John and Desmond go to the gym. They strengthen their hand muscles by stretching a spring.
During the stretching of the spring, the shape of the spring changes. The spring extends. We know that a force causes a change in the shape of a body. John and Desmond therefore act on the spring with a force.
John stretches the spring twice as much as Desmond. We therefore conclude that Desmond acts on the spring with a smaller force. If the force is slightly increased, the spring will stretch further.
After the gym, they go to the park. To stretch, they hang from a tree branch with their arms.
Their weight, with which they act on the branch, causes the branch to bend. We notice that Desmond's branch bends more than John's. We therefore conclude that Desmond is heavier than John.
In both of the above cases, a force acts on a body, which changes its shape under its influence. We say that the body deforms - it stretches or bends. From the magnitude of the deformation, we deduced which force was greater.
The change in body shape can therefore be used to measure force. The greater the force, the greater the change in body shape. When measuring force, we want the body to return to its original shape after the force has stopped acting. The deformation must be flexible. This feature has e.g. a steel spring twisted into a helix.
In this material, we will learn about a device for measuring force using a spring. We call it a spring balance. The law that describes the relationship between force and spring extension is Hooke's law.
If a force is applied to a spring, it stretches. Now, the question is, how much does the spring stretch?
Hooke's law states that the extension of a spring is:
directly proportional to the force that stretches the spring, and
inversely proportional to spring stiffness.
Force and extension are directly proportional. This only applies up to a certain extension. If the force acting on the spring is too large, the extension no longer follows the force in direct proportion. Plastic deformation may also occur. In this case, the spring will not return to its original shape after the force has stopped acting. It will remain extended permanently.
Hooke's law states that the force and extension of a spring are directly proportional. The law only applies up to a certain limit. We call it the limit of proportionality. If we continue to increase the force on the spring, the spring will no longer return to its original length after the force has stopped acting.
To measure force, we use a device called a spring balance.
Spring balance
The spring balance consists of a transparent tube in which are:
spring, and
extension scale.
Since force and extension are directly proportional, the spring balance shows force instead of extension. The marks on the balance are therefore written in newtons.
Normally, spring balances have a string in the middle of the spring, which prevents excessive stretching and thus the possibility of destroying the spring.
Spring balances use different hard springs. The harder the spring, the greater the force it can measure.
