The Principle of Conservation of Energy (Edexcel IAL Physics)

Spec point — 20

The Principle of Conservation of Energy

• The principle of conservation of energy is a law of Physics which always applies to a closed system

• To apply conservation of energy, heat losses are usually ignored during the calculation stage

• In reality there are always some energy losses from the system

  • These should be mentioned when comparing calculated, ideal values to real-life situations

• Conservation of energy is often applied in questions about exchanges between kinetic energy and gravitational energy

• Common examples include:

  • A swinging pendulum
  • Objects in free fall
  • Sports such as skiing or skydiving where gravity is causing motion and few drag forces apply

• The gravitational potential energy stored initially is transferred to kinetic energy, or vice versa

• This allows either;

  • Final velocity to be found from the distance the object moved, or
  • Height of a drop from the final velocity

Worked Example The diagram below shows a skier on a slope descending 750 m at an angle of 25° to the horizontal.

Calculate the final speed of the skier, assuming that he starts from rest and 15% of his initial gravitational potential energy is not transferred to kinetic energy.

Answer:

Step 1: Write down the known quantities

• Vertical height, h = 750 sin 25°
• E_k = 0.85 E_p

Step 2: Equate the equations for E_k and E_grav

E_k* *= 0.85 E_grav

½ mv² = 0.85 × mgh

**Step 3: Rearrange for final speed, v

** Step 4: Calculate the final speed, v

Exam Hint Gravitational energy:

• This equation only works for objects close to the Earth’s surface where we can consider the gravitational field to be uniform.

Kinetic energy:

• When using the kinetic energy equation, note that only the speed is squared, not the mass or the ½.
• If a question asks about the ‘loss of kinetic energy’, remember not to include a negative sign since energy is a scalar quantity.