cahier physique - physics workbook

DO THE LAB

One of the most basic principles related to the elasticity of solids is Hooke's law. THis law is named after Robert Hooke, who first discovered the principle in the seventeenth century while inverting the balance spring for spring driven clock.

If you pull a spring down, with a force F and the spring is stretched by an amount of X (the elongation) then : F = K X. The stress applied( your pulling force) is proportional to the elongation of the strping (X) (or strain). or F = K X/ K is a constant and depends on the kind of the spring you are stretching. K is called the spring constant. THe units are N/m

F / X = K F (in N) is the force applied (compressing or stretching) or stress
X (in m)is the change in length or elongation or strain
K is the elastic constant of the spring constant in N/m
Write this equation on an index card.

EXERCICES try not to look at the hints. unless you are confused.

1)A force of 5.00 N is applied to a spring whose elastic constant is 0.250 N/m. Find its change in lenght.
draw a picture.
Hints: F = K ∆x solve for ∆x

2)A force of 3.00lb stretches a spring 12.0 in. What force is required to stretch the spring 15.0 in. ?
hints: F = K ∆x solve for F

3) In a Hooke's lawe experiment, the following weights were attached to a spring, resulting in the following elongations:

weight (N)	elongation (cm)
50	2.0
75	3.3
105	4.2
125	5.0
150	6.0
175	7.4
225	9.5
275	11.1

A) Plot the graph of weight versus elongation.
hints: x = elongation (horizontal axis) and y = weight (vertical axis)

B) Draw the best straight line through the data. (going through the origin)

C) From the graph, what weight corresponds to an elongation of 7.5 cm?
hints: using the graph, if x = 7.5 what is y ?

D) From the graph, what elongation corresponds to a weihgt of 270 N?
hints: using the graph, if y=270 what is x?

E) From the graph, determine the spring constant ?
hints: F = K ∆x or y = K x so K is the slope of the graph = change in y / change in x

4) What was the original lengh of a spring constant 96.0 N/m that is stretched to 28.0cm by a 15.0 N weight ?
hints: F = K ∆x or W = K ∆x first solve for ∆x. Then ∆x = 28 - xo solve for xo

5) A force of 36.0N stretches a spring 18.0 cm. Find the spring constant (in N/m)
hints: F = K ∆x solve for K

6) Some students carry out an experiment to find out how a spring stretched when small loads were added to it.
A) Draw a diagram to show what is meant by the extension of the spring.
hint: draw 2 springs. one with a load and one without.
B) The resutls of the experiment are shown in the following table. One the readins in incorrrect. CAn you find it and correct it ?

x =load (N)	0	2	4	6	8	10	12	14
y = extension (mm)	0	16	32	58	64	80	96	112

hint: the relationship between the extension (y) and the load (x) is linear so y = K x
that means to get y you multiply x by the same constant.

C) Use these results to plot a graph

D) Use your graph to find (don't use the equation of the line, use the graph):
i) the extension when the load is 3N
hint: that is if x = 3N, find y
ii) the load which produces an extension of 40 mm

hint: that is find x with y = 40mm

D) Find the slope of the line. What does the slope represent ?

7) A fitness enthusiast is using chest expanders. THe table shows the force how the force he applies varies with the extension of the expanders.

extension (cm)	0	10	20	30
force (N)	0	40	80	120

A) Plot the points on a graph. Join the best straight line through the points.
hints: x = extension and y = force
B) Find the slope and the equation of the line.
hints: the relation is linear so y = K x K is the slope of the line or the ratio between y and x
C) What force would be required to stretch the expanders by 25 cm ?
hints: x = 25 cm, find y. you can use the graph of the equation y = K x

D) If the force is 60N, what is the extension? What is the extension is cm.

hint: sorry. by now you should be able to figure this out.

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