Work, Energy & Power

Question 1

What is work?

Answer 1

The product of force and displacement.

So if we generate a force to move an object over a given displacement, we have done work on that object.

Several forces may act at the same time, so work = total force x displacement

In fact, with many forces acting, work may be zero if resultant force is zero

Question 2

Examples of work

Answer 2

Work is done on our body during a jump, and on weight that we lift.

Work is also done on the water in a swimming or rowing stroke.

Work = F x d = N.m

Actual units are joules (J)

Question 3

What is power?

Answer 3

The product of force x velocity.

In order to lift a weight, work must be done; but an Olympic lifter needs the bar to move quickly to lift successfully.

Power = force x velocity
= N x m.s_1
Actual units are watts (W)

Question 4

Since Power is the rate of doing work, how else may it be calculated?

Answer 4

P = W/t

Since work = F x d,
power = Fd/t

We can measure F, d and t easily in most cases, so we can measure an athletes power easily.

Can also be angular:
Torque x angular displacement / time

Question 5

Energy. Anything that moves has…

Answer 5

Kinetic Energy (KE)

Question 6

Energy. Anything that has potential to move has…

Answer 6

Potential Energy (PE)

Question 7

Energy. All objects contain heat, known as…

Answer 7

Thermal Energy (TE)

Question 8

Kinetic energy is directly related to…

Answer 8

The mass and velocity of an object.

KE = 1/2mv^2

So big, fast objects have a high KE

Unit is joules, same as work

Question 9

Name three forms of Potential energy

Answer 9

Chemical
Gravitational
Elastic
Magnetic

Question 10

How do we measure gravitational PE?

Answer 10

Gravitational PE = mgh

• m = mass
  g = acceleration due to gravity
  h = height of object

So we can measure the gravitational potential energy of an object easily.

Question 11

Total Energy =

Answer 11

KE + PE + TE

Work (W) done on the system is: W = /\KE + /\PE + /\TE

Question 12

Elasticity…

Answer 12

is the tendency to return to shape after distortion (steel is elastic)

Question 13

Compliance…

Answer 13

is the tendency to be deformed when a force is applied (dough)

Question 14

Stiffness…

Answer 14

is the inverse of compliance: k = F/x, or force per elongation

Question 15

PE in hopping

Answer 15

PE is stored in the elastic tendon, which then recoils with KE

Question 16

Equation for elastic potential energy.

Answer 16

E = 1/2kx^2

So if we stretch the tissue further, or stretch a stiffer tissue as far, more energy is stored

Question 17

Countermovements…

Answer 17

allow PE storage

e.g. dip before a jump

Question 18

Recoil of elastic tissues leads to…

Answer 18

increased KE

Question 19

Work/energy ratio is known as?

Answer 19

Efficiency.

How much metabolic energy (from ATP) it takes to perform a given amount of work.

Question 20

Energy per metre or energy for a given velocity.

Answer 20

Economy.

Improving power output and efficiency/economy are often goals of technique optimisation.

Question 21

Summary

Answer 21

• The ability to perform work, at high power outputs, using least energy, is often important for sporting success
• The performance of movement requires work to be performed
• The speed (i.e. Power) and efficiency with which work is performed is optimised by exchanging the form of energy (e.g. Gravitational, elastic, kinetic)