Chapter 5: Work, Power, & Energy

Objectives:
  • Define:
    • Mechanical Work
    • Energy
    • Kinetic Energy
    • Gravitational Potential Energy
    • Strain Energy
    • Power
  • Distinguish the differences between positive and negative work
  • Explain the relationship between mechanical work and energy




Work is the product of force and displacement.
U = Fd
  • U = Work done on an object
  • F = Average force exerted on the object
  • d = Displacement of an object along the line of action of the average force

A lineman in football exerts an average force of 500N against the opposition while moving through a displacement of 2m in the direction of the force. How much work did the lineman do to the opposition?

How much work does a weightlifter do?

Energy is the capacity to do work
  • Potential Energy
    • Gravitational Energy=-mgh
    • Strain Energy=0.5kΔx2
  • Kinetic Energy=0.5mv2



How do we generate greater velocities?

Energy transfer in trampolining

Why does a high jumper use a run-up?
(Alma Richards - High Jump Gold Medal 1912 Olympics 1.93m)
(Platt Adams - Standing High Jump Gold Medal 1912 Olympics 1.63m)

2012 London Paralympic Games

Why did a shot putters starting postion change?

Adam Nelson decribing work-energy in the shot put.


U = ΔGPE + ΔKE + ΔSE

Work & energy in the javelin


Question: If a baseball pitcher wants to be able to pitch 82 mph, how much force should he train with (Assume he pushes the 0.15 kg ball through a displacement of 1.20 m during the pitch).

Conservation of Mechanical Energy
The total mechanical energy of an object is conserved if no external forces act on the object other than conservative forces (gravity, strain, electric, and magnetic).

Which of the following situations would have conservation of mechanical energy?
  • Diver in the air
  • Skater spinning on the ice
  • Hanglider
  • Gymnast in a giant swing

Pole Vaulting energy (men, women)

Power is the rate of doing work (measured in Watts)
P = U/t or P = F(v)

In a vertical jump test, a 60-kg student jumps 60 cm, whereas a 90-kg student jumps 45 cm. Assuming both jumpers applied force the ground for the same amount of time, which jumper was more powerful?