Work and power
What are the units for work and power?
Joule (Work) and Watt (Power)
A wheelbarrow is what class of lever?
second class; Input force, output (RESISTANCE) force, fulcrum
State the Law of Conservation of Energy
Energy cannot be created or destroyed, just changed from one form to another
Name 4 different forms of energy
Sound, radiant, thermal, electrical, nuclear, chemical, mechanical
Describe why a machine will always output less energy than what you input.
Some of the energy you input is changed into a nonmechanical form, like thermal (friction) and sound
When you push on a wall with 20N of force but it doesn't move, how much work was done
0 Joules; Work = Force x DISTANCE
Name all six simple machines
Lever, pulley, wheel and axle, inclined plane, wedge, screw
What is the difference between kinetic and potential energy?
kinetic is moving; potential is at rest
____________ fire pit _____________
(energy input) (energy output)
chemical (input); radiant and thermal (output)
How could you increase the efficiency of a knife?
Sharpen the blades; the taller and thinner a blade, the greater the MA, which increases efficiency. (You'd lose less of your input energy to friction)
If you generate 30W of power in 50 seconds, how much work was done?
1500 J
Do simple machines lower the total amount of work? Explain why or why not?
No, but they lower effort (input) force while increasing the (input) distance, making work "easier" because YOU are applying less force
If a moving elevator is halfway down a building, what can we infer about the potential and kinetic energy.
they are each 50%
What is the potential energy of a rock that has a mass of 50 kg and is at ground level?
no potential energy; GPE = mass x free-fall x HEIGHT
How much potential energy does a hiker have if he weighs 100 kg and is 15 m up?
14,700 joules
What is the efficiency of a machine that outputs 20 Joules with an input of 15 Joules.
133% Cannot exist; no machine is 100% efficient. Friction and sound decrease efficiency....so there cannot be any machine that's more than 100% efficient!
Name a household item for a 3rd class lever.
broom
describe how a bouncing ball keeps transferring between kinetic and potential energy.
as it bounces up=potential as it falls to the ground=kinetic
A car speeds by with a velocity of 80 m/s west and a mass of 500 kg. What is the car's kinetic energy?
1,600,000 joules
Explain how a class one lever can be adjusted to increase MA.
To increase MA, move the fulcrum further away from your input force. If you increase the input distance, you decrease the amount of force you need, increasing MA.
A cat pushes a ball with a force of 50N if the ball traveled 20m in 10 sec, how much power was produced?
100 watts
Explain why the MA goes up with the addition of pulleys in a block and tackle system.
With the additional pulleys added, there needs to be more rope. Any time there is more rope, there is an increase in distance. With an increase to distance, less force is applied, increasing the MA.
Name three energy transformations in a standard Iphone.
electrical, sound, radiant, chemical (from battery)
Knowing what you know about potential and kinetic energy, explain why the first hill of a roller coaster is always the highest.
Energy can be in different forms, such as kinetic energy, potential energy, sound energy and heat etc. When you are sitting on the roller coaster, because of the lift motor, it will be lifted to a very high position. Relative to the ground, the roller coaster has a very large potential energy. The higher position it is, the larger potential energy it has. After passing the highest position, it goes down the hill, both the speed and the kinetic energy increase. By conservation of energy, the potential energy will decrease and transform into kinetic energy.
When the roller coaster has just passed the first "valley", it has a lot of kinetic energy, so it can climb up the second "hill". By conservation of energy, it can climb up to a height equal to that it went down before. However, due to the friction in the machines, the total mechanical energy of the roller coaster will decrease. As a result, the first "hill" of many roller coasters are the highest, and the following hills will have decreasing heights.
Describe how to determine which class of lever an object is.
First determine where the input (EFFORT) force, output (RESISTANCE) force, and fulcrum are located. Then "look in the middle and be FRE!" If Fulcrum in the middle, it's first class because F is first letter. If Resistance is in middle its second class because R is second letter in FRE. If Effort is in middle it's third class because E is third letter.