Forces

• Vector quantities: have a magnitude and a direction


  force; velocity; displacement; acceleration; momentum
• Scalar quantities: only have magnitude


  speed; distance; mass; temperature; time
• Vectors are usually represented by an arrow: length shows the magnitude, and the direction of the arrow shows the direction of the quantity
• Contact force


  When two objects have to be touching for a force to act: friction; air resistance
• non-contact force

  magnetic force; gravitational force; electrostatic force
• When two objects interact, there is a force produced on both objects. An interaction pair is a pair of forces that are equal and opposites (Newton's third law)
• Gravitational force


  the force of attraction between masses
• Weight is the force acting on an object due to gravity; measured using a calibrated spring balance or newtonmeter
• The weight of an object depends on its mass and the strength of the gravitational field it is in.
• Mass is not a force. An object has the same mass whether it's on Earth or the moon; it's measured in kilograms with a mass balance
• Equation for weight of an object


  Weight (N) = Mass (kg) x Gravitational field strength (N/kg)
• Resultant force


  The overall force on a point or object
• Work done


  When a force moves an object through a distance, energy is transferred and work is done on the object
• Equation for work done


  Work done (J) = Force (N) x Distance (m)
• Scale drawings to find resultant forces


  Draw all the forces acting on an object; then draw a straight line to join the two forces; measure the length of the resultant force on the diagram to find the magnitude and the angle to find the direction of the force
• An object is in equilibrium if the forces on it are balanced


  If all of the forces acting on an object combine to give a resultant force of 0, the object is in equilibrium
• Work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object
• The extension of a stretched spring is directly proportional to the force applied


  Force (N) = Spring constant (N/m) x extension
• Equation for elastic potential energy


  Elastic potential energy (J) = 0.5 x spring constant (N/m) x extension (m)
• Speed is how fast an object is moving with no regard to the direction. It is measured in metres per second (m/s); velocity is speed in a given direction
• distance travelled (m) = speed (m/s) x time (s)
• Acceleration: the change in velocity in a certain amount of time


  Acceleration (m/s squared) = change in velocity (m/s) / time (s)
• Uniform acceleration means a constant acceleration


  final velocity (m/s) - initial velocity (m/s) = 2 x acceleration (m/s squared) x distance (m)
• Distance-time graphs

  gradient = speed; flat sections are where its stationary; straight uphill sections mean it's travelling at a steady speed; curves represent acceleration or deceleration
• Velocity-time graphs

  gradient = acceleration; flat sections means travelling at a steady speed; / uphill is acceleration and downhill is deceleration; curve means changing acceleration; area under graph is distance travelled