Work and Energy. What is energy? Defined as “ability to do work” But, what is work? Work = Force * displacement When work is done, energy is transferred.

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Presentation transcript:

Work and Energy

What is energy? Defined as “ability to do work” But, what is work? Work = Force * displacement When work is done, energy is transferred –(“someone” gets paid and “someone” pays energy) –If we do work on an object, we transfer energy to the object

Two Types of Energy Kinetic Potential

Kinetic Energy An oxygen atom has a mass of 2.66 x g. If an oxygen atom were moving at 200 m/s, what would the kinetic energy of that atom be? A 3-kg ball is traveling at a constant speed. If you had to transfer 30 J of energy to the ball to cause this motion, what must the velocity of the ball be?

Gravitational Potential Energy How much energy does an object gain because of its height above the earth (ground)? –Recall work = force * displacement

Law of Conservation of Energy Total energy in any process is constant. The energy may be transferred or transformed, but not created nor destroyed.

Conservation of Energy I hold a 3 kg ball 2m above the ground. –What is its potential energy? –What is its kinetic energy? –What is its total energy? I drop the ball –What is its total energy as it falls? –What happens to the ball’s PE as it falls? It is being transformed to KE –What is it’s velocity when it has fallen 2m? AFTER the ball rests on the ground, –What is its potential energy? –What is its kinetic energy? –What has happened to its energy? It has been transferred

Conservation of Energy I hold a racquetball 3 m above the ground. –What is its potential energy? –What is its kinetic energy? I drop this racquetball. What happens to its potential energy? –It is transformed into KE What is it’s velocity when it has fallen 1 m? 2m? 3m? (Ignore air resistance and friction.) –Assuming a mass of 0.20 kg, after 1m, v= 4.43 m/s down; after 2m, v=6.26 m/s down; after 3m, v=7.67 m/s down

Conservation of Energy A 1 kg ball is traveling along a frictionless sidewalk with a constant velocity of 3 m/s. Suddenly, the ball encounters a 0.5 m dip in the sidewalk and then continues rolling along a flat section of the sidewalk. –What is the total energy of the ball before it encounters the dip? –What is the total energy of the ball after it encounters the dip? –What is the velocity of the ball after it has fallen 0.3 m down the dip? (Ans = 3.86 m/s down) –What will the speed be for the ball after it has dropped to the bottom of the dip? –What has happened to the ball’s KE? PE?

Conservation of Energy A 5 kg object is sliding 2 m/s at the top of a frictionless hill that is 3 m high. –What is the object’s total energy? –What will be the object’s speed at the bottom of the hill? –The object then slides up another frictionless hill that is 1.2 m high. How fast will it be going at the top of this hill?

A B Tracks A and B are of equal length and have hills of the same curvature except A curves up and B curves down. If two identical balls are rolled simultaneously with the same initial velocity, which will reach the end of its track first? a.A b.B c.same