Download 7 - Work and Energy Student Notes

Science 10 Unit B: Physics
Part 2: Work and Energy
Work- takes place any time energy is transferred.
Energy
Mathematically, we can write this as
- is the capacity to do work on an object or the
environment, to change either in some way.
- can be converted from one form to another
- a change in energy is called work.
ex) A 1500 g bag of potatoes is dropped from 1.5 km.
What is the gravitational potential energy of the bag?
ex) An electron has a kinetic energy of
5.69 x 10-14 J and a speed of 2.5 x 108 m/s. What
is the mass of the electron?
2 Kinds of Energy
Where:
work is measured in joules (J) or
newton-metres (Nm)
force is measured in newtons (N)
displacement is measured in metres (m)
Work is a scalar, meaning it only has
magnitude. However, the direction of
the force and displacement are
important.
We calculate two kinds of energy in this unit:
Gravitational Potential Energy
- it takes work to elevate an object
- the higher we move an object, the more work it takes
James Joule
The energy gained by raising an object against the
gravitational force of the Earth is called gravitational
potential energy (PE or Ep for short).
*𝑔⃑= 9.81 m/s2. This is the acceleration due to gravity (the
acceleration objects fall to the ground at). It is a constant
(found in your data booklet) and does not count towards sig
digs!
ex) How much work is done by gravity when lifting a 25 N
______ to a height of 15 m?
ex) A 50 g golf ball has a gravitational potential energy
of 500 J. To what height is it hit?
ex) A 10 kg cat climbs a tree to a height of 20 m. What is
the gravitational potential energy of the cat?
Kinetic Energy
- the energy of motion
- the energy needed to accelerate
a body to a certain velocity
ex) What is the mass of a car with speed 3.2 km/h
and kinetic energy 250 J?
ex) What speed of a 12 g bullet moving with to
have a kinetic energy of 1.2 kJ?
ex) How much work is done by gravity after lifting the
_______ to carry it horizontally 250 m?
ex) To what height does 1.00 kg of water need to be raised
to give it 1.0 MJ of energy?
ex) A 10.0 kg melon is moving at 15 m/s. What is the
kinetic energy of the melon?
ex) How much work is used in pushing a box with 15 N of
force for 1.5 km?
Practice: Work and Energy
Practice Problems 1-4
Practice: Work and Energy
Practice 8-11
The Law of Conservation of Energy
- energy cannot be created or destroyed.
- the total energy of a system is always the same.
ex) A 10 kg ball is dropped from 3.0 m.
a) What is the potential energy of the ball at a height
of 3.0 m?
A system is a group of objects that we are interested in
studying (ex: a car and passenger).
What kinds of energy will the roller coaster have at
position A, B and C?
b) What is the kinetic energy of the ball the moment
before it hits the ground?
Efficiency
ex) An automobile's engine consumes 2500 kJ of
chemical energy from gasoline, but only 325 kJ is
turned into kinetic energy. What is the percent
efficiency of the engine?
Coal-Burning Power Station
A  ____________________________
B  ____________________________
ex) A hand-crank electric generator energy is 95%
efficient. How much kinetic energy has to be cranked
into the generator to produce 300 J of energy?
If the roller coaster starts off with 100 J of energy at
point A, how much will it have at point B or C: _____
c) What is the speed of the ball the moment before it
hits the ground?
How about in a pendulum? What kinds of energy does
the pendulum bob (the ball at the end of the swinging
rope) have at each point?
C  ____________________________
D  ____________________________
Pros:
Cons:
The Laws of Thermodynamics state that no machine
can be 100% efficient. Energy is always converted to
friction, light, heat, some sort of "non useful" form.
ex) A ball of unknown mass is dropped from 2.0 m.
What is the speed of the ball the instant before it hits
the ground?
Theoretical (and not possible) machines that are 100%
efficient are called perpetual motion machines.
Energy Conversions in Power Stations
Nuclear Power Station
A  ____________________________
B  ____________________________
C  ____________________________
D  ____________________________
Hydro-electric Power Station
If the bob starts off with 52 J of energy at point A,
how much will it have at point B or C: __________.
Pros:
Cons:
A  ____________________________
B  ____________________________
Practice: Conservation of Energy Practice WS
C  ____________________________
D  ____________________________
Pros:
Cons:
Practice: Problems 1-3 pg 216-217