Space exploration: rocket thrust and Newton's third law, weight and mass on different bodies, the use of satellites and the idea of a satellite as a projectile, and the risks and benefits of space travel including re-entry heating.

What this key area is asking

The SQA wants you to explain how a rocket produces thrust using Newton's third law, calculate weight on different bodies using $W = mg$ while knowing that mass is unchanged, understand a satellite as a projectile, and discuss the risks and benefits of space travel including re-entry heating.

Rocket thrust and Newton's third law

A rocket lifts off only when the upward thrust is greater than the downward weight, so that there is an unbalanced upward force. Using $F = ma$ on the resultant (thrust minus weight) gives the acceleration. As the rocket burns fuel its mass falls, so for a steady thrust the acceleration increases as the flight goes on. The SQA often asks for the resultant force and acceleration at lift-off.

Weight on different bodies

Each planet and moon has its own gravitational field strength: about $9.8 \text{ N kg}^{-1}$ on Earth, $1.6 \text{ N kg}^{-1}$ on the Moon and $3.7 \text{ N kg}^{-1}$ on Mars. Your mass is the same on all of them, but your weight is largest where $g$ is largest. This is why astronauts can bound across the Moon: their weight there is about one sixth of its Earth value.

Satellites as projectiles

Satellites are used for communication (relaying TV and phone signals), weather monitoring, navigation (GPS) and scientific observation. A geostationary satellite orbits once every $24$ hours above the equator, so it appears to stay over the same point on Earth, which is ideal for communications.

Risks and benefits of space travel

Space travel brings real risks: the huge fuel and energy needed for launch, the danger of the launch itself, exposure to radiation in space, and intense heating on re-entry. When a spacecraft returns it travels very fast through the atmosphere; friction with the air converts kinetic energy into a large amount of heat, so a heat shield is needed to stop the craft burning up. The benefits include satellites for communication and weather, scientific knowledge about other worlds, and technologies (such as new materials) developed for space and then used on Earth.

Try this

Q1. State Newton's third law. [1 mark]

• Cue. For every action force there is an equal and opposite reaction force.

Q2. An astronaut has a mass of $75 \text{ kg}$. Calculate the weight on the Moon, where $g = 1.6 \text{ N kg}^{-1}$. [2 marks]

• Cue. $W = mg = 75 \times 1.6 = 120 \text{ N}$.

Q3. Explain why a heat shield is needed when a spacecraft re-enters the atmosphere. [2 marks]

• Cue. Friction with the air converts the craft's large kinetic energy into heat; the shield protects the craft from burning up.