topic p5- forces

• • scalar quantities- only have magnitude and no direction- speed, distance, temperature, mass, time, power
  • vector quantities- have a magnitude and a direction- force, velocity, displacement, acceleration, momentum
• • force- a push or pull on an object caused by it interacting with something
  • direction of arrows shows direction of quantity
  • length of arrow shows magnitude
• contact forces- objects have to be touching

  • friction
  • air resistance- collisions between object + air particles
  • tension
  • normal contact force
• non-contact- objects don't need to be touching

  • gravitational force
  • electrostatic force
  • magnetic force
• gravitational force is the force of attraction between masses
• • weight- force that acts on an object due to gravity- weight is measured using newton meter
  • mass amount of stuff in an object
  • weight and mass are directly proportional
• • centre of mass- point at which an objects weight appears to act
• • object weight depends on strength of gravitation field at object location
  • object mass is the same value anywhere in the universe
• weight (N)= mass (kg) X gravitational field strength (N/kg)
• speed and direction affected by resultant forces
• equilibrium- all forces acting on object balance out
• • free body diagram- shows all forces acting on an isolated object, arrows show relative magnitudes and directions of forces acting
• • a resultant force is the overall force on a point or object
  • overall effect is found by adding those going in the same direction and subtracting any going in the opposite
• • work done- when a force moves an object through a distance, energy is transferred + work is done on the object
• work done (J)= force (N) X distance (M)- w=fs
• vertical= 1500-1500=0N

  horizontal= 1200-1000=200N

  resultant force= 200N
• bearing- angle measured clockwise from the north direction
• resolving forces- component forces acting together have same effect as the single force
• resultant force- a single force that can replace all the forces acting on an object to give the same effect as all the original forces acting together
• two ways to calculate resultant force

  1. add forces point in same direction, subtract forces pointing in opposite direction- f1-f2=resultant force
  2. draw forces to scale- measure length of resultant force to find it's magnitude and angle to find it's direction
• • changing shape (deformation)- more than one force has to act on a stationary object to change its shape- bending, stretching + compressing transfers energy
• two types of deformation

  1. elastic- object goes back to original shape
  2. inelastic- object doesn't go back to it's original shape
• work done + elasticity

  • force stretches or compresses object
  • work is done
  • energy transferred to object's elastic potential energy
  • (larger spring constant store more energy transferred)
• f=ke

  force (N)= spring constant (N/M) X extension or compression (M)
• force and extension graph

  • spring expanding
  • force becomes too big
  • limit of proportionality
• • extension- increase in length of a spring when its stretched
• investigating link between force + extension (practical)

  1. measure using a ruler the natural length of the spring and add a strip of tape to the bottom to increase accuracy
  2. add a mass to the spring, record the mass and measure the new length of the spring, the extension is the change in length
  3. repeat the process until you have enough measurements
  4. plot a force- extension graph of your result, they are directly proportional until limit is reached
• e=1/2ke ^2

  elastic potential energy (J)- energy stored in spring= 1/2 X spring constant (N/M) X extension (M) ^2
• • distance (scalar)- how far an object has moved
  • displacement (vector)- the distance + the direction in a straight line from an object's starting point to it's finishing point
• speed-scalar- how fast you're going with no regard to direction- objects, sound and wind rarely travel at a constant speed
• s=vt

  distance travelled (m)= speed (m/s) X time (s)
• velocity- vector- speed in a certain direction
• factors affecting speed

  • age
  • ability
  • distance
  • type of ground
  • temperature
  • pressure
• typical speeds

  • walking- 1.5
  • running- 3
  • cycling- 6
  • car- 25
  • train- 30
  • plane- 250
  • sound- 330
• • acceleration- change in velocity in a certain amount of time
  • deceleration- negative acceleration (shows an object is slowing down)
• a= av/t

  acceleration (m/s^2)= change in velocity (m/s)/ time (s)
• uniform acceleration means a constant velocity

  • acceleration of an object due to gravity is uniform for objects in free fall
  • it's roughly equal to 9.8m/s^2 near the earth's surface
• v^2-u^2=2as

  final velocity (m/s) - initial velocity (m/s)= acceleration (m/s^2) X distance (m)
• distance-time graph

  • distance (m)- y axis
  • time (s)- x axis
  • steady speed
  • stationary
  • accelerating
  • decelerating
  • gradient= speed
• velocity-time graph

  • velocity (m/s)- y axis
  • time (s)- x axis
  • steady speed
  • stationary
  • increasing acceleration
  • steady speed
  • constant acceleration
  • gradient= acceleration or deceleration
  • the area under any graph is equal to the distance travelled