Energy and Mechanical Energy Energy Review Remember: Energy is the ability to do work or effect change. Usually measured in joules (J) One joule represents the energy needed to move an object 1 m of distance using 1 N of force: 1 J = 1 N x 1 m 1
Forms of Energy Type of Energy Description Examples of Sources Elastic Electrical Thermal Radiation Energy stored in an object due to its compression or extension Energy from the ordered movement of electrons Energy from random motion of particles in a substance Energy found in and carried by electromagnetic waves compressed spring Stretched elastic battery heating element Fire light bulb microwave Sun cellphone Fire radio/tv Chemical Energy stored in molecular bonds food fuel Wind Energy from the movement of air wind Sound Energy found in and carried by sound waves music Hydraulic Energy from the movement of water Waterfall Nuclear Energy stored in the nucleus of an atom Sun Law of Conservation of Energy Energy can be transported from one place to another Energy transfer refers to the movement of energy from one place to another (ex from a power station to our homes) Energy transformation refers to the changing of energy from one form to another (ex. During photosynthesis, solar energy is changed into chemical energy) 2
The law of conservation of energy says that energy can neither be created nor destroyed; it can only be transferred or transformed. This means that the total amount of energy in an isolated (closed) system always remains constant Potential vs. Kinetic Energy Potential energy is stored energy - it has "potential" to do something There are 3 different types of PE: 1) Elastic potential energy - elastic can store energy and convert it to motion when it is released. 2) Gravitational potential energy - is the energy an object has when we raise it off the ground(or some other reference height). 6 3
3) Chemical potential energy is the energy stored in chemical bonds of molecules Examples of potential energy: By stretching a rubber band, you give it potential energy. A vase on a shelf has stored potential energy. A football being held by a quarterback has potential energy until it is thrown and it turns into kinetic energy. 7 Gravitational potential energy is affected by height and mass (in kg) : E p = mgh (m x g x h) where m = mass (in kg) h = height (m) g = 9.8 N/kg (gravitational field intensity on Earth a constant) can be increased by 1) Increasing height 2) Increasing mass 8 4
Objects A and B have equal mass. A was lifted vertically from the ground, B was moved up the slope. Less force was used to move B, but the distance moved was greater. When A and B are at the top of the incline, which has the greater potential energy? They have the same potential energy because they are at the same height and have the same mass 9 Calculate the potential energy of a 1 kg rock raised to a height of 1m: E p = mgh E p = 1 kg x 9.8 N/kg x 1m E p = 9.8 J Calculate the potential energy of a 2 kg rock raised to a height of 1m: E p = mgh E p = 2 kg x 9.8 N/kg x 1m E p = 19.6 J Calculate the potential energy of a 1 kg rock raised to a height of 2 m: E p = mgh E p = 1 kg x 9.8 N/kg x 2 m E p = 19.6 J 5
Kinetic energy is the energy of motion. Kinetic energy is measured by how much work is done to put an object in motion or to stop the object. Examples of kinetic energy: A basketball player has kinetic energy. The movements that she does show the energy that is being displayed while she is moving. When you are running, walking, or jumping, your body is exhibiting kinetic energy. Free powerpoint template: www.brainybetty.com 11 Kinetic energy is due to an object moving with a velocity (v in m/s) (speed). It can also be affected by mass (in kg) E k = ½ mv 2 Kinetic energy can be increased 1) Increasing speed 2) Increasing mass 12 6
An object or person can have both potential and kinetic energy, Example: A waterfall has both kinetic and potential energy. The water at the top of the waterfall has stored potential energy. Once the water leaves the top of the waterfall, the potential energy is changed into kinetic energy. 13 Calculate the kinetic energy of a 2500 kg car travelling at 50 km/h (about 14 m/s): E k = ½ mv 2 E k = ½ (2500 kg) (14 m/s) 2 E k =245 000J Calculate the kinetic energy of a 2500 kg car travelling at 100 km/h (about 28 m/s): E k = ½ mv 2 E k = ½ (2500 kg) (28 m/s) 2 E k =980 000 J Calculate the kinetic energy of a 5000 kg car travelling at 50 km/h (about 14 m/s): E k = ½ mv 2 E k = ½ (5000 kg) (14 m/s) 2 E k =490 000 J 7
Theoretically potential energy should equal kinetic energy but it doesn t Friction often converts some of the potential energy into thermal energy. 8