Newtons laws and gravity

• Falling objects eventually reach terminal velocity - where their resultant force is zero
• Stopping distances depend on speed, mass, road surface and reaction time
• Newton's First Law


  An object remains in the same state of motion unless a resultant force acts on it
• If the resultant force on an object is zero, a stationary object stays stationary and a moving object continues to move at the same velocity
• Inertia


  The tendency of an object to continue in its current state (at rest or in uniform motion) unless acted on by a resultant force
• Objects with uniform motion

  • A car travelling at constant speed
  • A runner at their top speed
• When a car accelerates


  The driving force from the engine is greater than the resistive forces, so the resultant force is not zero
• The horizontal forces on a submarine are balanced, so the horizontal resultant force is zero, meaning there is no horizontal acceleration
• The vertical forces on a submarine are balanced, so the vertical resultant force is also zero, meaning there is no resultant vertical acceleration
• If the submarine is moving, it is impossible to tell which direction it is moving from the forces alone, only that it will continue in the same direction at the same speed
• Falling objects eventually reach terminal velocity - where their resultant force is zero
• Stopping distances depend on speed, mass, road surface and reaction time
• Newton's Second Law


  Resultant force = mass × acceleration (F = m·a)
• Force (F) is measured in newtons (N), mass (m) is measured in kilograms (kg), acceleration (a) is measured in metres per second squared (m/s²)
• Acceleration of an object

  • Proportional to the resultant force on the object
  • Inversely proportional to the mass of the object
• Inertial mass


  A measure of how difficult it is to change the velocity of an object. It is defined as the ratio of force over acceleration.
• It is important to be able to estimate speeds, accelerations and forces involved in road vehicles
• Vehicle details


  • Family car
  • Lorry
• Maximum legal speed on a single carriageway for a family car is ~27 m/s, mass is ~1,600 kg, acceleration is ~3 m/s²
• Maximum legal speed on a single carriageway for a lorry is ~22 m/s, mass is ~36,000 kg, acceleration is ~0.4 m/s²
• Falling objects eventually reach terminal velocity - where their resultant force is zero
• Stopping distances depend on speed, mass, road surface and reaction time
• Newton's Third Law


  Whenever two objects interact, they exert equal and opposite forces on each other
• Newton's Third Law


  • The forces act on two different objects at the same time
  • Every action has an equal and opposite reaction
• Examples of force pairs

  • Pushing a pram
  • Car tyre on a road
  • Satellite in Earth orbit
• Explaining Newton's Third Law

  1. Contact forces between objects that are touching

  2. Non-contact forces between objects that are not physically touching
• Gravity


  One of the most important forces in the universe
• Gravitational field


  A field that exerts a force on objects with mass
• Weight


  The force acting on an object due to gravity
• Gravitational field strength (g)

  The force per unit mass exerted by a gravitational field, measured in newtons per kilogram (N/kg)
• The Earth's gravitational field strength is 9.8 N/kg
• The Moon's gravitational field strength is 1.6 N/kg
• Weaker gravitational field


  Smaller weight of an object
• Astronauts on the International Space Station appear to be weightless, but they are still within the Earth's gravitational field
• Astronauts seem weightless because they and the space station are constantly falling towards the Earth