Lecture 7, Work Power, Energy Flashcards
Mechanical Work
amount of energy transferred by a force acting through a distance
how much force did it take to move the system
work (W) = force (N) x displacement (m)
units: joules (J) - work is measured in J
- forces and movement (creating a force that results in movement)
- energy is the ability to do work (looks at different ways we can create those forces)
- energy and work is the same thing
- work is a scalar quantity even though force and displacement are vector (work is a dot scalar - not just a number and sign as
Positive Work and Negative Work
we have both positive and negative values
- positive work means both vectors are alignment with one another and they are moving in same direction - shortening muscles (concentric) - movement occurs in the same direction as the force - concentric muscle action
- negative work - movement occurs in the opposite direction as the force (does not matter if it is not completely opposite it just has to be different that the way force is pushing) - biceps brachii is controlling movement but is still being lengthened as the stool is going down but the brachii is trying to pull it up(eccentric muscle action
Dot Scalar - Work
- number measurement
- magnitude only
- measure of how closely vectors align
Measuring Work
- how do we calculate human work?
◦ hard to measure the forces generated by the muscles
◦ we measure the weight of the object moved - can you have work without movement?
◦ no movement = no work
◦ this is why we use the term “mechanical” work - but does that mean that the muscle “isn’t working”?
- work ≠ force
- isometric muscle action (force is there but no work has occurred)
Mechanical Work (internal vs external)
raising the arms (internal work - moving the body) - if you are moving you, inside the system to move parts of the system (calculate internal mechanical work)
pushing the stroller (external work - moving another system) - how far the stroller has moved
Power (formula)
the time-rate of doing work
scalar quantity
power (p) = work / time
power (p) = force x displacement / time
power (p) = force x velocity
units: watts (J/s)
- how fast can you generate work and how much work are you generating within a certain time period
- way of qualifying your work - assessment
- number only as we only care about time here
- force velocity tradeoff - fast but weak or strong but slow
Power (what is it?)
- power is how quickly or slowly work is done
◦ relying on either time or velocity - if you had to move a stack of books from one table to another
◦ the goal is to maximize power output - should you move one book at a time
◦ require a small force (which would allow for a quick movement) - should you move the entire stack at once
◦ require a large force (which would slow down the movement) - what is most amount of force i can generate and fast without seeing a tradeoff and losing one or the other
- power often requires a trade-off between force and velocity
Measuring Work and Power
ergometry is a science that measures work (and power) - the measurement of energy
- measuring power using a treadmill
◦ force: the weight of the runner
◦ distance: measured distance on the treadmill
◦ time: duration of the performance
- measuring power using a bicycle ergometer
◦ force: resistance against the motion of the flywheel
◦ distance: measure a point on the flywheel
◦ time: duration of the performance
- to see how much force, energy and power they can generate
Mechanical Energy
energy is the ability to do work (if work is the ability to create force)
- scalar quantity
there are different kinds of energy
- kinetic energy (KE) - means forces and movement (the amount of friction and resistance you are exposed to)
◦ capacity to do work taking motion into account
◦ translational or rotational
◦ the ability to do work because of your movements
- KE = ½mv2 - kinetic energy is a different way of exploring momentum - the effect movement has on only object
- units: joules (J)
Kinetic Energy
- kinetic energy is the ability to due work when moving or the ability to apply a force so there is a displacement because of movement
- moving bowling ball will knock down the stationary pin
◦ the heavier the ball used, the more work you can do
◦ the faster the ball is thrown the more work you can do - by virtue of the fact that the ball is moving it has the ability to knockdown the balls
Potential Energy
energy is the ability to do work
- scalar quantity
there are different kinds of energy
- potential energy (PE) - your position and place relative to another object and typically vertical position
◦ capacity to do work taking position into account
◦ due to gravity
- PE = magh
- units: joules (J
Potential Energy (2)
- potential energy is the ability to due work because of position or the ability to apply a force to create movement is dependent upon the position of an object
- a hammer dropped onto a nail will push the nail into the wood
◦ the higher the hammer is held from the nail, the more work you can do
◦ the heavier the hammer you use, the more work you can do - the bigger and heavier the hammer the more force and movement you can create
- can take about how fast your are moving and where your position is
Relationships Between Work and Energy (in sports
- the athletes applies a force upon landing
◦ this creates a reaction force
◦ the reaction force stops the athlete from moving - a high reaction force could injure the athlete
◦ the athlete sinks into the sand
◦ the forces are applied over a larger distance - as displacement increases the impact forces decreases
◦ so the reaction force decreases - we should call sand pits “energy absorbing”
◦ kinetic energy is absorbed
Work-Energy Theorem
- energy can be transferred from one form to another
◦ W = ΔPE + ΔKE
◦ W = Δ(magh) + Δ(½mv2 - as a ball is dropped from a height, it loses potential energy
- as the ball drops, the faster it travels gaining kinetic energy
- this assumes that no deformation occurs to the systems
- you look at the impact of both kinetic and potential energy
- as an object goes up it gains potential energy but it loses kinetic energy when it goes down it loses potential but gains kinetic as velocity is speeding up
- want to look at how both can be converted to one another
Conservation of Mechanical Energy
- energy CANNOT be created or destroyed
◦ energy can be transferred from one system to another
◦ energy can be transformed from one type to another - when gravity is the only acting external force, a body’s mechanical energy (C) remains constant
◦ C = ΔPE + ΔKE
◦ C = magh + ½mv2 - when objects fall or are projected, we can use this information to calculate either velocity or displacement of the object
- big C is total mechanical energy that is constant and does not change
