Efficiency Law of conservation of energy always applies. No use of energy is 100% perfect. As work is performed Some of the energy performs the productive work (the work you want done). Some energy is wasted as heat, non-productive energy, or undesired work.

A bouncing ball loses energy with each consecutive bounce (wasted energy). The productive work is work against gravity—the bounce moving the ball upward after the impact with the floor. The wasted energy is heat, sound, vibrational KE on the floor, and work against shape on the ball.

If a cylinder was rolled across the floor, it will progressively slow with distance because of energy loss to friction The productive work is mechanical work by translational KE. The wasted energy is heat created by friction force. The amount of heat created is proportional to the distance moved.

The first hill of a rollercoaster must always be the highest hill The first hill of a rollercoaster must always be the highest hill. The initial energy is GPE, and must be very large. The coaster cannot gain additional KE on the track to overtake the initial GPE of the tallest hill. The train progressively loses KE to friction the farther it moves along the track. The train’s speed will decrease with distance.

Human body’s main energy source is food (chemical potential energy). Productive work = cell division and generation, growing, physical work, organ system function, and muscle movement. ~ 60 % of the food energy is wasted as body heat (37º)

Efficiency: The ratio/percentage of productive work performed relative to the total energy required to perform that work. Productive work is the desired outcome of using the energy. The work you want to complete by using energy.

Efficiency = 100% Productive work (J) = Energy input (J) All of the energy used produces the expected productive work. No wasted energy Impossible! Efficiency < 100% Productive work (J) < Energy input (J) Some of the energy used produces the expected productive work. Some energy is wasted as heat or other non-productive energy.

The automobile relies on gasoline as the starting energy source The automobile relies on gasoline as the starting energy source. Productive work is moving the car from one place to another (mechanical energy) Thermal Mechanical KE Translational KE Thermal (friction) Rotational KE Chemical energy (gasoline) = translational KE + Rotational KE + Mechanical KE + Thermal

Start with 10,000 J of gasoline 5000 Joules of energy move the car (productive work) 5000 Joules of energy is wasted as heat (hot engine, friction) and non-productive work. 4000 J 10,000 J 1000 J 1000 J 3000 J 1000 J The automobile is 50% efficient.

Calculate the efficiency of the ball’s bounce. You drop a rubber ball from a height of 2.0 meters above the ground. The ball bounces 1.4 meters. The mass of the ball is 2.0 kg Calculate the efficiency of the ball’s bounce. Calculate the amount of wasted energy. 2.0 m 1.4 m

Lazlo pushes a box with a net force of 100 N for a distance of 30 meters. Lazlo expends the total of 5000 J of energy as he pushes. Calculate the efficiency of pushing the box Calculate the amount of wasted work as friction.

An elevator motor uses 200,000 Joules of energy to lift the elevator from the ground floor to the 3rd floor. The mass of the elevator is 1600 kg. Height between the ground floor and the 3rd floor is 9 meters. Calculate how much work was performed. Calculate the efficiency of the elevator motor. Calculate the amount of wasted energy as heat.

Wasted Work By Friction Friction is a dissipative force that transforms KE into heat (Q). Slows objects, reduces motion. At the contact between the moving object and the surface over which it moves. Acts in the direction opposite of the object’s motion Friction Force

Law of conservation of energy Without Friction or Heat With Friction and Heat Work Energy Theorem Work by Friction creates an equal amount of Heat

A 10 kg wooden block is pushed with an initial velocity of 8 A 10 kg wooden block is pushed with an initial velocity of 8.0 m/s across a horizontal wood floor. The block of wood slows to a stop in a distance of 4.2 m. Calculate the work of friction acting upon the block. Calculate the friction force slowing the block. Calculate the quantity of heat generated by friction. v0 = 8.0 m/s vf = 0 m/s d = 4.2 m

Starting from rest, a 25 kg wooden block slides down an inclined plane to the bottom. The sloping surface of the incline plane is 18 m long, and the rise height is 6 m. The velocity of the block at the bottom of the incline plane is 7.9 m/s. Calculate the work of friction acting upon the block. Calculate the friction force slowing the block. Calculate the quantity of heat generated by friction. v0 = 0 m/s d = 18 m vf = 7.9 m/s h = 6 m

The rollercoaster train moves down the slope of the first hill, then up to the summit of the second hill. The mass of the train is 100 kg. The height of the first hill is 50 m and the height of the second hill is 25 m. The velocity of the train at the summit of the second hill is 26.4 m/s. The length of track between the first hill and the second hill is 125 meter. Calculate the work of friction between the rails and the train’s wheels. Calculate the average friction force slowing the train. Calculate the quantity of heat generated by friction. h = 50 m v = 0 m/s h = 25 m v = 26.4 m/s