Static friction is friction between two surfaces
  which are not moving past each other.

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What is sliding friction?

• Sliding friction occurs when a force is great
  enough to overcome the static friction.

• What is rolling friction?
• Rolling friction is the friction which enables
  wheels to turn and objects to move. If there were
  no friction, turning wheels would not enable an
  object to move.

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Air Resistance another type of friction??

• Air resistance is a force which opposes a moving
  object. This can be easily seen with falling
  objects. When a piece of paper and a crumple
  piece of the same type of paper are dropped, they
  fall at the same rate.
• A penny and a feather dropped in air will not
  fall at the same rate, but a penny and feather
  dropped in a vacuum (no air) will.

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What is terminal velocity?

• Forces cause objects to accelerate (2nd Law).
• When the force of gravity on a falling object
  equals the force of the air resistance going
  against gravity, the forces balance out and the
  object stops accelerating.
• The object will travel at a constant velocity
  the terminal velocity.

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What is Gravity?

• Gravity is a force of attraction between two
  objects.
• Law of Gravitation any two masses exert an
  attractive force on each other. The amount of
  attraction depends upon two things the mass of
  the objects and the distance between the objects.

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What is gravitational acceleration?

• When objects fall they accelerate toward the
  ground.
• Using Newtons 2nd Law we can see this
  relationship F ma
• The force caused by gravity acts upon a certain
  mass to cause it to accelerate toward the ground
  at a constant rate.
• Acceleration due to gravity is 9.8 m/s2

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Gravity causes weight

• The weight of an object is caused by the force of
  attraction between the earth and objects on the
  surface of the earth.

Gravitation force is equal to weight, therefore
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How much does a person with a mass of 70.0 kg
weigh on the earth?
Weight mass X 9.8 m/s2 Weight 70.0 kg X 9.8
m/s2 Weight 686 N Remember that 1 N (1 kg)
(1m/s2)
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What is weight?

• When an astronaut moves away from the earth they
  lose weight. Weight is caused by the force of
  attraction between the earth and objects on its
  surface. The greater the distance an object has
  from the earth, the less the force of attraction
  which would exist.

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What is the difference between mass and weight?

• Mass is the amount of matter in an object. This
  does not change in an object as it moves away
  from the earth.
• Weight is caused by gravity. Therefore, the
  greater the distance, the less the force of
  attraction. The less the distance the greater the
  force of attraction.
• Your weight is dependent upon the mass of the
  earth. If you were able to go to the planet,
  Jupiter, your weight would be 27 time greater,
  because the mass of Jupiter is 27 times greater.

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What causes weightlessness?

• Even in the space shuttle, there is a force of
  attraction exerted by the earth on the shuttle
  and its contents.
• When the space shuttle temporarily falls toward
  the earth, the contents of the space shuttle
  appear to be weightless, but in fact they are
  falling with the space shuttle. This is called
  freefall.

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What is projectile motion?
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When an object is thrown into the air, two forces
and two motions affect the movement of the
object. The object may be thrown horizontally
with a certain force, but the vertical force of
gravity combines to affect the motion.
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What is centripetal force?

• centripetal means to move toward the center.
• Centripetal acceleration is the acceleration of
  an object toward the center of a curve or
  circular path.
• Centripetal force is a force toward the center of
  a circle which holds the moving object in its
  circular path.

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What is Newtons 3rd Law of Motion

• For every action there is an equal but opposite
  reaction.
• When one object exerts a force on a second
  object, the second one exerts a force on the
  first that is equal in size and opposite in
  direction.
• Action-Reaction Law

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Two objects in contact with one another and
moving must use the action-reaction law. Rocket
propulsion involves the action-reaction law. The
explosion from one end of the rocket causes a
force in that direction. This causes an
unbalanced force in the opposite direction
causing the rocket to accelerate. Video Clip!
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What is momentum?

• The momentum of an object is equal to the product
  of its mass and its velocity. The unit for
  momentum is kg m/s

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What is the Law of Conservation of Momentum?

• When object collide the momentum of one object is
  transferred to the other object.
• View this video to see a demonstration of
  conservation of momentum.
• Notice that the soccer ball has greater momentum
  since it has a larger mass. Its momentum is
  transferred to the smaller ball which has a
  smaller mass and will therefore have a larger
  velocity. Try this with a soccer ball and a
  tennis ball!