Work, Energy, and Power

Question 1

Work

Answer 1

product of force and the displacement in the direction of that force

U = Fd

Question 2

Weightlifting and work

Answer 2

zero net work because the overall displacement is zero

no mechanical work done but there was physiological work done by the lifter

Question 3

+/- work

Answer 3

• 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)

Question 4

positive work in the human body

Answer 4

• 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

Question 5

negative work in the human body

Answer 5

negative work is done when the muscles contract but the limb moves in the opposite direction of the muscle force
- eccentric contraction

Question 6

work (general, not +/-) in the human body

Answer 6

• muscle contractions can do zero mechanical work

- isometric contraction –> muscle is contracting but limb is not moving

Question 7

Energy

Answer 7

capacity (or ability) to do work

- heat, light, sound, chemical, and mechanical

Question 8

forms of mechanical energy

Answer 8

kinetic, potential

Question 9

kinetic energy

Answer 9

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

Question 10

kinetic energy vs. momentum

Answer 10

KE = 1/2 mv^2

- the units are the same (KE = kgm^2/s^2, same as P = N.m)

Question 11

KE is easier to determine than work because…

Answer 11

• determining KE is easier than work because mass/velocity is easier to measure than force

Question 12

Potential Energy

Answer 12

• gravitational potential energy

- strain energy (deformation of an object)

Question 13

gravitational potential energy

Answer 13

related to the weight of an object and it’s height above the ground
PE = mgh OR PE = Wh (W = weight)

Question 14

Strain Energy

Answer 14

-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

Question 15

in sports, energy is possessed by athletes and objects due to their…

Answer 15

motion (kinetic energy), their position above the ground (gravitational potential energy), and their deformation (strain energy)

Question 16

Energy conservation

Answer 16

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

Question 17

pole vaulter example

Answer 17

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

Question 18

Work-Energy Relationship

Answer 18

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

Question 19

Work-Energy Relationship in Depth (3 step - initial force, consequence, reaction)

Answer 19

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

Question 20

Impulse-Momentum Relationship in Depth (3 step example - initial force, consequence, reaction)

Answer 20

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

Question 21

Work-Energy Relationship example (throwing a ball; wrist only vs. whole arm)

Answer 21

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

Question 22

Power

Answer 22

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

Question 23

Power can also be described as…

Answer 23

P = U/t 
where U = Fd
so P = Fd/t and d/t = v
so P = Fv
Power = Force X velocity

Question 24

In sports: power can decide rates in things like running or cycling

Answer 24

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

Question 25

Power production in muscles

Answer 25

• 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

Question 26

to produce the most power we need to…

Answer 26

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