Law of Conservation of Energy

Law of Conservation of Energy

Essential Question:

It’s the LAW! • Matter/ energy can NOT be created or destroyed • But…it can be converted from one form to another- transformation!

Give some examples.. • An energy conversion that the utility company does for us. • An energy conversion that is due to the sun. • An energy conversion that changes Potential energy into Kinetic Energy

Define: Kinetic Energy Potential Energy

Kinetic Energy Energy of motion – Whole object or molecules (temperature)

Potential Energy Stored energy – Position, stretched shape, height above ground, chemical bonds

Formula! • Potential energy- P.E. • P.E. = mgh • m = mass of object (kg) • g = acceleration due to gravity 9.8 m/s2 • h = height in meters • Units are joules. Kg m2/s2

How much potential energy is contained in a 2.0 kg rock being held on top of a 200m tall tower? • P.E. = mgh • P.E. = 2.0 kg x 9.8 m/s2 x 200m = 3,920 Joules

Solving variables… • How would you solve for mass or height of an object? • A 5.0 kg ball has 4,500 J of energy. How high up is the ball? • P.E. = mgh, so … P.E./mg = h • 4500 J / (5.0 kg x 9.8 m/s2) = h h = 91.84 m

Formula Kinetic Energy KE = ½ mv2 KE = kinetic energy M = mass in kilograms V= velocity (speed) in meters/second Energy unit is joules

Calculate Example: 50 kg child runs at 5 m/s KE = ½ mv2 ½ x 50 kg x (5 m/s)2 ½ x 50 kg x 25 m2/s2 = 625 joules You try: 10 kg ball rolls at 2 m/s

Conservation of energy! • Energy is conserved. This means that energy is transferred from one form (or object) to another. • A rolling ball that hits another ball will transfer some/all of its energy therefore: • K.E.1= K.E.2 • Or: (½ mv2)1 = (½ mv2)2

Conservation of energy examples: • 1. A 3.0 kg ball is rolling along at 1.5 m/s and hits a second ball which weighs 6.0kg. How fast does the second ball move? • K.E.1 = K.E.2 Or: ½ mv2 = ½ mv2 • ½ 3.0 Kg (1.5 m/s)2 = ½ 6.0 Kg V2 • 1.5 Kg (1.5 m/s)2 = 3.375 J = ½ 6.0 Kg V2 • = 3.375/( ½ 6.0 Kg) = V2, • 1.125 = V • V = 1.061 m/s!

Solving for variables • How do we solve for mass or velocity of a moving object? • K.E. = ½ mv2 • How much does a ball with 12,000 J of energy travelling 2.0 m/s weigh? • Solving for mass: K.E. = ½ m v2 , • m = K.E. / ½ v2, so… 12,000 / ½ (2.0ms)2 = 6,000 kg

Solving for velocity • K.E. = ½ mv2 • A 2,500 kg car has 7500 J of energy. How fast is it going? • K.E. / ½ m = v2 ! *Squared! • 7500 J / ½ 2500kg = v2 = 6 m/s v= 2.45 m/s • So … √ 7500 J / ½ 2500kg = 2.45 m/s

Law of Conservation of Energy