Unit 3: Work, Energy & Power Flashcards
Parallel Force
A force in the same or opposite direction (θ = 0° or 180°)
Mechanical Work (W)
Force displacing object in direction of force or component of it
Work (W) quantity type
Scalar quantity
Work (W) variables
- W is work done (J)
- F is applied force (N)
- Δd is displacement (m)
Joule (J) unit in fundamental units
1 J = 1 N·m = 1 kg·m²/s²
Work (W) formula
W = FΔd
Application of W = FΔd formula: Only when
- Applied force and displacement are in same direction
- Force on object (F) is constant
Two-dimension analysis of work
Component of applied force causing displacement is required.
Positive Work
- Force and displacement are in same direction
- Speed of object tends to increase
Negative Work
- Force and displacement are in opposite directions
- Speed of object tends to decrease
Zero Work
Situation in which no work is done on an object
Conditions for Zero Work (only one of them is required)
- F = 0
- Δd = 0
- F ⊥ Δd
Force-Position (F-d) graph
- x-axis: Object’s displacement (d)
- y-axis: Magnitude of force (F)
- Work = x-axis(y-axis) = FΔd
Assumptions for F-d graphs
- θ = 0° or 180°
- Positive or negative work is above or below d-axis
- F can be constant or change over displacement
When force varies in magnitude during displacement
- W = F(av)Δd cos θ
- F(av) is the average force across displacement
Energy (E) [definition]
- The capacity (ability) to do work
- Comes in many forms
Energy (E) [numerical quantities]
- Scalar quantity
- Measured in joules (J)
Kinetic Energy, E(k)
Energy possessed by moving objects
Variables of E(k)
- E(k) (J)
- m (kg)
- v (m/s)
E(k) equation
E(k) = mv²/2
Work-Energy Principle
Work done on object is related to change in kinetic energy
Work-Energy Principle Formula
W(net) = ΔE(k) = E(kf) - E(ki)
Reference Level
- Designated level to which objects may fall
- Considered to have gravitational potential energy of 0 J
Potential Energy
Stored energy from object’s position related to forces in its environment
Gravitational potential energy, E(g)
Energy possessed by object due to its position relative to Earth
Variables for E(g) formula
- E(g) (J)
- m (kg)
- g is gravitational field strength (N/kg or m/s²)
- h is height relative to a reference (m)
Formula of E(g)
E(g) = mgh
Mechanical Energy, E(mech)
Sum of object’s kinetic and gravitational potential energy
E(mech) formula
E(mech) = E(k) + E(g)
Types of Energy (other than the ones you already learned) [A]
- Chemical Potential
- Elastic Potential
- Electrical
Types of Energy (other than the ones you already learned) [B]
- Nuclear
- Radiant
- Sound
- Thermal
Energy Transformation
Conversion of energy from one form to another
Law of Conservation of Energy
- Energy is neither created nor destroyed
- When energy changes from one form to another, no energy is lost
LOCOE Formula
E(start) = E(during) = E(end)
E(out)
Useful energy provided by a device
E(in)
Energy required to operate device
Efficiency
Ratio of E(out) to E(in)
Characteristics of E(in) and E(out)
E(in) > E(out), E(out) < 100% in all cases
Efficiency formula
efficiency = E(in)/E(out) (x 100%)
Most common form of waste energy
Thermal energy
Sankey Diagram (purpose)
- Diagram representing flow of energy throughout a process
- ΣE(out) = E(in) (law of conservation of energy)
Sankey Diagram (widths)
- Width on left represents total E(in)
- Widths of arrows on right represent amount of E(in) used.
Power (P)
Rate at which energy is transformed or work is done
Power (P) formula
P = ΔE/Δt = W(net)/Δt
Variables of Power Equation
- P is power (W)
- ΔE or W(net) is work done or energy transformed (J)
- Δt is the time (s)
1 W in derived units
= 1 J/s = 1 N·m/s = 1 kg·m²/s³
