Download Work Energy Theorem and Conservation of Energy - ND

Physics 20 Energy – Work Energy Theorem and Conservation of Energy Work-Energy Theorem to find changes resulting from adding or subtracting energy from
a system.
Does Equal velocity at the bottom
mean equal time to get there?
Law of Conservation of mechanical energy to calculate changes that occur in a system
where no energy is added or removed.
Law of Conservation of Energy
Energy is not created or destroyed but it can be converted from one form to another:
∆Ek + ∆Ep + ∆ET = 0 where ET is thermal energy (work done against friction Wf)
To properly account for all energy changes, the law of conservation of energy includes ________ ______________________forces
such as friction.
When friction is involved work is done against friction and some of the mechanical energy is converted into thermal energy.
∆ET = Wf = Ffd
The gain in thermal energy = ________ _______ __________________ ___________.
Energy is not created or destroyed but it can be converted from one form to another
Example 1: Loop - the - Loop
Example 2: Pendulum
1) In a loop-the-loop experiment, what minimal velocity must the
object have at the bottom of the loop in order to go around the
25.0 cm loop at the top?
2) If a 1.50 m pendulum, using a 50.0 g bob, swings through an
angle of 20o from its highest point to its lowest point, what
velocity will it have as it moves through the lowest point?
Closure:
1) A 250 kg roller coaster car travels past points A and B with
speeds shown in the diagram. How much heat energy is produced
between these points?
2) What horizontal force must be exerted to accelerate a 25.0 kg
object from rest to 5.00 m/s over a 20 m surface for which the
coefficient of friction is 0.340?
1 S Molesky @ Notre Dame Physics 20 Energy – Work Energy Theorem and Conservation of Energy Work-Energy Theorem
Wnet = ∆Ek + ∆ET + ∆Ep + ∆Ee
You have to be careful when dealing with change of energy, as
kinetic energy decrease (-) when potential energy increases (+)
Favgd = ½ m (vf2 –vi2) where F is the force that is either adding or
subtracting energy from the system.
Concept statement format of solving problems;
- Energy statement
- Substitute equations for each part
- Break down equations to the level of the known information
- Isolate the desired variable
- Substitute the known values
- Answer with proper significant digits and units
Forces can be categorized as internal forces or external forces.
There are many sophisticated and worthy ways of explaining and
distinguishing between internal and external forces. Many of
these ways are commonly discussed at great length in physics
textbooks. For our purposes, we will merely say that external
forces include applied forces, normal forces, tensional forces,
friction forces, and air resistance forces. For our purposes,
internal forces include gravitational forces, magnetic forces,
electrical forces, and spring forces.
When work is done upon an object by an ___________________force, the total mechanical energy (KE + PE) of that object is
______________. If the work is "positive work", then the object will gain energy. If the work is "negative work", then the object will
lose energy. The gain or loss in energy can be in the form of potential energy, kinetic energy, or both.
Under such circumstances, the work which is done will be equal to the change in mechanical energy of the object. External forces =>
_________ _____________ __________________
When work is done upon an object by an __________________l force (for example, gravitational and spring forces), the total
mechanical energy (KE + PE) of that object ________________ _____________ . In such cases, the object's energy changes form.
For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object
is transformed into kinetic energy. Yet, the sum of the kinetic and potential energy remains constant. This is referred to as energy
conservation .
When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total
amount of mechanical is conserved. Internal forces => __________________ _____ ____________.
Example 1: A 40.0 kg, young boy swings out on a rope above his
favourite swimming hole. How fast will he be travelling at the
bottom of his swing if he starts out 5.00 m above the water and
ends up 0.85 m above the water?
Example 2: A 500 N skateboarder (with board) rolls 5.26 m
down a 25o incline for which the force of friction is 60.0 N. What
will her velocity be at the bottom of the incline?
2 S Molesky @ Notre Dame