define Hooke's law

the extension of an object is directly proportional to the force provided it hasn't been extended beyond its elastic limit

on a graph, how do you know where the elatic limit is?

when the graph is no longer a straight line

if a spring obeys Hooke's law, does it behave elastically or plastically?

elastically

if a spring is beyond Hooke's law, does it behave elastically or plastically?

plastically

explain the meaning of elasticity

the object (e.g. the spring) return to its original state when the force is removed

explain the meaning of plasticity

the object will not return to its original state when the force is removed - it stays deformed

what does Hooke's law look like graphically?

A is the elastic limit so the line is no longer straight afterwards

give an example of an everyday object that behaves elastically

hairband

give an example of an everyday object that behaves plastically

bluetac

over which range does the spring obey Hooke's law?

summarise the characteristsics of the graph

range = 0N - 33N

elastic limit = A at 33N

until point A the graph is directly proportional: as the load increases, the extension increases in a proportional manner

after point A, the graph abandons the straight line and becomes curved

sketch a graph showing a single spring, a two springs in series and a two parallel springs being extended

A = single spring being extended

B = series spring being extended

C = parallel spring being extended

if a single spring (A) being extended has a gradient of 0.250 N/cm then calculate the gradient of two springs in series (B) and two parallel springs (C) being extended

A (single) = 0.250 N/cm

B (series) = 0.125 N/cm

C (parallel) = 0.500 N/cm

what is another word for strength of a spring?

stiffness

describe the relationship of A

gradient of A = 0.250 N/cm

as the load increases, the extension increases in a directy proportionally manner

the graph is linear with a straight line through the origin (directly proportional) with a gradient of 0.250 N/cm until point x where the extension increases more than the load increases

A obeys Hooke's law as force and extension are directly proportional

however, it does not obey Hooke's law at point x and beyond

describe the relationship of B

compare it to to A

gradient of A = 0.250 N/cm

gradient of B = 0.125 N/cm

in proportion to A, B's gradient is halved (0.125 N/cm)

two springs in series are half the strength of a single spring

B's graph is linear with a straight line through the origin showing it is directly proportional

B obeys Hooke's law as the load (force) increases as the extension increases in a directly proportional manner

describe the relationship of C

compare it to to A

gradient of A = 0.250 N/cm

gradient of C = 0.500 N/cm

in proportion to A, C's gradient is doubled (0.500 N/cm)

two parallel springs are double the strength of a signle spring

C's graph is linear with a straight line through the origin showing it is directly proportional

C obeys Hooke's law as the load (force) increases as extension increases in a directly proportional manner

rank the springs in order of strength (strongest first, weakest last)

C

A

B

on a Hooke's law graph, the steeper the gradient, the **...** the spring

on a Hooke's law graph, the steeper the gradient, the **stronger** the spring

D is an elastic band

describe the elastic band by referring to the graph

D does not obey Hooke's law as its gradient is not directly proportional

the elastic band is strong (steep gradient), then weak (flat graident), then strong again

draw a clear diagram of the equipment used in an experiment investing Hooke's law on a single spring

select equipment for investigating Hooke's law on a single spring

The experiment is set up as shown. A retort stand is attached to a clamp holding a Newton metre which is holding a spring. Another clamp holds a ruler. The spring is pulled down (extended). Each time the spring is extended one centimetre further. The load is recorded after each extension using the Newton metre.

control variable = the surface the experiment is carried out on

independent variable = the extension of the spring, measured by a ruler in cm

dependent variable = the load on the spring, measured by a Newton metre in Newtons (N)

how do you calculate the extension of a spring?

extension of spring = extended length — original length

how do you incur systematic error when investing Hooke's law?

not zeroing the Newton metre horizontally

can you improve systematic error?

no, but you can calculate the systematic error and then subtract it from each reading and continue accordingly

how can you improve parallax error when reading a Newton metre?

ensure that you are eye level with the equipment and filming yourself so you can clearly see

Look at Experimental Write-Up of Hooke's Law

What should a load/extension graph that obeys Hooke's law look like?

As extension increases, load increases

The line of best fit should go through he origin, being directly proportional and obeying Hooke's law

At the end of our graph there is a curve as the spring stops behaving elastically and started behaving plastically which means that the extension increased more than the load increased

How do you investigate Hooke's law with a spring?

1. Set up the apparatus. Make sure you have plenty of extra masses and measure the weight of each (with a balance)

2. Measure the length of the spring (e.g. with an accurate mm ruler) when no load is applied. Ensure the ruler is vertical (e.g. set square) and measure the spring at eye level. (This is the spring's natural level)

3. Add one mass at a time and allow the spring to come to rest, then measure the new length of the spring. The extension is the change in the length from the original length. Adding a marker to the top and bottom of the spring might make measuring lengths easier. Repeat this process until you have enough measurements (no fewer than 6)

4. Once you're done, repeat the experiment and calculate an average value for the length of the spring for each applied weight. This will make your results reliable