Define work and energy
Notes 1
Define Work and Energy
Physics Definitions
Investigation 7 Energy Experience
Classifying Work and Energy (Pages 282 - 283)
Positive, Negative, and Zero Work (Page 284)
Calculating Work
Essential Questions
How does physics define work and energy?
How do you know if a force is doing work or not?
How do you determine when work is positive or negative?
How do you calculate work?
When forces do work, what does that mean about energy transfer?
Energy
Definition:
Energy is a "".
We can calculate how much energy is "" or "".
Energy is a quantity that we can keep track of.
Measurement of Energy
Energy can be measured with:
The for energy is the .
Other units of energy:
1 food calorie (kcal) = _
1 BTU =
1 kWh = ___
1 eV = 1.60218 x 10^{-19} J
James Prescott Joule:
English physicist famous in electricity and thermodynamics.
Energy Transfer Observations
"_" can be observed when energy is transferred:
anything observable
Principles of Energy
Energy is neither created nor destroyed.
Forms of energy include:
Heat
Light
Electricity
Energy Storage and Transfer
Energy T-Chart:
Stored
Transferred
Work
Definition: Work is done when energy is transferred through a force acting on an object.
In order for energy to be transferred through work, 2 things must be true:
The object must __.
The and the must be .
Conditions for Work Done
Work is done (energy transferred mechanically) only when:
The result of the force is applied to the object in question.
Example scenarios:
Person does work on the box.
NO work is done on the backpack.
Person does work on the grocery bag.
NO work is done on the grocery bag.
Implications of Work Done
When these two conditions are met, the work done can be expressed mathematically:
Work done is equal to the times the .
EQUATION? How to Calculate Work
Types of Work
Positive Work: Energy is being added to the object from external forces.
Negative Work: Energy is being taken out of the object from external forces.
Zero Work: No change in the object's energy due to the force; examples include:
External force: pushing a broom
External force: stopping a sliding basket
External force: holding a box
Notation on Forces:
F & D are in the ___.
F & D are in the .
F & D are _.
Calculating Work - Examples
Diagram A:
Force (F) = 100 N
Mass = 15 kg
A 100 N force is applied to move a 15 kg object a horizontal distance of 5 meters at constant speed.
Diagram C:
An upward force is applied to lift a 15 kg object to a height of 5 meters at constant speed.
Forces at An Angle
Non-Parallel Forces (pulling at an angle):
Only the component of force that is to the is doing work.
is a Greek letter denoting the angle between a force and the displacement.
Note: the vertical component does no work.
Equation: If rewriting this equation, what would it be?
Equation with Symbols
Work equation components:
F
Angle θ
Fcos(θ)d
Calculating Work - More Examples
Example 1 (Diagram A):
Force (F) = 100 N
Mass = 15 kg
A 100 N force is applied to move a 15 kg object a horizontal distance of 5 meters at constant speed.
Example 2 (Diagram B):
A 100 N force applied at an angle of 30° to the horizontal to move a 15 kg object at a constant speed for horizontal distance of 5 m.
Example 3 (Diagram C):
An upward force applied to lift a 15 kg object to a height of 5 meters at constant speed.
Final Example:
A 10-N force is applied to push a block across a friction-free surface for a displacement of 5.0 m to the right.
F_norm = 20 N
F_app = 10 N
F_gray = 20 N
An approximately 2-kg object is sliding at constant speed across a friction-free surface for a displacement of 5 m to the right.
F_norm = 20 N
F_gray = 20 N