Forces and motion (physics)

• Speed is defined as the distance travelled per unit time
• If the speed of something is changing, it is accelerating
• Acceleration of free fall near to the Earth is constant
• Velocity is the speed in a given direction
• Acceleration is the change in velocity per unit time
• In a distance-time graph:

  • The gradient is velocity
  • Negative gradient means returning back to the starting point
  • A horizontal line means it is stationary
  • If the distance is zero, it is back at the starting point
  • A curved line means that the velocity is changing, and it is accelerating
• In a velocity-time graph:

  • The gradient is acceleration
  • Negative gradient (i.e. negative acceleration) is deceleration
  • If the speed is zero, it is at rest
  • A horizontal line means constant speed
  • The area under the line is the distance travelled
  • A curved line means that the acceleration is changing
• A vector has magnitude and direction

  A scalar has just a magnitude

  Examples:

  Scalars: Distance, Speed, Time, Energy

  Vectors: Displacement, Velocity, Acceleration, Force
• Forces can change the speed, shape, or direction of a body and are measured in Newtons (N)
• Friction is a force between two surfaces which impedes motion and results in heating

  Air resistance is a form of friction
• Newton’s first law states that an object has a constant velocity unless acted on by a resultant force
• Newton’s second law states that Force = mass × acceleration
• Newton’s third law states that every action force has an equal and opposite reaction force
• Mass is measured in kilograms (kg)

  Weight is a gravitational force
• The weight of an object acts through its centre of gravity
• Elastic deformation is when the object returns to its original shape when the load has been removed

  Hooke’s law states that F = kx
• The moment of a force is a measure of its turning effect, measured in Newton metres (Nm)
• Momentum is the product of mass and velocity, measured in kilogram metres per second (kgm/s)
• Force exerted on an object is equal to its change in momentum over time
• Safety features in cars work by increasing the time taken for the people in the car to come to rest, reducing the force
• In a collision, the total momentum before is equal to the total momentum afterwards, known as the principle of the conservation of momentum
• The recoil speed of a gun after firing a bullet can be calculated using the principle of conservation of momentum