Work, Energy, and Power Flashcards
Work
product of force and the displacement in the direction of that force
U = Fd
Weightlifting and work
zero net work because the overall displacement is zero
no mechanical work done but there was physiological work done by the lifter
+/- work
- positive work is done when a force displaces an object in the same direction as the force (throwing a ball)
- negative work is done when the object is displaced in the direction opposite to the force (catching a ball, lowering a weight)
positive work in the human body
- muscles do mechanical work
- contracting = pulling on the point of attachment - positive work is done when muscles shorten and moves the limb in the direction of contraction
- concentric contraction
negative work in the human body
negative work is done when the muscles contract but the limb moves in the opposite direction of the muscle force
- eccentric contraction
work (general, not +/-) in the human body
- muscle contractions can do zero mechanical work
- isometric contraction –> muscle is contracting but limb is not moving
Energy
capacity (or ability) to do work
- heat, light, sound, chemical, and mechanical
forms of mechanical energy
kinetic, potential
kinetic energy
energy due to motion
- collisions: one object hitting another does work on the second object and vice versa
- capacity for work depends on the velocity and mass of the object
- sort of like momentum but also not
kinetic energy vs. momentum
KE = 1/2 mv^2
- the units are the same (KE = kgm^2/s^2, same as P = N.m)
KE is easier to determine than work because…
- determining KE is easier than work because mass/velocity is easier to measure than force
Potential Energy
- gravitational potential energy
- strain energy (deformation of an object)
gravitational potential energy
related to the weight of an object and it’s height above the ground
PE = mgh OR PE = Wh (W = weight)
Strain Energy
-due to the deformation of an elastic object
- deformed pole in pole vault
- the greater the deformation the greater the strain energy
SE = 1/2 kx^2
k = spring constant
x = amount of deformation
in sports, energy is possessed by athletes and objects due to their…
motion (kinetic energy), their position above the ground (gravitational potential energy), and their deformation (strain energy)
Energy conservation
total amount of energy in a system remains constant; though it can be changed from one form to another or transferred from one object to another
pole vaulter example
as he runs towards the bar, he gains kinetic energy
as he bends the pole, he does work and stores elastic potential energy
this elastic energy is then converted to gravitational potential energy as the pole does work on the vaulter to lift them above the ground
Work-Energy Relationship
raising an object does work, but it also changes the potential energy of it
- moving something horizontally will not changes the PE, but it will do work
- this work will accelerate the object now giving it KE
Work-Energy Relationship in Depth (3 step - initial force, consequence, reaction)
1) + work is done by exerting a force over a certain distance. the work is converted to kinetic energy and the object now has a velocity
2) the object will continue to travel with a constant velocity for a distance
3) negative work will be done when the object encounters a force that acts on it for a certain distance and decreases the velocity to zero
Impulse-Momentum Relationship in Depth (3 step example - initial force, consequence, reaction)
1) to get an object moving a force is applied for a certain amount of time (over a certain distance) The impulse will cause a change in momentum of an object and change its velocity
2) the object will continue to travel with a constant velocity for a distance
3) when the object encounters a force that acts on it for a certain amount of time (over a certain distance) the objects momentum will decrease and the velocity will become zero
Work-Energy Relationship example (throwing a ball; wrist only vs. whole arm)
throwing with only the wrist allows for little distance and time for the force to build up resulting in a very short, weak throw.
using your whole arm you can wind up and bring your shoulder and biceps and triceps and all the other muscles in to play over a longer time and larger distance resulting in a much farther, more powerful throw
Power
rate of doing work
P = U/t
- described as the ability to exert a force over a distance in a certain amount of time
-can be considered how fast or how slow work is done
- units are watts/joules/horsepower
Power can also be described as…
P = U/t where U = Fd so P = Fd/t and d/t = v so P = Fv Power = Force X velocity
In sports: power can decide rates in things like running or cycling
Cycling: gear: high force, pedal slowly; low force, pedal quickly
Running: stride length vs stride rate: long stride, large force, slower rate; short stride, small force, high rate
Power production in muscles
- is an important limitation to how we perform a skill
- contraction velocity (cv) increases our ability to produce force decreases
- cv decreases, force production increase
to produce the most power we need to…
decide which variable we need to maximize (speed or weight)
- for humans, the best choice would be a medium force and a medium velocity (35-60% of the muscles max contraction velocity)
- it also depends on the distribution of slow twitch and fast twitch muscles that a person has
