Chapter 10 Energy, Work, and Simple Machines Quiz 10

Chapter 10 Objectives Describe the relationship between work and energy Display an ability to calculate work done by a force Identify the force that does work

Chapter 10 Objectives Differentiate between work and power and correctly calculate power used Demonstrate knowledge of why simply machines are useful Communicate an understanding of mechanical advantage in ideal and real machines

Chapter 10 Objectives Analyze compound machines and describe them in terms of simple machines Calculate efficiencies for simple and compound machines

Work Work = Change in Energy. No work done means no change in energy. Energy is conserved, but given from one object to the other Work is the transfer of energy by means of forces. The work done on the system can be positive (energy taken) or negative (energy lost) and is equal to the change in energy of the system.

Work Work = Force times distance Work is the product of the forces exerted on an object and the distance the object moves in the direction of force. Positive work is done when the motion of force is in the direction of the movement. Negative work is done when the motion of force is in the opposite direction of the movement.

Work An object slides down a surface which has friction. What force does positive work? What force does negative work? Work as is energy, is conserved, so if a positive work is done, there has to be negative work or a net work (increase in kinetic energy)

Power Power is how fast work is done Work divided by time Measured in Watts 1 Watt = 1 Joule / second 1 Horsepower is approx 746 Watts

Simple Machines Trade Force for Distance MA of 1 means both the Load and the Force move the same distance, and force = load MA of 2 means the Force moves 2x as far but is half as large as the load MA of ½ means the Load moves 2x as far and a force 2x the load is needed

Efficiency Useful Energy Out / Energy put in If you put 200 J of energy into a machine, but it only puts out 180 J of work, it is 90 % efficient Energy losses (energy not being used to accomplish purpose) include Sound Heat Friction

Mechanical Advantage Lever: Ratio of Effort to Resistance distance from fulcrum This is true for all 3 types of levers Pulley: MA of 1 for fixed, MA > 1 for movable pulleys (determine by number of support ropes) Wheel and Axle: Ratio of Wheel to Axle radius

Mechanical Advantage Screw: Ratio of circumference to pitch Inclined Plane: Ratio of slope to height Flat plane = MA of infinity Vertical plane = MA of 1 Wedge: Ratio of slope to base Same as inclined plane, but serves purpose of separation/change in direction of forces

Human Body Many levers in human body Biceps are good example of type 3 lever (poor MA) How strong is the human bicep? Your body uses 2,000 Calories (1 C = 4,184 J) How big of a light bulb are you?

Work Questions When lifting weights, a student bench presses 135 lbs. The student lifts the weights 40 cm off their chest. If the human body is only 25% efficient at converting chemical energy to mechanical energy, how many calories will the student burn lifting the weight? 1 food calorie = 4,186 J

Work Questions A box of mass 15 kg is at rest on a flat surface. If the value of Uk between the box and the surface is 0.25, how much energy will be required to push the box a total distance of 12 m?

Work Questions A box of mass 18 kg is at rest on an inclined plane of 20 degrees. If the value of Uk between the box and the plane is 0.43, how much work is required to push the box a total distance of 6.0 m up the hill?