Download Energy

Work
Kinetic Energy
Potential Energy
 Work is done when
 There is an application of a force
 There is movement of something by that force
 Work = force x distance (W=Fd)
 SI Unit = Joule (J) = Newton meter (Nm)
 Work-Energy relationship
 Whenever work is done on an object, its energy is
changed
 Whenever energy is changed from one form to another
work is done
 Work-Energy Theorem
 Change in energy = work done
 Gravitational Potential Energy
 due to the height of an object with mass
 Elastic Potential Energy
 The energy in springy things
 Kinetic Energy
 due to motion
 Energy is measured in Joules (J)
 Kinetic Energy
The energy of an object that is due to the
object’s motion is called kinetic energy.
 Kinetic energy depends on speed and
mass.
1
KE  mv 2
2
1
2
kinetic energy =  mass   speed
2
 A dog can run at a speed of 16.0 m/s. What is the KE of the
dog if its mass is 20.0-kg?
Potential Energy is the energy associated with an
object because of the position, shape, or condition
of the object.
Gravitational potential energy is the potential
energy stored in the gravitational fields of
interacting bodies.
Gravitational potential energy depends on height
from a zero level.
PEg = mgh
gravitational PE = mass  free-fall acceleration
 height
 Legend has it that Isaac Newton “discovered" gravity when
an apple fell from a tree and hit him on the head. If a 0.20kg apple fell 7.0 m before hitting Newton, what was its
change in PE during the fall?
 The total amount of energy in a system is constant in a
closed, isolated system.
 The energy can change form.
 Energy is neither created nor destroyed.
 Kbefore + Ug before = Kafter + Ug after
 Identify the relevant points in the problem
 At one point is you know enough to calculate the energy,
at the other point you want to know something.
 Write down the conservation of energy formula
 Write down the forms of energy at each point.
 Substitute in the formulas for each type of energy.
 Substitute numbers with units.
 Solve for the answer – check units.
 Maya is changing the tire of her car on a steep hill 20.0 m
high. She trips and drops the 10.0-kg spare tire, which rolls
down the hill with an initial speed of 2.00 m/s. What is the
speed of the tire at the top of the next hill, which is 5.00 m
high? (Ignore the effects of rotational KE and friction.)
 Momentum is always conserved in a collision if there is
no outside force.
 Mechanical Energy is also conserved in an elastic
collision.
 pinitial = pfinal ; KEinitial = KEfinal
 Mechanical Energy is not conserved in an inelastic
collision.
 pinitial = pfinal ; KEinitial > KEfinal