Bending, Expanding, and Pushing Back

Science · Forces & Motion · ages 12-13

Name ______________________ Date ____________

Why do bridges have expansion gaps?
- To let water drain through
- To allow for thermal expansion and contraction
- To make the bridge lighter
- To reduce wind resistance
A metal spring stretches when you hang a weight from it. When you remove the weight the spring returns to its original length. This is an example of:
- Elastic deformation
- Plastic deformation
- Thermal expansion
- Fluid pressure
In still water of constant density, the extra pressure at 10 m depth is about 100 kPa. What is the approximate extra pressure at 20 m in the same water, in kPa?
Answer: ______________
If you bend a piece of clay and it stays bent, that is elastic deformation.
Circle one: True False
Atmospheric pressure is about 101 kPa. The water adds 100 kPa of gauge pressure at 10 m depth. What is the total absolute pressure there, in kPa?
Answer: ______________
Two nearby pools contain still water of the same density, at the same depth and surface air pressure. Their bottom pressures must differ if their shapes differ.
Circle one: True False
An ideal hydraulic lift has frictionless pistons at the same height. Ignore piston weights. Their areas are 10 cm² and 500 cm². A 200 N push on the small piston adds pressure to the enclosed liquid. What added force acts on the large piston, in newtons?
Answer: ______________
The chart shows extra pressure in still water in relative units. At which depth, in meters, is this pressure highest?
Answer: ______________
A poorly designed railway track has no gaps or other allowance for thermal expansion. What can happen to its rails on a very hot day?
- Nothing will change
- The rails will shrink and pull apart
- The rails will expand and can buckle or bend the track
Jay bends a metal spoon slightly and it springs back straight. Then he bends it hard, and it stays bent in a new shape. What happened?
- The spoon was inelastic deformation the whole time
- It went from elastic to inelastic deformation
- The spoon will spring back to straight eventually

Answer key

For grown-ups. Fold this page away before handing over the rest.

Bending, Expanding, and Pushing Back W1-mt_fkwcCB5px7-s1

To allow for thermal expansion and contraction · Bridge materials usually expand when heated and contract when cooled. Expansion gaps allow these changes and reduce the risk of buckling or cracking.
Elastic deformation · The spring returns to its original shape after the force is removed, which is the definition of elastic deformation.
200 · In the stated constant-density water, extra pressure is proportional to depth. Doubling depth from 10 m to 20 m doubles 100 kPa to 200 kPa.
False · A permanent shape change is inelastic (plastic) deformation, not elastic. Elastic means it springs back.
201 · Absolute pressure = atmospheric pressure + gauge pressure = 101 kPa + 100 kPa = 201 kPa.
False · Both pools have the same density, gravity, depth and surface pressure. Their bottom pressures are equal even though their shapes differ.
10000 · The added pressure is equal at both pistons. Added force on the large piston = (200 N / 10 cm²) x 500 cm² = 10000 N.
10 · The tallest bar is at 10m. Pressure keeps climbing the deeper you go underwater.
The rails will expand and can buckle or bend the track · The metal tries to expand. With no allowance for expansion, compression builds and can buckle the rails.
It went from elastic to inelastic deformation · The small bend stayed elastic and reversed. The hard bend passed the spoon's limit, so it entered inelastic (plastic) deformation and kept its new shape.