EasyPhysicsHigh School (11–12)Mechanics › Elasticity and Mechanical Properties of SolidsConceptualBoard Exam
Answer, and give a reason. Stress and pressure have the same unit…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Answer, and give a reason.
- Stress and pressure have the same unit. Are they the same thing?
- Why does strain have no unit?
- A hanging wire is replaced by one of the same metal and thickness but twice as long, with the same load. What happens to the stress?
Need a reminder? The key ideaShow
Stress is the restoring force per unit area, , in pascal (). Strain is the fractional change, with no unit: for stretching. A pressure on every side is a hydraulic stress; a force along a face is a shear stress.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) No. Pressure is an outside push on a surface, always at right angles to it. Stress is the restoring force per unit area inside a body, and it can be a pull, a push or a shear.
Step 2. (b) It is a length over a length (or a volume over a volume), so the units cancel.
Step 3. (c) It stays the same: the same force on the same area. The strain is the same too; the longer wire just stretches twice as far.
Answer: (a) No. (b) It is a ratio. (c) Unchanged.
Watch out. Doubling the length doubles the stretch, not the stress or the strain.
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