Principles of Physics - Foederer. All energy in existence is already here. It cannot be created or destroyed, only transferred among systems. So, E total.

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Presentation transcript:

Principles of Physics - Foederer

All energy in existence is already here. It cannot be created or destroyed, only transferred among systems. So, E total before transfer = E total after transfer

A 0.100kg egg is dropped from the top of the bleachers which are 5.5 m high. How fast was it travelling when it hit the ground if there was no air resistance? PE→KE Since energy is conserved, PE top = KE bottom mgh =1/2 mv 2 (0.100 kg)(9.8 m/s 2 )(5.5 m) = 1/2 (0.100 kg)v 2 v = 10.4 m/s

How does energy change form or enter/leave a system? Work is done on an object or by an object  Work = change in energy  a force is applied to the object to change its motion or condition W = Fd W = work (J) F = force (N) d = distance (m) **This force could be applied, friction, gravitational, etc. for the distance that it is causing the change

Open system – energy can be transferred out of the system to another system. Example  A box slides down an incline. Its PE changes to KE ◦ There is less KE at the bottom of the incline than there was PE at the top ◦ Energy was lost as heat from friction as the box slid

A car with mass 200 kg has a motor that supplies a force of 950 N. The car travels a distance of 6.3 m when the motor is turned on. During the 6.3 m distance 5, 557 J of work is done on the car by friction. What is the velocity of the car after travelling 6.3 m? W→KE Since energy is lost because of friction, W motor - W friction = KE Fd – W friction =1/2 mv 2 (950N)(6.3 m) – 5557 J= 1/2 (200 kg)v 2 v = 2.1 m/s

Closed system – all energy stays within the system Example  The box slides down an incline with no friction. Its entire PE changes to KE.

Work is done on system to raise bob to position A (W = E total ) ◦ Work is relative to its height at equilibrium (C) As pendulum is being raised, work energy is changing to PE When at position A all work is now PE (E total = PE) As pendulum swings from its initial position A and back again, PE changes to KE and back E total = PE A = PE B + KE B = KE C = PE D + KE D = PE E Excellent example of conservation of energy