Kin 216: Work, Power and Energy

Another way of describing how forces create movement

Energy

Difference between positive and negative work

Positive work = Force and displacement are in the same direction

Negative work = Force and displacement are in the opposite direction

Identify the different types of energy

Kinetic energy = capacity to do work taking velocity into account

Potential energy = capacity to do work taking height into account

Explain how energy is transferred

Energy is transferred when force acts on an object causing displacement.

Explain how energy is transformed

Energy from one type (KE) to another (PE) or v/v

Takeoff of a ball: KE is maximum, PE = 0

At the peak: PE = maximum, KE = 0

Thus, energy was transformed as the ball was rising up in the air from KE to PE

What is work

Applying forces to a system to move the object

Transferring energy from one system to another

W = fd

what are two ways you increase power?

Can do more work when the mass of the object is smaller (thus less force to move)

Or if you can decrease the d (the distance through which you need to move it)

Mechanical work

Using force to create movement

How much force did it take to move the system

Is work scalar or vector

Neither, work is dot scalar

Dot scalar

Magnitude only. 

Has a sign but it doesn’t mean direction. Sign means how are the two vectors aligned i.e. reflect whether or not vectors are going in the same direction or different

Positive work

Movement occurs in the same direction as the force

Pushed to the left, moved to the left 

Concentric muscle action: muscle shortens, body part moves in the same direction

Negative work

Movement occurs in the opposite direction as the force 

If vectors going in opposite direction, work is negative

Eccentric muscle action: muscle is lengthening

What’s the point of negative work

It slows down action protecting you and the object you’re holding

Lifting the barbell up. What kind of work? What creates the action

Positive work; muscle creates the action

Lowering the barbell down. What kind of work? What creates the action?

Negative work; gravity creates the action

Can you generate forces but have no movement occur? Give an example

Yes. Concept of work and force are two different things

Work means movement has occurred

You can generate forces, but if no movement occurring, no work 

e.g. isometric work

Two types of work

Internal work: moving the body

External work: moving another system

Describe power. Is it scalar or vector? UoM?

Rate of doing work or how quickly you can express strength. Taking into account time 

Power is all about the number so it’s scalar

P = W/t

UoM Watts

Power tradeoff

Apply low level of work quickly or huge amount of force but not so fast

Emphasize speed of the movement or the force

What’s the goal of power

To maximize power output

What’s the most amount of force that i could generate w/o sacrificing velocity

What is ergometry?

Science that measure work and power

Two examples of equipment we can use to measure power

Treadmill and Bicycle Ergometer

What is energy?

Energy is the ability to do work

Work = energy

Energy is a scalar quantity

Kinetic energy

Ability to do work when moving

Use movement to create forces that will then create movement of that same object 

KE =1/2 mv²

Potential energy

Capacity to do work taking height into account

Ability to do work because of position 

PE = ma_gh

what is required to do work

to do work requires change in energy

when you’re at a constant velocity (static state) are you doing work?

No, because you have no change in energy

In order to do work, to generate force you must

give up something:

loss of velocity in one object that creates movement in another

loss of height in one object that creates movement in the other

Example of energy change by transforming from one type to another

Take off (KE = max, PE = 0)

Maximum height (KE = 0, PE = max)

What is the point of sand pits?

They absorb incoming force, which reduces the reaction force coming back on the performer. Decreases chance of injury

Describe the conservation of mechanical energy

When no external forces present, only gravity, then total energy system has available to it C is constant

Energy cannot be created or destroyed 

C = PE + KE

C =

Total mechanical energy of a system

The two kinds of mechanical energy are KE and PE
C = KE + PE
When you jump, the KE and PE changes but the total mechanical energy of the system remains the same

Work, power, energy summary

Work: moving objects by generating forces

Power: how quickly you can do that 

Energy: using velocity or position (height or displacement) to create movement

Scalar, vector or dot scalar: Work, power and energy

Work = dot scalar. magnitude. sign shows if vectors are aligned (f and d same direction) or not

Power = scalar

Energy = scalar

Internal vs external work example

Internal work = moving your arm to stretch your body

External work = pushing a stroller. Stroller is the system, you’re apply force to move it

What is the purpose of eccentric muscle action i.e. negative work?

To slow movements down

Explain how platform diver’s energy changes from the top of the diving board to the when they hit the water

Top of the diving board: Max PE, No KE

As they dive down, KE increasing, PE decreasing

When they hit the water, KE = max, PE = 0