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Up and down slopes

Forces and motion · level 2, lesson 4 of 6

A question to start

Riding a bike up a hill is hard work. Coming down, you hardly need to pedal. Why? And why do mountain paths go in zigzags instead of straight up?

The idea

Gravity pulls everything straight down. On flat ground, the ground holds you up and gravity does not push you along. On a slope, part of gravity's pull points down the slope.

  • Going down, that part of gravity helps you. A ball let go on a slope rolls down on its own.
  • Going up, it pulls you back. You have to push harder to climb.
  • The steeper the slope, the bigger that pull along it.
gravity: downdown the slope

A zigzag path is less steep than a straight line up. You walk farther, but each step is easier. Roads up mountains do the same.

On Mars, rovers climb hills too. Curiosity has spent years driving up Mount Sharp, a mountain inside Gale crater, by picking paths that are not too steep. Gravity on Mars is weaker, so a slope pulls back less hard than on Earth. But the ground there is often loose sand and gravel, which makes climbing harder.

Worked examples

1. A ramp for a wheelchair. A long, gentle ramp is easier to push up than a short, steep one, even though it goes to the same height.

2. A marble run. A marble starts rolling on a slope by itself, and goes faster on a steeper slope.

Try it

  1. Which way does gravity pull: straight down, or along the slope?
  2. Why does a ball roll down a slope on its own?
  3. Why do mountain paths zigzag?
  4. Name a mountain on Mars that a rover is climbing.
Answers
  1. Straight down. Part of that pull points along the slope.
  2. Part of gravity's pull points down the slope, and nothing holds the ball back.
  3. To make each part of the path less steep, so it is easier to climb.
  4. Mount Sharp, inside Gale crater (Curiosity).

Common mistake

"A slope makes gravity stronger." Gravity is the same; the slope only changes how much of it points along the ground. On flat ground none of it does; on a cliff, all of it does.

Where it leads