Forces phys 2

• Contact force


  Force that requires objects to be touching for it to act
• Non-contact force


  Force that does not require objects to be touching for it to act
• Examples of contact forces


  • Friction
  • Air resistance
  • Tension in rope
  • Normal force
• Examples of non-contact forces

  • Magnetic force
  • Gravitational force
  • Electrostatic force
• When two objects interact, a force is produced on both objects
• The interaction pair of forces are equal and opposite
• Weight


  Force acting on an object due to gravity
• Mass


  Amount of 'stuff' in an object
• Weight depends on the strength of the gravitational field, but mass does not change with location
• Weight is measured in newtons, mass is measured in kilograms
• When an object is elastically deformed


  It can return to its original shape and length after the force is removed
• When an object is inelastically deformed


  It cannot return to its original shape and length after the force is removed
• Elastic potential energy


  Energy stored in an object when it is elastically deformed
• Spring constant


  Measure of the stiffness of a spring
• The work done in stretching a spring is proportional to the square of the extension
• Scalar quantity


  Physical quantity with only magnitude, no direction
• Vector quantity


  Physical quantity with both magnitude and direction
• Examples of scalar quantities

  • Speed
  • Distance
  • Mass
  • Temperature
  • Time
• Examples of vector quantities

  • Force
  • Velocity
  • Displacement
  • Acceleration
  • Momentum
• Velocity is a vector quantity, speed is a scalar quantity
• The acceleration of an object is directly proportional to the resultant force acting on it, and inversely proportional to its mass
• Newton's first law states that an object at rest will remain at rest, and an object in motion will remain in motion, unless acted upon by a non-zero resultant force
• Newton's second law states that the acceleration of an object is directly proportional to the resultant force acting on it, and inversely proportional to its mass
• Newton's third law states that for every action, there is an equal and opposite reaction
• Inertia is the tendency of an object to resist changes in its state of motion
• The greater an object's mass, the greater its inertia
• When two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction
• In an equilibrium situation, the forces acting on an object are balanced
• A book resting on the table is in equilibrium. The weight of the book is equal to the normal contact force
• The two forces acting on the book are the weight and the normal contact force, which demonstrate Newton's Third Law
• Revision must be done with tea
• The Newton's Third Law force pairs trip people up, so make sure you understand exactly what the forces are acting on and causing the movement (or lack of it)
• A car travels at a constant velocity along a road, so that it is in equilibrium
• A pair of forces that demonstrate Newton's Third Law in this situation

  • Driving force
  • Friction force
• You can investigate how Mass and Force Affect Acceleration
• Investigating Motion


  1. Set up apparatus with trolley and light gate

  2. Connect trolley to pulley and hook with known mass

  3. Measure acceleration of trolley system

  4. Repeat with different masses on trolley and hook
• Newton's Second Law


  • F=ma, where F is the weight of the hanging masses, m is the mass of the whole system, and a is the acceleration of the system
• Adding masses to the trolley

  Increases the mass of the whole system but the force applied stays the same, leading to a decrease in acceleration
• Transferring masses to the hook

  Increases the accelerating force without changing the mass of the whole system, leading to an increase in acceleration
• Light gate


  Can be used to measure the acceleration of a trolley