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Stuck in the sand

Forces and motion · level 2, lesson 5 of 6

A question to start

Have you seen a car stuck in soft sand on a beach? Its wheels spin and spin, and it sinks deeper. Two rovers on Mars, Spirit and Opportunity, got stuck like that. What went wrong?

The idea

On hard ground, a wheel pushes back on the ground, and the ground does not move. So the ground pushes the wheel forward, and the rover goes.

In soft, loose sand, the sand moves when the wheel pushes on it. The wheel throws sand backwards instead of moving forwards. And because the sand is soft, the wheel sinks into it. Now it has to climb out of its own hole with every turn. If it keeps spinning, it digs itself deeper.

hard ground: it goessoft sand: it digs in

Opportunity drove into a sand ripple in April 2005, nicknamed "Purgatory". Its wheels dug in. The team on Earth tested moves on a copy of the rover in a sandbox, then backed it out a few centimetres at a time. It took about five weeks to get free. Opportunity went on driving for 13 more years.

Spirit broke through a hard crust into soft soil in 2009, at a place called "Troy". One of its six wheels had already stopped working. After months of trying, NASA gave up driving it, and Spirit stayed there as a fixed science station until it fell silent in 2010.

Worked examples

1. Pushing a car out. Putting a board or a mat under a stuck wheel gives it something firm to push on, so the ground can push the wheel forward again.

2. Going slowly. Spinning the wheels fast in sand only digs deeper. Gentle, slow turns help a wheel climb out.

Try it

  1. Why can a wheel push a rover forward on hard ground?
  2. What goes wrong in soft sand?
  3. How long did it take to get Opportunity out of "Purgatory"?
  4. What did the team on Earth use to test their moves first?
Answers
  1. The ground does not move when the wheel pushes back on it, so the ground pushes the wheel forward.
  2. The sand moves back when the wheel pushes it, and the wheel sinks in and digs itself deeper.
  3. About five weeks.
  4. A copy of the rover in a sandbox.

Common mistake

"Stuck wheels need more power." More spinning usually digs a deeper hole. What helps is firm ground to push on, less weight on the wheel, or very slow and careful turning.

Where it leads