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- 12th Grade
Physics unit four notes
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Lesson 2.1Work is the energy transferred to an object by a force that moves it a certain distance.
The equation for work is:
W = (F)(d)
W = work done in Joules (J)
F = force exerted in newtons (N)
d = displacement in metres (m)
If the angle is stated, then you use this formula:
W = (F) (d cos Ө)
θ = angle between the direction of displacement and the direction of the force
Δd = magnitude of displacement (m)
If the angle is 90 degrees, then the cosine function will always equal 0. (horizontally is zero work
Zero displacement means that there is 0 work done which means zero work.
Sometimes, an object that is being displaced experiences a force in a direction opposite to its
direction of motion. The most common example of this is the force of kinetic friction. This is also
called negative work.
Energy is the potential to do work.
Two types of energy:
Gravitational potential energy: (GPE)
The energy possessed by an object due to its gravitational attraction to another object.
Equation:
Eg = mgh (e = energy, m = mass, g = gravity{9.8], h = height [metres])
Answer in joules
Kinetic energy:
The energy an object has because of its motion. The amount of kinetic energy depends on its mass
and speed.
Ek = ½ mv2 ( m = mass [kg], v = speed [m/s2])
, The work-energy theorem states that the total work done on an object is equal to the object’s
change in kinetic energy, provided there is no change in any other type of energy, such as
gravitational potential energy.
When given 2 speeds, you use this formula to find work:
W = Ek1 - Ek2 which also equals W = ½ mv2 - ½ mv2
In some situations, some friction is unavoidable, resulting in the formation of another energy called
thermal energy.
Thermal energy is the energy related to the motion of atoms in materials or, in other words, heat
energy. Quantitatively, thermal energy is proportional to the force of kinetic friction and the
displacement moved by the object.
The formula for thermal energy is:
Eth = (ukFn) d
Which is also basically
Eth = (uk)(mg)(d) = [uk is coefficient of kinetic friction]
Conservation of energy: You can classify energy into two types:
Conservative system:
No mechanical energy is lost but energy can be converted from one form to another.
- A roller coaster on a frictionless track
Non-conservative system:
Mechanical energy is wasted or “lost” in the form of thermal energy, (i.e. heat due to friction)
- A car slows down as the brakes are applied.
The law of conservation of energy:
- Energy can never be created nor destroyed, but it can be converted/transferred from one
form to another.
Which can also be displayed as:
Etotal initial = Etotal final
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