Week 4 - Work, Power & Energy

Question 1

Describe work (8 points)

Answer 1

• in biomechanics, work is the amount of force applied on a an object multiplied by the displacement that the force is applied over
• work is performed when a force is applied through a distance
• Work is a scaler quality. Has a magnitude, but no direction.
• Formula: w = f x s
  
  • W = work
  • F = force (N)
  • S = displacement (m)
• Measured in Joules (J)

Question 2

List steps to calculate work (3 points)

Answer 2

1. What is the average force that has been applied?
2. What is the displacement the force has been placed over?
3. The direction of the force: positive or negative

Question 3

Describe positive ad negative work (2 poins)

Answer 3

• Positive work work occurs when the direction of the force application and the displacement of the object are in the same direction
• Negative work occurs when the direction of the force application and the displacement of the object are in opposite directions

Question 4

List examples of work (10 points)

Answer 4

• Bench press raising = positive work
• Bench press lowering = negative
• Baseball pitch/throw = positive work
• Baseball catch = negative work
• Running is an example of negative and positive work
  
  • Before runner heel strikes the ground (when they first come in contact with the ground), their center of gravity will be at its highest point
  • GRF will be pushing up, but the runner’s center of gravity will drop as the move into a mid-stance position during the heel strike.
  • Because the body movement is pushing down while the GRF is moving upwards, this phase of running will be negative work.
  • As the runner transitions to mid-foot position through to the toe-off phase, just before they are completely airborne in the run, the height of their centre of gravity increases.
  • At this point, the GRF is moving in the same direction as the centre of gravity movement, making this type of work positive.

Question 5

Describe the mechanical work performed by muscles (6 points)

Answer 5

• Whether the work performed is negative or positive is dependent on the type of muscle action…
  
  • Concentric contractions involve positive work
  • E.g. Upward phase of a bicep curl
  • Eccentric contractions involve negative work
  • E.g. Lowering a barbell back to the floor.
• Isometric contractions produce no mechanical work as there is no change in muscle length and hence no displacement.

Question 6

Calculate muscle mechanical work of a bench press repetition (9 points)

Answer 6

• Total work = 0 J
• Why? Once we go down and then up, our displacement for the entire movement is 0. Hence, no matter what force is applied, when displacement = 0, work = 0.
• Alternatively, in the downward phase,
  
  • Work = 1000 (force) x -0.7 (Displacement).
  • Work = -700 J
• In the upward phase,
  
  • Work = 1000N X 0.7
  • Work = 700 J
• Hence, total work equals positive work and negative work which is equal to 0

Question 7

Describe power (8 points)

Answer 7

• Power is the rate of doing work, or how much work is done in a specific amount of time
• Expressed in Watts (W).
• Formula: P = Work / Time (watts)
• Another formula: P = f x v
  
  • Work = Force x Displacement
  • Hence Power = (Force x displacement) / time
  • Displacement/ time equals Velocity
  • Power = Force x Velocity

Question 8

How can athletes increase power? ( 3 points)

Answer 8

• Increase force
• Increase the velocity
• Increase both (best case scenario)

Question 9

Describe the force-velocity relationship ( 7 points)

Answer 9

• If you make a muscle contract so as to get maximum force out of it, then it only contracts very slowly.
• If you make it contract as quickly as possible, then you do not get much force.
• For example:
  
  • A 1RM deadlift displays high force but is a slow movement
  • A power clean has slightly less force and therefore slightly more velocity
  • A vertical jump would ideally have the jumper use as much force as they possibly can by pushing into the ground as hard as they possibly can. However, the force produced will still be lower than any sort of weight lifting sport and therefore will have a much higher velocity
  • Sprinting will have the greatest velocity out of these examples, since the movement is in a horizontal direction and does not push against gravity as much as the other examples. Therefore, the force will be the lowest amount

Question 10

Describe the power-velocity relationship (2 points)

Answer 10

• Typically, peak power will be achieved at 1/3 of maximum contraction velocity or maximum isometric force
• Does train slightly between individuals and also depends on type of training they are doing

Question 11

Why does a sprinter need power? (5 points)

Answer 11

• Need to apply large amount of force. But because they are moving at high velocity as well, the contact time with the ground is very small.
• Need to produce force quickly to benefit their performance
• Therefore, need to be powerful so they can perform high amounts of force in small periods of time
• The faster they run, the more crucial power is
• Better sprinters produce more force in a shorter amount of time, not the strongest or most forceful

Question 12

Describe sport examples of power (5 points)

Answer 12

• The generation of power is critical for:
  
  • Jumping activities
  • Take –offs for aerial activities
  • Starts in events that want to move the body over a set distance in the shortest time
  • Continuous events in which there is an intermittent application of force

Question 13

Describe ways to measure power (10 points)

Answer 13

• Direct measurement via force plate and Linear position transducer.
  
  • Force plate will read force produced
  • Linear position transducer is a ‘string’ that moves up and down
  • Linear position transducer will calculate the velocity of the ‘string’ moving up and down
  • Multiply both together to calculate power
• More common/ Easier tests
  
  • Vertical jump test for vertical power
  • Standing Broad Jump to assess horizontal and vertical power
  • Plyometric push up
  • These tests have a velocity component and athlete needs to produce the force in a short amount of time

Question 14

Describe energy (5 points)

Answer 14

• In mechanics, Energy is defined as the capacity to do work.
• Three forms of energy:
  
  • Kinetic energy: when an object is moving
  • Potential energy: based off the objects position in terms of how it will be affected by gravity
  • Strain or elastic energy

Question 15

Describe kinetic energy (8 points)

Answer 15

• Energy due to linear motion
• A moving body has energy and the capacity to do work.
• Formula: K.E = 1/2m v2
• Where:
  
  • K.E = Kinetic Energy
  • m = mass of body
  • v = Velocity
• K.E is measured in Joules

Question 16

Describe potential energy (10 points)

Answer 16

• Energy due to position
• P.E is stationary
• When body hits ground, it does work on the ground
• Formula: P.E = m x g x h
• Where:
  
  • P.E = Potential Energy
  • m = mass of body
  • g = gravity
  • h = height above ground
• P.E is measured in Joules

Question 17

Describe conservation of mechanical energy (6 points)

Answer 17

• For an airborne body (no air resistance) total energy is conserved (remain constant)
• Formula: C = (P.E + K.E)
• Where:
  
  • C = total mechanical energy
  • P.E = potential energy
  • K.E = kinetic energy

Question 18

Provide and explain an example of K.E and P.E relationship (8 points)

Answer 18

• Throwing a ball straight up:
• Ball stationary on ground
  
  • K.E = 0
  • P.E = 0
• At release, Velocity is highest, hence, K.E is at its highest. Because it is just off the ground, meaning it has very little height, P.E is small
• As the ball rises, gravity is decelerating the ball, hence, KE, due to velocity decreasing, is decreasing. As the ball is getting further off the ground and gaining more height, P.E is increasing.
• As the ball is at the top, it is stationary (momentarily), hence, K.E = 0. P.E is at its maximum as it is at its maximum height.
• On the way down, gravity accelerates the ball, hence, K.E is increasing and will be at its highest just before it hits the ground. As the ball is getting lower, potential energy is decreasing.

Question 19

Describe strain or elastic energy (7 points)

Answer 19

• Energy due to elasticity of bodies (return to original shape after being deformed)
• “Recoil” has capacity to do work
• Formula: S.E = 1/2k x (x)2
• Where:
  
  • S.E = strain energy
  • k = stiffness
  • x = distance of deformation

Question 20

Provide and explain an example of S.E moving into K.E (7 points)

Answer 20

• Example: pole vaulting:
  1. The vaulter applies force to the pole which causes the pole to bend
  2. The more the pole bends, the more force is required to add additional bend.
  3. The bend results in the storage of strain energy in the pole
  4. The vaulter maneuvers into a position in which they do not supply enough resistance to maintain the bend in pole.
  5. The pole then releases the stored S.E in the form of K.E.
  6. The correct timing of the return of energy from the pole enables the vaulter to jump higher.

Question 21

Describe the work-energy relationship (5 points)

Answer 21

• The work produced on an object will equal the change in energy on that object.
• Looking at the definitions of work and energy, there are a few similarities:
  
  • Work = means by which energy is transferred from one object to another
  • Energy = capacity to do work
  • Unit of measure for work and energy = Joule

Question 22

Provide an example of the work-energy relationship (7 points)

Answer 22

• Example: a bench press repetition
  
  • Work = Force x Distance
    = 700 J of work done
  • Kinetic energy = unchanged
  • P.E = m x g x h
    = 700 J
• Archery
  
  • Striking exerts force over distance (work) = energy given to arrow
  • All K.E lost when arrow hits target = work done, change in energy

Question 23

Describe work-energy relationship applications (13 points)

Answer 23

• This relationship is important in two certain scenarios:
  
  • Apply work to increase energy
  • Apply work to decrease energy
• To increase energy:
  
  • Work-energy relationship – large change in kinetic energy requires application of a large force over a long distance
  • Work = Force x Displacement
  • Example: shot put
• To decrease energy:
  
  • Can also be used to explain techniques used in transferring (or absorbing) energy
  • Average force you must exert depends on how much energy must be absorbed and over how long a distance you can apply the force
  • Force = Work/ Displacement
  • Example: catching a ball, landing from a jump
  • Work-energy relationship is at play in various protective equipment as well (crash mats, running shoes, gloves, air bag). These equipment items apply the work over an increase in displacement as they deform during impact. This leads to a decrease in force.