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Forces in newtons

Rockets and landing · level 3, lesson 1 of 6

A question to start

A rocket engine's push is measured in kilonewtons or meganewtons. What is a newton? How big a push is that?

The idea

A newton (N) is the unit of force. To get a feel for it: hold a small apple, about 100 g, in your hand. The push you need to hold it up against gravity on Earth is about 1 newton.

Weight in newtons. On Earth, each kilogram weighs about 9.8 N (call it 10 for quick sums). On Mars, each kilogram weighs 3.7 N.

\[\text{weight} = \text{mass} \times \text{pull of gravity per kilogram}\]
Thing Mass Weight on Mars Weight on Earth
An apple 0.1 kg 0.37 N 0.98 N
A child 30 kg 111 N 294 N
The Perseverance rover 1,025 kg 3,800 N 10,000 N
A Falcon 9 rocket, fuelled 550,000 kg not launched there 5,400,000 N

Big forces use bigger units: 1 kilonewton (kN) is 1,000 N, and 1 meganewton (MN) is a million newtons.

Rocket engines. The thrust of an engine is its push, in newtons:

Engine or rocket Thrust
A small model rocket motor about 5 to 10 N on average
One Merlin engine (Falcon 9) about 850 kN at sea level
Falcon 9 first stage (9 engines) about 7,600 kN
Saturn V first stage (5 engines) about 34,000 kN (34 MN)

Worked examples

1. Your weight. A 40 kg person weighs \(40 \times 9.8 = 392\) N on Earth and \(40 \times 3.7 = 148\) N on Mars.

2. Holding up a bag. A 5 kg bag of rice needs a pull of \(5 \times 9.8 = 49\) N to hold it on Earth, but only \(5 \times 3.7 = 18.5\) N on Mars.

3. Kilonewtons. The Falcon 9's 7,600 kN is 7,600,000 N: enough to hold up about 7,600,000 apples, or 775 tonnes on Earth.

Try it

  1. About how much does a 100 g apple weigh in newtons on Earth?
  2. Find the weight of a 70 kg astronaut on Earth and on Mars.
  3. How many newtons is 2.5 kN?
  4. An engine pushes with 850 kN. How many kilograms could it hold up on Earth (use 10 N per kg)?
Answers
  1. About 1 N.
  2. Earth: \(70 \times 9.8 = 686\) N. Mars: \(70 \times 3.7 = 259\) N.
  3. 2,500 N.
  4. \(850{,}000 \div 10 = 85{,}000\) kg (85 tonnes).

Common mistake

Giving weight in kilograms. Kilograms measure mass, how much stuff. Weight is a force and is measured in newtons. Everyday scales say kilograms, but they are working out your mass from your weight, assuming you are on Earth.

Where it leads