10. Work & Power Flashcards
What is Work?
Amount of energy transferred by a force acting through a distance
Work (W) = Force x displacement
Units: Joules (J)
Is work a vector or scalar qunatity?
Neither, work is a Dot Scalar
If F and d are in the same direction: + (R,R)(U,U)
If F and D are in opposite directions: - (R,L)((U,D)(R,U)
What is Positive Work?
Movement occurs in the same direction as force
- Concentric Muscle Actions (lifting barbell up)
- Muscles create action
What is Negative Work?
Movement occurs in the opposite direction as force
- Eccentric Muscle Action (lowering barbell down)
- Gravity creates action
eccentric movements important for control and safety
Imagine doing a bench press lifting a 40kg barbell 50 cm up and then down. How much work is done?
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
Is work being performed during an isometric contration?
Work ≠ force
Work is not being perfomed but
force is still generated by the muscle
What is Internal Work?
What is External Work?
What is Power?
Power is how quickly or slowly work is done
○ Relying on either time or velocity
What is the formula for Power?
What is the best way to move a stack of books from one table to another?
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 (allowing for a quicker movement)
Should you move the entire stack at once
- Require a large force (which would slow down the movement)
Jim arrives at his office building at 8:29 am and needs to get to a meeting on the third floor that starts at 8:30 am
● There are 60 stairs between the ground floor and the third floor (each step is 30 cm)
● Jim powers his 80 kg body up the stairs using 240 W
● Will he make his meeting in time?
YES
Power = Work/Time
Time = Work / Power
Time = 80(9.81)(18)/240
= 58s
What is Ergometry?
a science that measures work (and power)
How can a treadmill measure power?
Measuring power using a treadmill
- Force: the weight of the runner
- Distance: measured distance on the treadmill
- Time: duration of the performance
How can a bicycle ergometer measure power?
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
