Transcript PA2001: Energy and Momentum

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
Dr Mervyn Roy, S6
Energy and Momentum
PA2001: Time and Energy
Work
A force does work when it acts on an object moved through a distance.
Motion in 1D with constant force:
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
Energy and Momentum
PA2001: Time and Energy
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.
Energy and Momentum
PA2001: Time and Energy
Work
Energy and Momentum
PA2001: Time and Energy
Work done by a variable force
Energy and Momentum
PA2001: Time and Energy
Work done by a variable force
Work done = area under the graph of Fx against x
Energy and Momentum
PA2001: Time and Energy
Power
Power is defined as the rate of doing work
Units are Joules/second (or Watts)
Energy and Momentum
PA2001: Time and Energy
Power and acceleration
At constant power, acceleration decreases with velocity
e.g. Ford Fiesta
max power = 50 hp
mass = 1000 kg
At 70 mph.
Acceleration =
Energy and Momentum
PA2001: Time and Energy
Potential energy
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
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.
Energy and Momentum
PA2001: Time and Energy
Potential energy
define
e.g. spring
Energy and Momentum
PA2001: Time and Energy
Momentum is conserved
Before collision
Energy is conserved
Energy and Momentum
After collision
PA2001: Time and Energy
Examples
Inelastic collision
Energy and Momentum
PA2001: Time and Energy
Examples
Elastic collision
Energy and Momentum
PA2001: Time and Energy
Pressure in a Fluid
P=F/A
When 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.
area, A
Variation of pressure with depth
Po
h
P
Energy and Momentum
PA2001: Time and Energy
Archimedes Principle
Fs
PoA
Fs
Fs
Buoyant force equal to
the weight of the
displaced fluid
B
PA
Energy and Momentum
mg
mg
PA2001: Time and Energy
F = mg - B
Energy and Momentum
PA2001: Time and Energy
Examples
Energy and Momentum