- Created by: Charlottelg
- Created on: 08-12-18 15:41
Hooke’s Law says extension is proportional to forc
Metal wire supported at the top with a weight attached at the bottom stretches. Weight pulls down with force (F), producing an equal and opposite force at the support.
Robert Hooke discovered extension of a stretched wire (x) is proportional to the load or force (F).
F = kx (k=force or stiffness constant in Nm^-1)
Stretching a material creates tension across it. Forces of tension act along same line as forces stretching material but in the opposite direction at each end - they ‘pull’ on the object at either end.
Hooke’s Law also applies to springs
Metal springs change length when a pair of opposite forces are applied.
Extension of a spring is proportional to force applied. If forces are compressive, spring is squashed - extension is negative.
For springs, k can also = spring stiffness or spring constant.
Tensile forces stretch the spring.
Compressive forces squash the spring.
Hooke’s Law works for compressive forces as well as tensile forces. For a spring, k has the same value.
All other materials obey Hooke’s Law up to a point,
Hooke’s Law stops working when the load is great e
Graph showing force against extension for a typical metal wire or spring.
First part shows Hooke’s Law being obeyed - straight line relationship between force and extension.
Force becomes great enough - graph begins to curve. Metals obey Hooke’s Law up to the limit of proportionality (P).
Elastic limit is exceeded - material will be permantley stretched. All forces removed - material will be longer than in the beginning.
Beyond elastic limit - material stretches further for a given force.
Some materials (e.g rubber) only obey Hooke’s Law for really small extensions.
A stretch can be elastic or plastic
Materials show elastic deformation up to their elastic limit, and plastic deformation beyond it.
Deformation = elastic - material returns to original shape.
Material is put under tension - atoms of material are pulled apart from one another. Atoms can move slightly relative to their equilibrium positions without changing position. Load is removed - atoms return to their equilibrium distance apart.
Deformation = plastic - material is permanently stretched.
Some atoms move position relative to one another. Load is removed - atoms don’t return to original postitions.
You can investigate extension by stretching an obj
Set up clamp stand with clamp and hang object under investigation off clamp and attach weights to it. Have a ruler attached to the clamp stand to measure extension.
Add masses one at a time, measure the new length then calculate extension: extension = new length - original length.
Plot a graph of force against extension. Where line of best fit = straight - object obeys Hooke’s Law, gradient = k. Loaded the object beyond limit of proportionality - graph starts to curve.