Forces and Motion

• Terminal velocity

  At terminal velocity, the weight of the object due to gravity is balanced by the frictional forces, and the resultant force is zero
• Forces on a skydiver

  1. Immediately on leaving the aircraft, the skydiver accelerates downwards due to the force of gravity. There is no air resistance acting in the upwards direction, and there is a resultant force acting downwards so the skydiver accelerates towards the ground.
  2. As the skydiver gains speed, their weight stays the same but the air resistance increases. There is still a resultant force acting downwards, but this gradually decreases.
  3. Eventually, the skydiver's weight is balanced by the air resistance. There is no resultant force and the skydiver reaches terminal velocity.
• Newton's First Law of motion

  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, this means:

  • a stationary object stays stationary
  • a moving object continues to move at the same velocity (at the same speed and in the same direction)
• Newton's Second Law of motion

  Newton's Second Law of motion can be described by this equation:

  resultant force = mass × acceleration

  F=ma

  This is when:

  • 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)

  The equation shows that the acceleration of an object is:

  • proportional to the resultant force on the object
  • inversely proportional to the mass of the object

  In other words, the acceleration of an object increases if the resultant force on it increases, and decreases if the mass of the object increases.
• Newton's Third Law of motion

  Whenever two objects interact, they exert equal and opposite forces on each other.

  This is often worded as 'every action has an equal and opposite reaction'.
• Stopping distance


  stopping distance = thinking distance + braking distance

  This is when:

  • thinking distance is the distance a vehicle travels in the time it takes for the driver to apply the brakes after realising they need to stop
  • braking distance is the distance a vehicle travels in the time after the driver has applied the brake
• What is speed defined as?


  The rate at which an object covers distance
• What type of quantity is speed?


  Speed is a scalar quantity
• What is the formula for speed?


  Speed = Distance / Time
• How is velocity different from speed?


  Velocity includes direction
• What is the formula for velocity?


  Velocity = Displacement / Time
• What does acceleration measure?


  The rate at which velocity changes
• What is the formula for acceleration?


  Acceleration = Change in Velocity / Time Taken
• When is acceleration considered positive?


  When an object is speeding up
• What does a flat line on a distance-time graph indicate?


  The object is stationary
• What does the gradient of a distance-time graph represent?


  The speed of the object
• What does a horizontal line on a velocity-time graph indicate?


  The object is moving at constant velocity
• What does the area under a velocity-time graph represent?


  The distance travelled
• What does Newton's First Law state?

  An object will remain at rest or continue moving at constant velocity unless acted upon by a resultant force
• What does Newton's Second Law describe?

  The relationship between acceleration, force, and mass
• What is the formula for Newton's Second Law?


  F = ma
• What do the symbols in the formula F = ma represent?


  F is force in Newtons, m is mass in kilograms, a is acceleration in m/s²
• What does Newton's Third Law state?

  For every action, there is an equal and opposite reaction
• What is a resultant force?


  The overall force acting on an object
• What happens when forces are balanced?


  The resultant force is zero, and the object remains at rest or moves with constant velocity
• What is weight defined as?


  The force due to gravity acting on a mass
• What is the formula for weight?

  Weight = Mass × g
• What is the value of g on Earth

  9.8
• What is friction?


  The force that opposes motion between surfaces in contact
• What is air resistance?


  A type of friction acting on objects moving through the air
• How is mass defined?


  The amount of matter in an object
• How is weight measured?


  In Newtons (N)
• What is momentum?


  A measure of how difficult it is to stop a moving object
• What is the formula for momentum?

  Momentum = Mass × Velocity
• What does the conservation of momentum state?


  In a closed system, total momentum before a collision equals total momentum after the collision
• What happens when an object reaches terminal velocity?


  The object falls at constant speed when air resistance balances gravitational force
• What is work done defined as?


  Work is done when a force moves an object over a distance
• What is the formula for work done?


  W = F × d
• What is kinetic energy?


  The energy of a moving object
• What is the formula for kinetic energy?


  KE = \frac{1}{2} mv^2