Forces

• Vector quantities = with a magnitude and direction

  include force, velocity, displacement, acceleration, momentum
• Scalar quantities = have only magnitude and no direction

  include speed, distance, mass, temperature, time
• vectors are represented by an arrow- the length shows the magnitude
• the weight of an object depends on the strength of the gravitational field strength

  
  

  weight can be measured using a calibrated spring balance or a newtonmeter
• free body diagrams:

  -a skydiver's weight acts on pulling him to the ground

  -the drag (air resistance) acts on him in the opposite direction to his motion
• an object is in equilibrium if the forces on it are balanced
• levers increase the distance from the pivot at which the force is applied

  Since M = FD this means less force is needed to get the same moment

  This means levers make it easier to do work
• displacement is a vector quanitity- it measures the distance and direction in a straight line from an object's starting and finishing points

  e.g. if you walk 5m North, and 5m South- your displacement is 0m but your distance is 10m
• a person walking- 1.5 m/s

  a person running - 3 m/s

  a person cycling - 6 m/s

  a car - 25 m/s

  a train - 30 m/s

  a plane- 250 m/s
• the speed of sound = 330 m/s

  the speed of light = 3x10^8 m/s
• drag is the resistance you get in a fluid, e.g. air resistance

  the most important factor in reducing drag is keeping the shape of the object streamlined --> this allows fluid to flow easily across it and reduced drag
• when an object first begins falling, the force of gravity is much more than the frictional force slowing it down, so it accelerates

  as the speed increases, the friction builds up. This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force --> therefore the resultant force is zero

  It would have reached its maximum speed or terminal velocity and fall at a steady speed
• -maximum speed = When you are going as fast as you can (eg; in a car), it's at its maximum or its top speed

  -terminal velocity = the maximum velocity that an object can attain when it is falling through a fluid, such as air or water.
• Newton's first law = if the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force is zero, it will continue moving at the same velocity
• Newton's second law = F=MA, the larger the resultant force, the greater the acceleration (force and acceleration are directly proportional)

  However acceleration is inversely proportional to the mass of an object
• inertia = the tendency of an object to remain at rest or continue moving at a constant speed

  an object's inertial mass measures how difficult it is to change the velocity of an object, this can be worked using Newton's second law (F = MA)
• Newton's third law= when two objects interact, the forces they exert on each other are equal and opposite
• braking = maximum force is applied to the brakes
• reaction time can be measured using a stopwatch or a timing device and calculating change in velocity
• in a closed system, the total momentum before an event (e.g. a collision) is the same after the event --> this is called conservation of momentum
• car safety features:

  • crumple zones --> crumple on impact, increasing the time taken for the car to stop
  • seat belts --> stretch, increasing the time taken for the wearer to stop
  • air bags --> inflate, the compressing air slows the person down gradually