Forces and Motion

The fast and the slow

Stories from Physics for 11-14 14-16 IOP RESOURCES

  • Cracks in glass plates can travel at speeds from 1 m/s to 1,500 m/s depending on the length of the crack and the stresses in the glass. One particular form of glass, known as Prince Rupert’s Drops, displays some particularly unusual properties. The teardrop-shaped beads have a fine tail and are produced by dripping molten glass into cold water. The head of the drop can withstand being crushed in a vice (it is reported drops can withstand compressions of 15,000 N), yet breaking the tail results in the whole drop shattering. The accepted explanation, developed in 1665 by Robert Hooke, argues that the differential cooling of the drop leads to compressive stresses in the surface of the drop which contrast with the high tensile stress in the interior. Cracks in the drops have been recorded with high-speed cameras to travel at speeds in the range 1,450-1,900 m/s.
  • In 1999, researchers used an ultra-cold atomic gas to reduce the speed of light to 17 m/s.
  • Ultra-fast video technology has revolutionised the way in which the passage of light can be visualised. A team has developed a technique that captures images at a rate of 100 billion frames per second, which enables the propagation of light to be recorded. 
  • Scientists have calculated estimates for the potential fastest speeds with which a ballpoint pen could write. The answers are dependent on the viscosity of the ink so change with ambient temperature: 181 m/s in the Sahara Desert and 192 m/s in Siberia.
  • The immense forces at galactic centres result in stars being accelerated to great velocities. One such ‘hypervelocity star’ was detected in the Milky Way halo travelling at around 850 km/s.
  • In 1919, a large tank in Boston burst releasing a wave of molasses that travelled at 56 km/hr, killing 21 and injuring 150 people.
  • The fastest speed a human has ever travelled is 11 km/s, reached by the three men crew of Apollo 10 during re-entry to the Earth’s atmosphere.
  • Two researchers at the University of Minnesota debated whether swimmers would travel faster or slower in higher viscosity liquids than water: most experts in fluid mechanics they consulted felt the swimmer would travel slower in the more viscous liquid. Eager to find an empirical answer to the problem, the scientists mixed 310 kg of gum with water and added the fluid to a 650 m 3 swimming pool, leading to a liquid with a viscosity double that of water. Their results indicated that the change in viscosity did not alter swimming speed. They propose this outcome might be explained as most of the swimmer’s drag is due to their form and viscous drag is responsible for only 10% of form drag.


appears in the relation SUVAT Equations
can be represented by Motion Graphs
has the special case Wave Speed
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