pa2001: time and energy energy and momentum work and energy power potential energy conservation of...
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PA2001: Time and Energy
Energy and Momentum
Work and energy
Power
Potential energy
Conservation of momentum
Pressure in a fluid
Tipler Chapters 6,7,8 and 13
Energy and Momentum
Dr Mervyn Roy, S6
PA2001: Time and Energy
Energy and Momentum
Work is a scalar quantity that is positive if Δx and F is the same sign, and negative if they are opposite – units are Joules = Newton metres
Work
A force does work when it acts on an object moved through a distance.
Motion in 1D with constant force:
PA2001: Time and Energy
Energy and Momentum
Energy and work are related
• When work is done by one system on another energy is transferred between the two systems.
• Total energy is always conserved (read Tipler Chapter 7).
Types of Energy
• Kinetic energy – associated with the motion of an object.
• Potential energy – associated with the configuration of a system.
• Many types of potential and kinetic energy – e.g. Thermal energy.
PA2001: Time and Energy
Energy and Momentum
Work
PA2001: Time and Energy
Energy and Momentum
Work done by a variable force
PA2001: Time and Energy
Energy and Momentum
Work done by a variable force
Work done = area under the graph of Fx against x
PA2001: Time and Energy
Energy and Momentum
Power is defined as the rate of doing workUnits are Joules/second (or Watts)
Power
PA2001: Time and Energy
Energy and Momentum
At constant power, acceleration decreases with velocity
e.g. Ford Fiestamax power = 50 hpmass = 1000 kg
At 70 mph. Acceleration =
Power and acceleration
PA2001: Time and Energy
Energy and Momentum
Potential energy is energy stored within a physical system. • exists when there is a force which tends to pull an object back towards its original position when it is moved.
e.g. a stretched spring, or a weight suspended weight
Potential energy
If the spring is released or the weight dropped, the stored energy will be converted back to kinetic energy and the restoring force will accelerate the object back towards its original position.
PA2001: Time and Energy
Energy and Momentum
Potential energy
define
e.g. spring
PA2001: Time and Energy
Energy and Momentum
Momentum is conserved
Energy is conserved
Before collision After collision
PA2001: Time and Energy
Energy and Momentum
Examples
Inelastic collision
PA2001: Time and Energy
Energy and Momentum
Examples
Elastic collision
PA2001: Time and Energy
Energy and Momentum
Pressure in a Fluid
P = F / AWhen a body is submerged in a fluid, the fluid exerts a force perpendicular to the surface of the body at each point on the surface.
h
P
Po
area, A
Variation of pressure with depth
PA2001: Time and Energy
Energy and Momentum
Archimedes Principle
Buoyant force equal to the weight of the displaced fluid
PA
PoA
Fs
Fs
mg
Fs
mgB
PA2001: Time and Energy
Energy and Momentum
F = mg - B
PA2001: Time and Energy
Energy and Momentum
Examples