First Law of Motion: Inertia
Outline Newton’s First Law of Motion and inertia
An object will keep moving with the same speed and direction unless acted upon by an external force.
Analyse some situations involving Newton’s First Law and inertia including space travel
- Orbiting: Satellites and spacecraft orbiting Earth or other celestial bodies are constantly in motion, and their orbits are maintained by the force of gravity, which acts as the unbalanced force that keeps them from flying off into space in a straight line.
- Astronauts in Space: Astronauts on the International Space Station (ISS) experience the effects of inertia firsthand. If they release an object, it will float in place until it collides with something or another astronaut applies a force to it.
- Landing Rovers: When a lander and rover are ready to descend to a planet’s surface, their thrusters are used to create an unbalanced force that pushes them out of orbit and down to the surface.
Relate observations from a first-hand investigation to demonstrate Newton’s First Law to car safety and car design
We conducted a practical experiment where we crashed Play-Doh dummies into cars and then into a wall. The dummies were equipped with a barely functioning seatbelt and surprisingly, with it on they weren’t launched as far off the car. Typically due to inertia, the dummies would fly off onto the wall.
Outline some features of modern car design and driver behaviour that are applications of Newton’s First Law
Modern cars are equipped with seatbelts and airbags to protect both the driver and passengers in the event of a crash. These safety features prevent the rapid deceleration of the car from exceeding the deceleration experienced by the passengers. This imbalance of forces would otherwise cause passengers to crash through the windscreen or into the seat in front of them due to inertia.
Second Law of Motion: Force and Acceleration (F=ma)
State Newton’s Second Law of Motion
F=ma
The acceleration produced when a net force is applied to an object is directly proportional to the magnitude of the applied force, and inversely proportional to the mass of the object.
Analyse some situations involving Newton’s Second Law of Motion including space travel
No.
Perform some calculations using F=ma
What force is required to accelerate a 10kg mass at 2ms?
F=ma
F=10*2
F=20N
If a 100 N force is applied to a 5 kg mass, calculate its acceleration.
F=ma
100=5a
a=20m/s/s
Analyse aspects of car design intended to reduce the force on passengers during accidents
Crush zones in cars are designed to crumple upon impact, slowing down the car and reducing the force of impact on passengers. Similarly, airbags are designed to inflate rapidly upon impact, providing a cushion for passengers and reducing the force of impact by reducing the acceleration of the person (due to Newton’s First Law: inertia).
Third Law of Motion: Equal and Opposite Reaction
State Newton’s Third Law of Motion
Every action has an equal and opposite reaction.
Analyse some situations involving Newton’s Third Law of Motion including space travel
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Aristotle vs Galileo 💻🧠
Compare Aristotle and Galileo’s Ideas on Falling Objects
Aristotle’s View (384-322 BC)
- Heavier things fall faster — the speed is directly proportional to weight
- The speed of a moving object is in direct proportion to the applied force
- If you stop pushing, the object stops moving
- Continuous motion requires a continuous cause
Galileo’s View (1564-1642)
- Objects fall at the same rate regardless of mass (disproven the idea that heavier objects fall faster)
- Understood the importance of friction in affecting how things fall
- Objects with different masses dropped from the same height hit the ground at about the same time (as demonstrated at the Leaning Tower of Pisa)
Define the Term Field
A field is a region in space where a force is exerted on an object without direct contact. For example, the gravitational field around Earth exerts a pull on objects, causing them to accelerate toward the ground. Fields allow forces to act at a distance — you don’t need to touch an object for gravity to affect it.
Difference Between Mass and Weight
| Property | Mass | Weight |
|---|---|---|
| Definition | The amount of matter in an object | The force of gravity acting on an object |
| Constant? | Constant (doesn’t change anywhere) | Varies depending on location/gravity |
| Units | Kilograms (kg) | Newtons (N) |
| Formula | Intrinsic property | Weight = mass × gravitational acceleration (W = mg) |
| Mass is the amount of “stuff” in an object and stays the same whether you’re on Earth, the Moon, or in space. | ||
| Weight is the force due to gravity and changes depending on where you are. On the Moon, your weight would be less because the Moon’s gravity is weaker, but your mass would stay the same. |
Calculate the Weight of an Object
Example: What is the weight of a 50kg person on Earth?
Using the formula: W = mg
- Mass (m) = 50 kg
- Gravitational acceleration (g) ≈ 10 m/s² (on Earth)
W = 50 × 10 = 500 N
So a 50kg person weighs approximately 500 Newtons on Earth.