Forces

• SCALAR QUANTITES:

  examples - mass, temperature, speed, energy, distance, time,

  They only have magnitude (size)

  have no direction
• VECTOR QUANTITES:

  examples - displacement, weight, force, velocity, acceleration, momentum

  All have magnitude and direction
• a force is a push or a pull that acts on an object due to the interaction with another object.
• all forces have both magnitude and a direction.
• forces are in Newtons (N)
• contact forces:

  objects are physically touching

  examples:

  
  

  tension in a rope (tug of war) - force of tension is pulling on the puller

  
  

  friction (airplane landing) - force of friction between the runway and aeroplane

  
  

  air resistance (parachute) - air particles collide with the parachute so air resistance acts upwards

  
  

  normal contact force (lamp on a table) - lamps exerts a downwards force on the table (force is the weight) and the table exerts an upwards force on the lamp.
• non contact forces:

  two objects physically separated

  examples:

  
  

  gravitational force - attracts all objects to other objects (space station to earth and earth to space station)

  
  

  electrostatic force - force between two charged objects, objects with opposite charge experience an electrostatic force of attraction. (two objects are physically separated as the electrostatic force is non-contact)

  
  

  Magnetic force - force experience by certain objects in a magnetic field.
• Mass of an object tells us how much matter sn object has in it.

  the unit is kilogram (kg)

  mass of an object doesn't depend on where it is (e.g. on the moon will be the same mass)
• weight of an object is the force acting on it due to gravity.

  unit is newton (N)

  weight of an object does depend on where it is as the gravitational field strengths in different places differ
• weight (N) = Mass (kg) x gravitational field strength (N/kg)

  W = m x g

  weight of an object is directly proportional to the mass of an object

  so if we double the weight the mass also doubles
• we can determine an abjects weight by using a calibrated spring balance also known as a newtonmeter

  
• centre of mass is the weight of an object (the force due to gravity) can be considered to act at a single point.
• weight (N) = Mass (kg) x gravitational field strength (N/kg)

  W = m x g

  weight of an object is directly proportional to the mass of an object

  so if we double the weight the mass also doubles

  
• the man is applying a force of 20N acting to the right

  a friction force of 10N is acting to the left

  the result force is a single force that has the same effect as all of the original forces acting together

  
• to work out resultant force we subtract the smaller force from the larger force

  
• adding the friction with the road and air resistance gives us a total of 9000N

  resultant force is 1000N acting to the left

  
• a force of 800N due to the skydivers weight (the force due to gravity)

  upwards force of 800N due to air resistanceforces are balanced so resultant force is 0

  
• airplane flies at constant velocity at constant altitude

  
  

  weight: acts downwards towards the earth

  
  

  lift: force is same magnitude as the weight but acting in opposite direction

  
  

  Thrust: forward force provided by engines

  
  

  air resistance: acts in reverse direction

  
  

  since the airplane is at constant velocity the forward and backward arrows must be balanced shown by the lengths

  
• Resultant force is a single force that can replace several forces that act on an object with the same effect.
• first step is to draw 1N=1cm line then use a protector to measure out 30*

  
• second step is to draw the 8N line

  
• third step is we need to make a parallelogram so copy the 8cm line and position it at the head of the 10cm force vector

  make sure from line up it is still 30*

  
• fourth step is to connect the 10 cm line under and draw a line in-between then measure it and that is the resultant force.17.5cm=17.5N

  to double check you can measure the inner angles to make sure its as accurate as possible

  
• • draw x and y axis
  • use protractor draw 35
  • make a dotted line of arrows
  • measure and write in N eg 83N
  • rub out faint lines and make arrows on x and y axis

  
• • draw x and y axis
  • use protractor draw 20
  • make a dotted line of arrows
  • measure and write in N eg 70N
  • rub out faint lines and make arrows on x and y axis

  
• work done (J/N m) = force (N) X Distance (m)

  W = F x s
• • box is travelling at constant velocity
  • man is applying a forward force on the box
  • force of friction acting between the bottom of the box and the floor
  • friction of the box causes temperature of box to increase
  • chemical energy in the mans muscles has been transferred to the thermal energy store of the box

  
• • kinetic energy store of the car transfers to the thermal energy store of the brakes
  • temperature of the brakes increase and cr slows down as it looses kinetic energy

  
• calculate work done

  
• examples of Elastic materials:

  • slinky
  • rubber bands
  • rubber gloves
  • eraser
  • playground surface
  • tennis ball
• stretching forces are equal in magnitude are equal in magnitude but in opposite direction

  forces cause the object to stretch

  when forces are released the object returns back to its original length

  
• apply squeezing force

  the spring compresses

  and if we take away the forces it returns to its original length

  
• applying three forces causes the object to bend

  if we remove the force it returns to its original shape and length

  these changes are called elastic deformation

  
• to change san objects length or shape we have to apply more than one force

  if we only add one force to a stationary object then the forces are no longer balanced
• if we stretch inelastic materials they do not return to their original length when forces are removed like polymers

  scientists call it inelastic deformation
• Force = spring constant (N/m) x extension (m)

  F = k x e
• calculate force for spring

  
• elastic potential energy is stored in the object and work done is equal to elastic potential energy but only true is the object is not inelastically deformed.
• distance tells us how far an object moves.
• Displacement tells us the distance an object moves in a straight line from the starting point to the finish point.