K INETIC E NERGY AND W ORK. E NERGY Loosely defined as the ability to do work There are many types of energy, but the total energy of a system always.

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Presentation transcript:

K INETIC E NERGY AND W ORK

E NERGY Loosely defined as the ability to do work There are many types of energy, but the total energy of a system always remains constant – Law of Conservation of Energy

K INETIC E NERGY A moving object has the ability to do work and thus has energy

W = Fd F = ma W = mad v 1 2 = v ad W = ½ mv 2 2 – ½ mv 1 2

T RANSLATIONAL K INETIC E NERGY Work is the change in KE W = KE 2 – KE 1 W = ½ mv 2 2 – ½ mv 1 2 Therefore, KE = ½ mv 2

The net work done on an object is equal to the change in its kinetic energy If work done on the object is positive, then it is increasing its kinetic energy If work done on it is negative, decreasing its kinetic energy

Since work and kinetic energy are related, they are measured in the same unit – Joule (J) The amount of KE is directly proportional to the mass – mass doubles, KE doubles The amount of KE is directly prop to square of velocity – v doubles, KE quadruples

E XAMPLE 1 A 145 g baseball is thrown with a speed of 25m/s a) what is the KE? B) how much work is done on the ball if it started from rest?

E XAMPLE 2 How much work is required to accelerate a 1000kg car from 20m/s to 30m/s? If it takes a total of 25m for this acceleration, what is the force supplied by the car’s motor?

E XAMPLE 3 A car traveling 60km/hr can brake to a stop in a distance of 20m. If the car is going twice as fast, 120km/hr, what is the stopping distance?