Forces and Elasticity

Understanding Forces and Elasticity

• A force is a push or pull that one object exerts on another. It is measured in newtons (N).
• A force can cause an object to change its shape, speed, or direction of motion.
• Elasticity is the property of an object that makes it return to its original shape after being stretched or compressed.
• An object is said to be elastic if it regains its original shape after a force has been removed. If it does not regain its original shape, the object is described as inelastic or plastic.
• A spring is a common example of an elastic object, and the deformation of springs and other elastic objects can often be described using Hooke’s Law.

Hooke’s Law

• Hooke’s Law states that the force needed to extend or compress a spring by a certain distance is proportional to that distance.
• A more formal statement is F = kx, where F is the force, k is the spring constant (a measure of the spring’s stiffness), and x is the extension or compression of the spring from its rest position.
• The spring constant is measured in newtons per metre (N/m).
• Hooke’s Law applies as long as the elastic limit of the object has not been exceeded. Past the elastic limit, the object will not return to its original shape when the force is removed, and the law no longer applies.

Work Done and Elastic Potential Energy

• When a force is used to stretch or compress an elastic object, work is done against the elastic forces within the object. This work is stored as elastic potential energy.
• The amount of elastic potential energy in a stretched or compressed elastic object can be calculated using the formula: E = 0.5Fx where E is the elastic potential energy (in joules, J), F is the force applied to the object (in newtons, N), and x is the distance the object is stretched or compressed from its rest position (in metres, m).

Force and Extension Practical Work

• Investigations into the relationship between force and extension usually involve loading weights onto a spring and measuring the extension.
• They can be used to get a value for the spring constant from the slope of the force-extension graph and to identify the elastic limit of the spring.

Springs in Series and Parallel

• When springs are arranged in series (one after the other), the total extension is the sum of their individual extensions. When in parallel (side by side), the total force exerted is the sum of the forces exerted by each spring.
• Understanding the behaviour of springs in series and parallel can also help explain similar behaviour in electric circuits, which often form part of a physics syllabus.