motion, forces and energy

• a scaler quantity only has magnitude such as distance, speed, time, temperature, energy and mass
• a vector quantity has magnitude and direction such as force, weight, velocity, acceleration, momentum, electric feild strength
• density = mass over volume

  p=m/v
• weight= mass times gravitational feild strength

  w=mg
• the volume of a small irregular object if its sinks in water is measured by adding it to a measuring cylinder with water. then look at the water level before and after and subtract to get the volume
• the volume of a large irregular object if it sinks is measured by using a displacement can or eureka can and a measuring cylinder. fill them to the brim after adding the object some water will overflow into the measuring cylinder.
• the volume of a small irregular object if it floats use a sinker to help the object sink and subtract the initial and final volume of water in the measuring cylinder.
• speed is defined as the distance travelled per unit time

  speed= distance over time

  speed is a scaler quantity

  unit: m/s
• precautions for test for volume

  parallax error: has to be measured at eye level

  splashing of water: use string
• velocity is the speed in a given direction. It is defined as the rate of change of displacement.

  velocity is a vector quantity

  velocity = displacement over time

  unit: m/s
• average speed= total distance over total time
• acceleration is defined as the rate of change of velocity with time

  acceleration= final velocity - initial velocity over time

  a= v-u/t

  unit: m/s^2
• when an object is falling freely near the surface of the earth its acceleration is constant and is approximately equal to 10m/s^2
• gradient of a distance time graph gives you speed
• area of a speed time graph gives you distance
• the gradient of a velocity time graph gives us acceleration
• distance travelled by an object moving at constant speed is directly proportional to the time taken.
• speed is always positive or zero but velocity can be negative if it's going backwards
• the sum of more than one force is called resultant force
• the resultant force is calculated by working out the difference in opposing forces
• force= mass times acceleration

  f=ma

  f= m(v-u)/t

  unit: newton
• measurement errors:

  random error: cause by anomaly when you repeat the result

  systematic error: for example reaction time can cause systematic error
• the spring constant, k, is defined as the force per unit extension.

  force= spring constant times extension

  f=kx

  unit: N/m

  for a linear load-extension graph, the spring constant stays the same. spring constant= gradient

  on a load extension graph, the limit of proportionality is the point at which the graph is no longer linear (straight)
• extention, x = final minus initial length
• • elastic limit or limit of proportionality is the point at which hooke's law is no longer obeyed.
  • the material gets permanently deformed or is broken after this point
• the centripetal force is the force that enables an object to move in a circle
• the moment of a force is a measure of its turning effect. It is the turning effect of a force acting on an object
• moment= force times perpendicular distance from pivot

  unit: Nm
• at equilibrium the sum of clockwise moments= sum of anticlockwise movements
• an object is in equilibrium when there is no resultant force and no resultant moment on the object
• the center of gravity is the point where the object's weight seems to act
• a body is stable as long its centre of gravity remains vertically above its base. if this is not the case, the body will topple
• conditions for stability:

  • wide base
  • lower its center of gravity
• momentum= mass times velocity

  p=mv

  unit: kgm/s
• in an elastic collision both momentum and kinetic energy and conserved
• in an inelastic collision, only momentum is conserved. energy can be transferred to other forms such as thermal energy and sound energy causing a decr. in kinetic energy after the collision
• kinetic energy not conserved:

  total kinetic energy before collision ≠ total kinetic energy after collision
• kinetic energy= 1/2 x mass x velocity squared

  k.e= 1/2mv^2
• conservation of momentum:

  m1u1+ m2u2= m1v1+ m2v2

  total momentum before collision= total momentum after collision
• conservation of kinetic energy:

  1/2 m1u1^2 + 1/2 m2u2^2 = 1/2m1v1^2 + 1/2m2v2^2