2.1.6Space
Newton’s Laws and Space Travel
5 min read 6 parts Lesson 6 of 12
Section 1 of 6Rockets and the Third Law
Rocket engines are a direct application of Newton’s second and third laws.
Thrust
- A rocket forces hot gas downwards, and the gas pushes the rocket upwards with an equal force, called thrust.
Needs no air
- The thrust arises from the interaction with the exhaust gas, so rockets work in a vacuum.
Resultant
- On launch the resultant force is thrust minus weight, and acceleration is found from F = m a.
Mass change
- As fuel burns the mass decreases, so for the same thrust the acceleration increases.
Section 2 of 6Moving in Space
Moving in Space
First law
- Away from planets and without engines a spacecraft keeps moving at constant velocity in a straight line.
Engine burns
- Engines are used only to change speed or direction, saving fuel.
Slowing down
- To slow down, the engine must push in the direction of travel so the thrust acts against the motion.
Landing
- A lander uses thrust to cancel its weight and reach a controlled touchdown.
| Planet or moon | g (N/kg) | Weight of 10 kg (N) |
|---|---|---|
| Earth | 9.8 | 98 |
| Moon | 1.6 | 16 |
| Mars | 3.7 | 37 |
Section 3 of 6Weight Across the Universe
1
Mass stays the same wherever the object is, but the weight depends on the gravitational field strength.
2
Use W = m g with the local g value for the planet or moon.
3
On bodies with weaker gravity, such as the Moon, lifting and jumping are easier.
4
On a heavy planet the weight is larger, so the same engine force gives a smaller acceleration.
Section 4 of 6Thrust and Acceleration in Detail
Thrust and Acceleration in Detail
Launch
- The engine must provide thrust greater than the weight, or the rocket will not leave the ground.
Later in flight
- As fuel is used the mass is reduced, so the acceleration rises even if the thrust is constant.
In deep space
- With no air resistance, switching off the engines leaves the spacecraft at constant velocity.
Orbit changes
- Short engine burns give an unbalanced force that changes the speed and so the orbit.
Section 5 of 6Worked Example
Question
- A rocket of mass 5.0 × 10⁵ kg has a thrust of 7.5 × 10⁶ N at launch.
- Calculate its weight (g = 9.8 N/kg), the resultant force and the initial acceleration.
- Weight = m g = 5.0 × 10⁵ × 9.8 = 4.9 × 10⁶ N.
- Resultant = 7.5 × 10⁶ − 4.9 × 10⁶ = 2.6 × 10⁶ N.
- a = F ÷ m = 2.6 × 10⁶ ÷ 5.0 × 10⁵ = 5.2 m/s².
Answer
- Weight 4.9 × 10⁶ N;
- resultant 2.6 × 10⁶ N;
- acceleration 5.2 m/s² upwards.
Section 6 of 6Exam Checklist & Tips
2.1.6 – Key Points for the Exam
- Thrust The force that propels a rocket, produced by expelling gas.
- Resultant force The overall force after weight and thrust are combined.
- Weight The force of gravity on a mass.
- Gravitational field strength The force per kilogram at a point in a gravitational field.
Common Mistakes
- Thinking the rocket pushes on the launch pad or air.
- Using the thrust as the resultant force without subtracting the weight.
- Saying mass changes on another planet.
- Forgetting that the rocket loses mass as fuel burns.