INTRODUCTION - FILL BLANKS for full credits/ you can use a word more than 1 time.
A pulley is a simple ___________ used to multiply and/or change the direction of an applied _________.
Let's say you try to lift a weight called a load. The force you apply is called the input force or Finput.
The load or weight is called the output force or _________ . If the input force (your pulling) is
less than the output force (the load), the machine is multiplying your __________.
The __________ ___________ (MA) of a machine is the number of times that the machine multiplies the input _______.
The MA of the machine is therefore the ratio _________ / ___________.
So if the MA of a pulley is 3 and if the load is 30 pounds, you only need a force of ______ pounds to lift the load.
In that case, the machine multiplies your force by _________ ! you are ____ times stronger !
A machine makes you stronger, but there is a price to pay. Since energy has to be conserved,
work in (energy you put in the machine) = work out (energy used to lift the load ) + energy lost to heat (due to friction between cords and pulley)
Let's forget about the energy lost. Le'ts neglect friction you have:
WORK IN = WORK OUT
REmember ? work = force x _______ so:
F input x distance you pull = F output x distance the load moves (let's neglect friction)
of Finput x distance in = Foutput x distance out
so if F input < F output then we have distance ____ > distance _______
When using a hydraulic jack to lift your car, you need to pump many times to lift the car a very small ______.
THis is the price to pay. THe machine makes it easy to move a ____ but the price to pay is the distance you push or pull
is greater than the distance the ____________ moves.
Because of _______, not all the work put in the system (by pulling the string) is going to be used to
lift the ______. Some of the energy put in will be lost to heat (because of friction).
To measure how efficient a machine is in converting energy, we measure the efficiency = (work out/ work in ) x 100.
An ideal machine has an efficiency of ___ % and all the energy in = energy out. No energy is lost to heat.
This never happens, but we can try to get close to it by reducing ______ as much as possible.
In this lab, you will investigate the relationship between force in and force out, work in and work out,
,distance in and distance out as well as the energy lost to friction.
PROCEDURE -
A system of pulleys is called block-and-tackle. You will build the pulley systems shown below.
PROCEDURE
TABLE 1
| system pulley | scale reading = your pull= force input = F in (newtons) |
distance scale moved = distance you pulled = d in (meters) |
load = weight = F out (newtons) |
distance load (1kg) moved = d out (meters) |
| 1 | ____ g = _____N | 20cm=0.2m | 1kg=10N | _____cm = _____m |
| 2 | ___________ | ___________ | ___________ | ___________ |
| 3 | ___________ | ___________ | ___________ | ___________ |
| 4 | ___________ | ___________ | ___________ | ___________ |
STEP 1:
Build the first system with 1 simple pulley, a string and a load of 1 kg (load = 10N). Suspend one pulley to a ring stand
or to a meter stick that you can insert in the closet's handles. Pass a length of string over it, tying a hook made of paper clip to each end. Attach 1 kg mass one one of the string and a spring scale to the other end. Determine the force required to lift the weight. Record in the table but : If the spring scale is in Newton, this is fine. If the spring scale is in grams, divide by 100 to get newtons.
( weight (N) = mass (kg) x 10 m/s/s but mass(kg)= mass(g) / 1000 so weight (N) = mass(g) / 100 )
Pull the string a distance of 20 cm (call it din) and record the distance moved by the weight. call it d out.
convert to meters. Record every thing in the TABLE 1.
Does the single pulley magnify force or simply change the direction of the applied force ?
____________________________________________________________________________________________
STEP2:
Build the second system. You have now two pulleys in a block-and-tackle arrangement.
REpeat step1. Fill the TABLE 1 Is it easier to pull the load ? what is the price to pay ?
STEP3: build the third system. same questions.
STEP4: build the 4th system. same questions.
ANALYSIS
1) using TABLE 1 , fill TABLE2. Find the mechanical advantage of each system of the efficiency.
As you increase the numbers of pulleys what happens to the MA and efficiency ?
2) Determine the relationship between the number of strands supporting a block and tackle and its mechanical advantage.
3) For the last system, the MA is ________, your force is therefore multiplied by ______
but the distance you have to work (pulling the cord) is also multiplied by _________________.
Also, as you increase the numbers of strands passing in the pulleys, you also increase __________
decreasing the efficiency.
TABLE2, use data from TABLE1
| system pulley |
work input= Fin x d in ( joules) |
work output Fout x d out (joules) | Mechanical advantage Fout/Fin | efficiency (in %) work out/work in x 100 |
| 1 | ____N x ___m = ______J |
____N x ___m = _______J | __________ | ___________ |
| 2 | ___________ | ___________ | ___________ | ___________ |
| 3 | ___________ | ___________ | ___________ | ___________ |
| 4 | ___________ | ___________ | ___________ | ___________ |
GOING FURTHER - try without hint. it is easy.
MA = F out/ Fin = load/effort (F in = effort and F out = load and d in = the distance of rope you pull, d out = distance load moves
d out = MA x d in - work out = load x distance the load moves = load x d out - Work in = effort x distance of rope you pull = effort x d in
efficiency = work out / work in x 100
I) A force of 500N is exerted on the rope of a bock and tackle, and the rope is 10 meters, lifting a weight of 2,500 newtons
1.8 meters. What work was put into the machine? What was the work output ?
What was the efficiency ? What was the theoretical mechanical advantage ?
The actual mechanical advantage ?
II) consider the system below:
If the block has a mass of 120kg, how much is the tension T in the rope?
hint: find the weight of the block first. Since the block is in equilibrium,
force pulling down (weight ) = sum of forces pulling up. ( T + T)
III) Now consider the system below, that helps lifting things.
1) Use previous problem. How much force must you exert to lift the 120kg block ?
For you, it is like lifting _________ instead of 120 kg.
The mechanical advantage is _____, the machine multiply your force by _________.
If the block needs to be risen by 5 meters, you need to pull ______ of cord.
This is the price to pay.
2) Using this system, it takes 450N (effort or Fin) applied to the rope to lift a 80kg
(Fout) load 12 m.
A) Find the ideal mechanical advantage of the machine.
hint: find the weight of the load. Then find ratio load/effort
B) The amount of rope that has been pulled through the pulley.
hint: find d out. d out = MA x din
C) Find the work output
D) Find the work input
E) Find the efficiency
IV) Now consider the following system:
The load is 480N (F output) and it is too be lifted 6m (dout).
A) Find the effort (Finput) (the force you have to produce, your pull)
B) The effort distance (the distance of strand you need to pull, d in).
hint: the weight mg is balanced by the three cords. (T+T+T).
Your effort is only T.
V) Now consider the super system below:
1) This system has problably a lot of friction. STill, if you want to raise the load
by 5 m, how much rope for you have to pull ?
2) What is the ideal mechanical advantage of the system?
3)If the efficiency of the pulleys is 80%, how much effort is required to lift 750-lb load ?
hint: use load x d out / effort x d in = 0.8
use d out = MA x din