Edexcel IGCSE Physics 复习笔记 4.2.4 Work, GPE & KE

• Whenever any work is done, energy gets transferred (mechanically) from one store to another
  • This is a consequence of conservation of energy

   

• The amount of energy transferred (in joules) is equal to the work done (also in joules)

Energy transferred (J) = Work done (J)

• If a force acts in the direction that an object is moving, then the object will gain energy (usually in the form of kinetic energy)
• If the force acts in the opposite direction to the movement then the object will lose energy (usually as heat)

• Common forms of energy that work can be transferred to:
  • Kinetic Energy, when speeding an object up
  • Gravitational Potential Energy, when raising an object higher
  • Thermal Energy, in heating the object up
  • Sound Energy, in producing noise

   

• Take the example of an object which is pushed horizontally with a 100 N force a distance of 10 m
• The work done on the object is equal to:

W = Fd

W = 100 N × 10 m = 1000 N m

• This work transferred energy to the object in the form of kinetic energy
• The object started with no kinetic energy and now has 1000 J of kinetic energy

Part (a)

• • Work is done by the woman as she exerts a force on the rope to pull the bucket up
  • The work done on the bucket is due to overcoming the weight of the bucket for a distance of 15 m
  • As the bucket rises, the work done is stored as gravitational potential energy

Part (b)

Step 1: List all of the known values

• • Mass, m = 10 kg
  • Gravitational field strength, g = 9.8 N/kg
  • Height, h = 15 m

   

Step 2: Write the equation relating work, force and distance

Work = Force × Distance

Step 3: Write out the equation for weight and substitute it into the work equation

Weight = m × g

Work = m × g × h

• • Note: This is the equation for gravitational potential energy

   

Step 4: Calculate the work done on the bucket

Work = 10 × 9.8 × 15 = 1470 N m

Step 5: Convert the work done into energy transferred

Energy transferred in Joules = Work done in newton-metres

Energy transferred = 1470 J

• • The bucket gained 1470 J of gravitational potential energy