Experiment: Tension, an internal force
An internal force arise in response to external forces acting on a body.
This is similar to normal or friction forces which also only arise due to the action of some other force.
Internal forces are characterized as
- tension: arises from stretching or pulling apart, often associated with a string, cable or chain but also occurs in rigid bodies such as pulling a meter stick from both ends
- compression: from pushing or squeezing on a rigid body, opposite of tension
- shear: from two external forces offset, not directly opposite each other
- torsion: twisting
The internal forces relate to
- stress (force per unit area, useful was to characterize internal force)
- strain (effect of force as size deformation relative to original)
- creep (effect of force as permanent deformation due to stress)
- modulus (property of material, ratio of stress to strain, measures resistance to stress)
In this lab we are looking at tension (such as a string pulled) and compression (such as a meter stick pushed from opposite ends). For tension or compression, the internal force is along the axis of the body (based on two external forces acting) and is not given a direction except when it acts on the external force. Tension or compression internal force has a magnitude equal to the acting external force.
For this lab exercise, we are looking at tension (in a string or an elastic). The exercise could equally apply to compression (or shear, bending, torsion for that matter).
When you pull on a rope attached to a crate, your pull is somehow transmitted down the rope to the crate. Tension is the name given to forces transmitted in this way along strings, ropes, rubber bands, springs and wires. Note that (obviously) the rope by itself is unable to exert a force on the crate if your are not pulling on the other end. Thus tension forces are passive, they only act in response to an active force like your pull.
- If you apply a force to the end of a rope as in the picture above, is the whole force transmitted to the crate, or is the force at the crate smaller or larger than your pull?
- If the rope is longer, will the force applied to the crate be larger, smaller or the same as with the shorter rope? Does it matter how long the rope is?
- Suppose that instead of a rope, you used a bungee cord or large rubber band. Will the force applied to the crate be larger, smaller or the same as with the rope? Suppose that you use a strong wire cable?
- How is the force "magically" transmitted along the rope?
As in the figure, attach a rubber band between two spring scales. Without pulling, notice if there is a difference in readings between the scales. Now pull on one scale and observe the difference in readings between the two scales.
- Base on your readings of the scales, is the force transmitted down to the other end when you pull on one end of the rubber band? Explain.
- As you increase the force applied to the rubber band, what happens to the length of the rubber band? Propose a mechanism based on these observations to explain how the force is transmitted down the rubber band from one scale to the other.
- Indicate on a diagram the directions of the forces exerted by the rubber band on to the scales.
Replace the rubber band with short piece of string with loops at both ends and repeat the experiment. Now do the same for a longer piece of string.
- Based on the readings of the scales, when you pull on one end of the string, is the force transmitted undiminished down to the other end? Does it matter how long the string is? Explain.
- Did the string stretch at all when you pulled on it? Can you propose a mechanism for the transmission of the force along the string?
Setup a scale with a 1000g mass over a pulley as shown in the picture. Vary the angle the scale/string makes with the pulley and observe the scale reading.
- What happens when a string is hung over a pulley? Is the tension force still transmitted fully from one end of the string to the other?
- How does the force vary with the angle the scale makes with the pulley?