cahier physique - physics workbook

REVIEW: What is a torque ?
A torque (or turning effect) is what causes an object to rotate.
The same way a force is what causes an object to move in a straight line.
Let's say you want ot open the door of the Physics lab by pushing it:

The torque τ measures how efficient is your action in moving the door. The torque depends on your push (the force F) and on the distance L (called lever arm) between the axis and the point where the force is applied

PURPOSE: In this lab you will find the mathematical expression for torque. You will derive the relationship between torque, force and distance from the fulcrum. For the lab you will work with a meter stick that will work like a see saw. You will use masses to apply forces on the meter stick. The axis of rotation (fulcrum) will be located in the middle of the meter stick. You will determine the relationship between masses and distances from the fulcrum (axis of rotation) for a balanced see-saw (balanced meter stick).

DISCUSSION:
An object at rest is in equilibrium. The sum of the forces exerted is zero. The resting object also shows another aspect of equilibrium. Because the object has no rotation, the sum of the torques exerted is zero.
The see-saw is a simple machine that rotates about a pivot or fulcrum. It is a type of lever. Levers make work easier to accomplish a variety of tasks. Suppose you try to balance an elephant on a see saw. The elephant weighs 2000 kg and you weighs 50kg. The elephant is to stand 2m from the fulcrum. How far from the fulcrum you need to stand from the fulcrum, on the other side, to balance the elephant ? This lab will try to answer this question.

EQUILIBRIUM : The torque exerted by the elephant = torque exerted by the player.

PROCEDURE:
STEP1: CArefully balance a meter stick horizontally on a wedge or knife-edge. Suspend a 200g mass 10cm from the fulcrum. Suspend a 100g mass on the other side of the fulcrum at the point that balances the meter stick. REcord the masses and distances from the fulcrum in TABLE A.

STEP2: MAke more trails to fill in TABLE A. You can do this by using the masses of STEP 1 and changing their positions. For instance, you can move the heavier mass to a new location 5 cm farthre away, and then rebalance the meterstick with with th lighter mass. You can also change the value of the masses, Replace the heavier mass with another mass and rebalance the lever by moving the lighter mass. Record the masses and distances from the fulcrum in TABLE A. Be sure to take into account the mass of any hanger or clamp.

TRIAL	1	2	3	4	5	6
m1 = MASS 1 (g)	100
l1 = DISTANCE FROM FULCRUM (cm)
multiply m1 x l1
m2 = MASS 2 (g)	200
multiply m2 x l2
l2 = DISTANCE FROM FULCRUM (cm)

STEP 3: Try to find a pattern in the data of TABLE A: compare m1 x l1 to m2 x l2. Are these products equal ?
STEP4: convert the the pattern into a word statement.
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STEP 5: Now convert this statement into a mathematical equation. Be sure to explain what each symbol stands for.
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ANALYSIS

2) Can a heavier mass be balanced by a lighter one? EXplain how ?
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1) With the help of your partners , use the equation to find the distance a 50kg person should stand from
the fulcrum to balance the elephant/ Show your work.

2) If you are playing see saw- with your younger sister (who weighs much less than you), what can you do to balance the see saw? Mention at least two things.

3) Suppose you weigh 80 lbs and your brother weihs 40lbs. If your brother sits 6ft away from the fulcrum, where your brother should sit to balance you ?