Work, Energy, Power

Question 1

What is a definition of work?

Answer 1

The energy transferred to a body by a force

Question 2

What two requirements are there for work to be done?

Answer 2

1. A force needs to act on a body
2. Movement needs to occur

Question 3

What is the equation for work?

What are its units?

Answer 3

Force x distance

Nm = Joules (J)

Question 4

What is power?

What is the equation to calculate power?

What are the units of power?

Answer 4

The rate at which work is done

Work / time

J.s-1 = Watt (W)

Question 5

What is the formula for kinetic energy?

Answer 5

m.v2/2

Question 6

How is the formula for kinetic energy derived?

Answer 6

= F.d
=m.a.d
=m.((v2-v1)/t).(v1+v2).t/2)
(where v1 -= 0)
=m.v2^2/2

Question 7

What is the equation linking power to force and how is it derived?

Answer 7

Power = work/time

= F.d/t

=F.v
(either instantaneous or average)

Question 8

What is the formula for angular power?

Answer 8

T.w

Question 9

What is the sign convention for muscular power?

Answer 9

+ve - working concentrically

-ve - working eccentrically

Question 10

Definition of energy (from a mechanical perspective)?

Answer 10

The capacity of a body to do work

Question 11

What are the 3 types of energy in mechanics?

Answer 11

1. Kinetic energy
2. Potential energy
3. Elastic energy

Question 12

Formula for rotational kinetic energy?

Answer 12

I(w^2)/2

Question 13

Derivation of rotational kinetic energy formula?

Answer 13

= kinetic energy of sum of all individual parts (molecules of body)

= sum (mass(i).(v(i)^2)/2

= sum (mass(i)(w(i)^2)(r(i)^2)/2

= (w^2) sum (mass(i).(r(i)^2))/2

= (w^2).I/2

Question 14

Formula for potential energy?

Answer 14

m.g.h

Question 15

Formula for elastic energy?

Answer 15

stiffness.(change in length^2)/2

= k.(delta l ^ 2)/2

Where k = spring constant

Question 16

Derivation of formula for elastic energy?

Answer 16

Compressive force F = k . distance compressed (l)

Where k = spring constant

Energy = work done = integration of the force distance curve

= k . l^2 / 2

Question 17

What is the law of conservation of energy?

Answer 17

In a closed system energy cannot be created or destroyed

Question 18

What is a closed system?

Answer 18

A system which is isolated from its surroundings and does not experience any forces external to it

Question 19

What concept and formula do you use to determine the velocity of block which has slid h metres measured vertically down a slope of frictionless ice of varying gradient?

Answer 19

Law of conservation of energy

m.g.h = m.v^2/2

Question 20

What is the relationship between work done and energy

Answer 20

Change in energy = work done

Question 21

Explain what constitutes positive and negative work regarding muscular force.

What is the application of this with regard to a dumbbell curl?

Answer 21

Positive work = force applied in same direction as movement takes place (muscle working concentrically)

e.g. upwards movement of dumbbell curl

Negative work = force applied in opposite direction as movement takes place (muscle working concentrically)

e.g. downwards movement of dumbbell curl

Question 22

What is the definition of internal vs external work?

Give examples

Answer 22

Internal work - work which results from the motion of body segments (e.g. dumbbell curl)

External work - work which is done on the whole body (e.g. going up/down stairs)