Physics Explained
Pushes and crashes, swings, magnets and the northern lights, hot and cold, and why GPS needs Einstein, in plain language.
Pushes and crashes
- Kids
Things keep doing what they are doing until something pushes them. A heavy thing is harder to speed up than a light one. When two carts crash, whatever "oomph" one loses, the other gains.
Check yourself: If a big cart hits a small cart that is standing still, which one moves faster afterwards?
The small one: in a bouncy crash, a 2 kg cart at 3 m/s sends a 1 kg cart off at 4 m/s.
Swings and pendulums
- Kids
A pendulum 1 m long takes about 2 seconds to swing there and back, whatever it weighs. Longer pendulums swing more slowly, which is why old clocks used them to keep time.
Check yourself: Does a heavier swing go back and forth faster?
No. Only the length (and gravity) sets the time, as long as the swings are small.
Air resistance
- Teens
A ball thrown at 40 m/s (90 mph) would fly about 153 m with no air. With air pushing back, it only reaches about 94 m. Drag grows with the square of speed, so fast objects feel much more of it.
Check yourself: With air, is 45° still the best angle to throw for distance?
No: for this ball the best angle is about 41°, a bit lower.
Magnets and the northern lights
- Teens
The Sun blows out a wind of charged particles at about 400 km/s. Earth’s magnetic field makes them spiral along its field lines toward the poles, where they hit the upper air and make it glow: the aurora.
Check yourself: Why are auroras seen mostly near the poles?
That is where Earth’s magnetic field lines dip down into the atmosphere, guiding the particles in.
Why GPS needs Einstein
- Community college
GPS satellites carry atomic clocks. Because they are high up where gravity is weaker, those clocks gain about 46 microseconds a day; because they move fast, they lose about 7. The net gain of about 38 microseconds a day would add roughly 12 km of position error every day if it were not corrected.
Check yourself: Which effect wins for GPS: speed or gravity?
Gravity: the clocks run fast overall, so they are set slightly slow before launch.
Ready for the equations? Continue with the step-by-step math. · Try it in the Physics Lab