chapter 10 energy, work, and simple machines quiz 10
TRANSCRIPT
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?