Work in groups from today’s class. Show Dr. F your work on the following and then work on the puzzle at the end of this post.
Science 7 Pulleys–measuring work
In today’s DSN entry, begin with 3-5 sentences about your thoughts on the meaning of energy in science. What do you think are the most important ideas? Why?
Watch this short music video. How do you think energy is involved?
In order to build our ideas about energy, we will examine how forces are applied in pulley systems and how much work goes in and how much comes out. (Energy is the capacity to do work.)
Recall the different relationships between the fundamental quantities: mass, space (distance), time
- Velocity (speed with direction) = distance/time (Meters/Second)
- Acceleration = Velocity/time (Meters / Second²)
- Force = Mass x Acceleration (Kilogram x Meters / Second²) (Newtons)
- Momentum = Mass x Velocity (Kilogram x Meters / Second)
- Pressure = Force / Area (Newton / Meter²) (Pascal) (Pounds per square inch-psi; and Bar are units of pressure that are not part of the scientific metric unit system)
- Work = Force x Distance (Energy is the capacity to do work). (Joules)
- Power = Work/Time (Force x Distance / Time) (Joules/Second or Watts)
Pulleys are used to make work seem easier. There are two ways in which a pulley can make work easier.
- Pulleys can change the direction of the force
- Pulleys can multiply the force applied by spreading it over a longer distance.
There are three main types of pulleys: single – fixed pulley, single moveable pulley, and block and tackle – at least one fixed pulley and one moveable pulley in a system.
There are many online references for pulleys. Here is one: http://www.ropebook.com/
Build and operate the three systems (see photos). Examine and record the work input and work output of each pulley system and compare them to one another.
PROCEDURE: Single fixed pulley (see photos)
- You will first need to set up a single fixed pulley system as directed by your teacher.
- Determine the weight of the object being lifted by attaching it to a Newton spring scale and recording this value in row B.
- Attach the weight to one end of a string and run it up and around a single fixed pulley attached to the top bar. Attach the short end of the string to spring scale.
- Using a meter stick, note the height at which the spring scale is attached to the string. Pull on the scale so that it moves at a constant speed and record the reading on the scale in row E.
- Move the weight being lifted up .1m (10 cm) from the tabletop to the bottom of the object. Record this in row C.
- Determine the distance that the scale was moved by subtracting the final reading from the initial reading on the meter stick. Record this value in row F.
- Calculate out the remaining rows using the formulas provided and your data.
Single moveable pulley (see photos)
- Tie one end of a string to the top bar. Run the string through a pulley and attach the other end to a spring scale.
- Connect the pulley to the object being lifted and repeat steps 4-8 as you did for the single fixed pulley and record your data in the data table
Block and tackle pulley system (see photos)
- Tie a pulley to the top bar. Loop a string through this pulley. Tie one end of the string to the top of a second pulley and take the other end and loop it around the second pulley and then tie it to the spring scale. Connect the weight to the second pulley
- Repeat steps 4-8 as you did for the single fixed pulley and record your data in the data table
Complete the following data table:
|Single fixed pulley||Single moveable pulley||Block and tackle|
|Resistance force-Weight of object being lifted(N)|
|Resistance distance -Height that the object is lifted(m)||
|Work output (J) = force x distance(B x C)|
|Effort force (N) (Reading from spring scale as string is pulled)|
|Effort Distance How far scale is moved (m)|
|Work input (J) = force x distance (E x F)|
|Mechanical advantage (B/E)|
|Efficiency = work output/work input(D/G) x 100|
1. Which pulley system required the greatest effort force? Explain why.
2. Which type of pulley had the greatest mechanical advantage? Explain why this is.
(HINT: Think of which system you had to pull the most string through)
3. What would be an easier way to determine the mechanical advantage of a pulley system?
(HINT: Think of how many strings are holding up the weight)
4. Which pulley system was the most efficient? Is this what you expected?
5. Explain the best way that a mechanic could pull out a large truck engine by himself using the least possible amount of force.
6. Try another system with more than 3 pulleys. Record your ideas and your results.
7. In what sense does the pulley make the work easier?
8. Design a simple machine (which works) where the input work is less than the output work. If this is not possible, explain why you think so.
A single pulley. Input force is directed down, weight moves up.
Single moveable pulley. Lifting force moves weight upward.
Block and tackle. One moveable pulley and one fixed pulley. Downward input force lifts weight upward.
Some different ways to think about energy and energy transformations; the development of ideas about energy:
Dr. F will assign you an energy-work puzzle that involves motion and energy. Be prepared as a group to solve the puzzle.
(What is a calorie and who invented the idea: https://www.sciencedaily.com/releases/2006/11/061120060301.htm ?)
(Who invented the term work? Who was James Prescott Joule and what did he try to find out about the relation between work and energy/heat?)
- See this passage from Wikipedia to help you think about the puzzle <https://en.wikipedia.org/wiki/Calorie>:A calorie is a unit of energy. Various definitions exist but fall into two broad categories. The first, the small calorie, or gram calorie (symbol: cal), is defined as the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. The second is the large calorie or kilogram calorie (symbol: Cal), also known as the food calorie and similar names, is defined in terms of the kilogram rather than the gram. It is equal to 1000 small calories or 1 kilocalorie (symbol: kcal).Although these units relate to the metric system, all of them have been considered obsolete in science since the adoption of the SI system. The unit of energy in the International System of Units is the joule. One small calorie is approximately 4.2 joules (so one large calorie is about 4.2 kilojoules). The factor used to convert calories to joules at a given temperature is numerically equivalent to the specific heat capacity of water expressed in joules per kelvin per gram or per kilogram. The precise conversion factor depends on the definition adopted.In spite of its non-official status, the large calorie is still widely used as a unit of food energy. The small calorie is also often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.