Motion and Force

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Motion and Force
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What is force?

• Any action that can change the state of motion of
  an object.
• Has a magnitude and direction

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Fundamental forces
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• Vary in strength.
• SN is strongest, holds nuclei of atoms together,
  effective over a very short distance
• EM force is about 1/100 of the SN force
• Gravitational force is much weaker.
• Can act through contact or at a distance .
• Contact ? push, pull
• Field ? gravity, magnetic

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Balanced vs Unbalanced Forces

• Net force combination of all the forces acting
  on an object
• When there is a net force acting upon an object,
  the object accelerates in the direction of the
  net force.
• - No acceleration occurs if net force is 0!

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• Balanced forces (no net force) do not change
  motion.
• Unbalanced forces (net force gt 0) do not cancel
  completely, objects will accelerate in the
  direction of the greater force.
• Think of tug-of-war!

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Friction

• Force that opposes motion between two surfaces
  that are in contact.
• Occurs because the surfaces of objects are not
  perfectly smooth.

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Types of Friction

• Static friction ? resists the initiation of
  sliding motion between two surfaces that are in
  contact and at rest.
• Kinetic friction ? opposes the movement of two
  surfaces that are in contact and are moving over
  each other.

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• Static friction is greater than kinetic friction.
• Different types of kinetic friction sliding,
  rolling, fluid, etc.

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Friction and Motion

• Friction is necessary for many everyday tasks!
• Think of your typical day!
• Tires push against ground to make your car move.
• Brakes use friction to stop your car.
• Friction between hands steering wheel.
• Friction between pencil and paper allows you to
  write.
• Friction between your feet and floor allow you to
  push off when you walk.

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Check for understanding...

• Describe a situation in which unbalanced forces
  are acting on an object. How is it affected?
• Identify the type of friction.
• Two students push a box that is at rest.
• Box pushed by students is now sliding.
• Students put rollers under box and push it
  forward.

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Newtons?

• Force is measured in N.
• Newton is the force required to cause a mass of
  1kg to accelerate 1 m/s/s
• Force is calculated using F ma
• Force is equal to mass x acceleration.