10. Work & Power

Question 1

What is Work?

Answer 1

Amount of energy transferred by a force acting through a distance

Work (W) = Force x displacement

Units: Joules (J)

Question 2

Is work a vector or scalar qunatity?

Answer 2

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)

Question 3

What is Positive Work?

Answer 3

Movement occurs in the same direction as force

• Concentric Muscle Actions (lifting barbell up)
• Muscles create action

Question 4

What is Negative Work?

Answer 4

Movement occurs in the opposite direction as force

• Eccentric Muscle Action (lowering barbell down)
• Gravity creates action

eccentric movements important for control and safety

Question 5

Imagine doing a bench press lifting a 40kg barbell 50 cm up and then down. How much work is done?

Answer 5

At the end of cycle, no work has been performed

Question 6

How do we calculate human work?

Answer 6

• Hard to measure the forces generated by the muscles
• We measure the weight of the object moved

Question 7

Can you have work without movement?

Answer 7

No movement = no work

This is why we use the term “mechanical” work

Question 8

Is work being performed during an isometric contration?

Answer 8

Work ≠ force

Work is not being perfomed but
force is still generated by the muscle

Question 9

What is Internal Work?

What is External Work?

Answer 9

Question 10

What is Power?

Answer 10

Power is how quickly or slowly work is done
○ Relying on either time or velocity

Question 11

What is the formula for Power?

Answer 11

Question 12

What is the best way to move a stack of books from one table to another?

Answer 12

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)

Question 13

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?

Answer 13

YES

Power = Work/Time
Time = Work / Power
Time = 80(9.81)(18)/240
= 58s

Question 14

What is Ergometry?

Answer 14

a science that measures work (and power)

Question 15

How can a treadmill measure power?

Answer 15

Measuring power using a treadmill

• Force: the weight of the runner
• Distance: measured distance on the treadmill
• Time: duration of the performance

Question 16

How can a bicycle ergometer measure power?

Answer 16

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