Energy Transformations and Law of Conservation of Energy

“What is Energy?” Energy is what makes things ‘happen’ (turning the lights on, driving a car, using a Bunsen, etc.) Energy is measured in ______. There are many different forms of energy.

“Energy” has many different forms: Light energy Heat energy Sound energy Electrical energy Radiation energy Kinetic energy Nuclear potential energy Chemical potential energy Gravitational potential energy Elastic potential energy

Law of Conservation of Energy Energy can neither be created nor destroyed. Therefore the total energy in the system is always conserved. Energy can only be transformed or transferred.

The three main forms of Energy in Y12 Physics: 1. Kinetic Energy 2. Gravitational Potential Energy 3. Elastic Potential Energy

Kinetic Energy m = mass of the moving object v = speed of the moving object

Gravitational Potential Energy m = mass of the moving object g = acceleration due to gravity (9.8 ms-2 ↓) h = height of the object

Example: EK & EP Transformation A bowling ball (3 kg) is dropped from the top of a cliff. The cliff is 40 m high. Calculate the gravitational potential energy in the ball at the top. Calculate the kinetic energy of the ball at the bottom. Calculate the instantaneous velocity of the ball at the bottom.

HOWEVER, Therefore the total energy is still conserved! the kinetic energy at the bottom will be just a little bit less than the gravitational potential energy at the top – WHY???? Always some energy will turn into heat energy and sound energy, because of the friction (from the air, in this example). Therefore the total energy is still conserved!

A bullet of mass 30 g is fired with a speed of 400 ms-1 into a sandbag A bullet of mass 30 g is fired with a speed of 400 ms-1 into a sandbag. The sandbag has a mass of 10 kg and is suspended by a rope so that it can swing. Calculate the maximum height that the sandbag rises as it recoils with the bullet lodged inside.

Elastic Potential Energy & Hooke’s Law

Elastic Potential Energy k = spring constant (unit: ________) x = length of stretch or compression

Gravitational Potential Energy Kinetic Energy Elastic Potential Energy

Example: Trampoline A little kid is playing on a trampoline. At one point, he is at the lowest position where the trampoline is stretched down by 50 cm. The spring constant of the trampoline is 0.80. Calculate the elastic potential energy in the trampoline at this position. Calculate the maximum height the kid will reach, if he does not use any energy of his own.

Question: When a mass of 1.0 kg was hung on a spring it extended the spring by 40 cm. If the mass is doubled to 2.0 kg, how much would the spring be extended by? What if the mass is tripled?

Hooke’s Law or F = force applied to stretch or compress x = length of stretch or compression k = spring constant (unit: _______)

Example: A 94 kg child stands on a trampoline and causes the trampoline to sag by 2.90 m. Calculate the trampoline’s spring constant. Calculate the elastic potential energy stored in the trampoline.

Force (N) By using Hooke’s Law and the graph shown, work out the spring’s Spring Constant. Compression (cm)

A bungi jumper (75 kg) jumps off a bridge over a river A bungi jumper (75 kg) jumps off a bridge over a river. The spring constant of the bungi cord is adjusted so that the jumper’s head just touches the river at maximum stretch (30 m). If the natural length of the cord is 10 m, calculate the cord’s spring constant. State any assumption(s) you make.

A toy airplane (500 g) is hanging at the end of a spring A toy airplane (500 g) is hanging at the end of a spring. The spring is 48.0 cm long when hanging vertically. When the airplane is hung from the end of the spring, the length of spring becomes 80.0 cm. Calculate the spring constant. (M) Write a unit with your answer. (A) Calculate the energy stored in the spring when a second toy of mass 400 g is also hung along with the airplane. (M) The 500 g airplane is now hung on a stiffer spring, which has double the spring constant. Discuss how this affects the extension and the elastic potential energy in the spring. (E)

Worksheet NINE