Motion, Energy and Gravity I

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Motion, Energy and Gravity I

• Dr Bryce

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Class notices

• Chapter 4

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Aims

• We need to formally define motion.
• We need to understand the difference between
  weight and mass
• Newtons laws of motion
• Conservation

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Motion?

• Speed the rate at which an object moves, units
  are m/s
• Velocity speed and direction
• Acceleration the change in velocity, units are
  m/s2

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Gravity

• Produces acceleration, eg towards the centre of
  the Earth
• All falling objects accelerate at the same rate
  (if we ignore the effects of air resistance)
• On Earth the acceleration (g) is 10 m/s2, so with
  each second of falling the speed increases by 10
  m/s

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Acceleration of Gravity

• Galileo and Neil Armstrong demonstrated that g is
  the same for all falling objects regardless of
  their mass
• Although g on the Moon is much lower than on the
  Earth!

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Momentum (mmtm)

• Momentum mass velocity
• A net force changes momentum, usually a change in
  velocity (an acceleration)
• Angular momentum is the momentum of a spinning or
  orbiting object

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Thought QuestionIs there a net force? Y/N

• A car coming to a stop.
• A bus speeding up.
• An elevator moving up at constant speed.
• A bicycle going around a curve.
• A moon orbiting Jupiter.

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Thought QuestionIs there a net force? Y/N

• A car coming to a stop. Y
• A bus speeding up. Y
• An elevator moving at constant speed. N
• A bicycle going around a curve. Y
• A moon orbiting Jupiter. Y

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Conservation of momentum

• The total momentum of interacting objects cannot
  change unless an external force is acting on them
  
• Interacting objects exchange momentum through
  equal and opposite forces

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Mass and Weight

• Mass The amount of matter in an object
• Weight The force that acts upon an object
• An object in freefall is weightless not massless

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Thought QuestionOn the Moon

• My weight is the same, my mass is less.
• My weight is less, my mass is the same.
• My weight is more, my mass is the same.
• My weight is more, my mass is less.

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Thought QuestionOn the Moon

• My weight is the same, my mass is less.
• My weight is less, my mass is the same.
• My weight is more, my mass is the same.
• My weight is more, my mass is less.

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Isaac Newton 1642-1727

• Formulated laws of motion and gravity.
• The same laws on Earth are also applicable to
  planets and stars.
• Also famous for optics, calculus

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Newtons first law of motion

• An object moves at constant velocity, unless a
  net force acts to change its speed or direction.

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Newtons second law of motion

• Force mass acceleration

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Newtons third law of motion

• For every action there is an equal and opposite
  reaction

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Types of Energy

• Kinetic
• Radiative
• Stored or potential
• Energy can change types, but it cannot be created
  or destroyed.

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Thermal energy (Kinetic)
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Thermal Energy the collective kinetic energy of
many particles(for example, in a rock, in air,
in water)
Thermal energy is related to temperature but it
is NOT the same. Temperature is the average
kinetic energy of the many particles in a
substance.
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Temperature scales