Module 3 Work Energy Power Flashcards
Definition for work done?
force times displacement moved in the direction of the force
What happens when you multiply two vectors together?
you get a scalar quantity
Equation for work done at an angle?
W=Fxcosθ where theta is the angle between direction of force and direction of displacement
For an object moving horizontally, what does work done also =?
gain in kinetic energy
Prove that work done = KE
Simple equation showing conservation of energy when energy is transferred
total energy lost = total energy gained
For an object dropped from a height with no resistive forces, how do you find final velocity using the idea of kinetic energy?
mgh = 1/2mv^2
cancels out to give 2hg = v^2
What is the equation showing conservation of energy for an object dropped from a height with drag (assuming constant drag force?)
mgh= 1/2mv^2 + Fh
How to find rate of work done against resistive forces of an object falling at terminal velocity?
Loss of PE in one second = mgh where h is the terminal velocity
(no change in KE at terminal velocity)
When a projectile is launched at an angle, how do you find total PE gained at max height?
Gain in PE = loss of KE
At max height projectile still has horizontal velocity
Horizontal component of velocity = vcosθ
Plug into KE equation to give 1/2mv^2*cos(θ)^2
This KE remaining at max height
How to find braking distance using idea of work done by brakes = loss of KE?
Fx = 1/2mv^2
x = (mv^2)/(2F) where F is the braking force
Relationship between braking distance and mass of car?
directly proportional
Relationship between braking distance and initial speed of a car?
x proportional to u^2
Relationship between braking distance and braking force
x proportional to 1/F
What is the effect of wet or icy roads on braking distance?
reduces braking force as friction is decreased
RF = breaking force + drag + friction
Increasing braking distance
(Fx = 1/2mv^2)
What is heat described as?
work done against resistive forces
Braking on a slope
How to resolve the weight of an object down a slope?
component of weight down the slope = Wsinθ
Explain how braking up a slope reduces the braking distance
Explain the change in braking distance in terms of energy down a slope compared to horizontally
Loss of KE = gain in PE + heat
Fx = heat
Rearrange to give x, x is shorter
Explain how braking down a slope increases braking distance
Definition for power
rate of work done or rate of energy transfer
What is another equation for power?
P=Fv
P is power
F is force
v is velocity
How to calculate efficiency?
useful energy output/total energy input x 100
