Topic 2: Motion and Forces

• Scalar - A quantity that has magnitude only, no direction

  • distance
  • speed
  • mass
  • energy
• Vector - A quantity that has both magnitude and direction

  • displacement
  • velocity (speed in a stated direction)
  • acceleration
  • force
  • weight
  • momentum
• Distance/Time Graphs:

  • gradient of a slope is the acceleration or deceleration
  • no gradient means no movement
• Velocity/Time Graph:

  • gradient shows acceleration
  • no gradient means constant speed
  • area under the graph is distance travelled
• Trolley Practical: (Part 1)

  1. Adjust the height of the ramp until the trolley begins to move which reduces the effects of friction
  2. Measure the length of the card and attach it to the top of the trolley
  3. Weigh the combined mass then securely fasten a pulley to the opposite end of the ramp
  4. Place the trolley at the top of the ramp attached to the pulley by a string and held in place by sticky tape that can act as a marker
• Trolley Practical: (Part 2)

  5. Set up two light gates, one near the top and the other near the bottom, and attach them to a data logger

  6. Let go of the pulley with the mass and record the velocity and time it takes for the trolley to pass through each light gate

  7. Repeat the experiment with varying mass and create a scatter graph (y-axis is acceleration and x-axis is total mass)
• Typical speeds:

  • Walking - 1.4m/s
  • Running - 3m/s
  • Cycling - 5.5m/s
  • Car - 13m/s
  • Wind - 5 to 20m/s
  • Sound - 340m/s
• Acceleration - the change in velocity in a certain amount of time

  • constant acceleration is called uniform acceleration
  • objects are uniform (10m/s*2) in free fall because of gravity
• Newton's First Law - an object will remain at rest or continue at a constant velocity unless acted upon by a resultant force

  • constant velocity is when all forces are balanced (resultant force is zero)
• Newton's Second Law - the acceleration of an object is directly proportional to the resultant force acting on it
• Weight - the force of gravity acting on an object, measured in newtons (N)

  • force changes depending on gravity
• Centripetal Force - the force that causes an object to move in a circle

  • object travelling in a circle at a constant speed is constantly changing velocity
  • changing velocity means there must be a resultant force causing it to accelerate
• Inertial Mass - how difficult it is to change the velocity of an object via the ratio of force over acceleration

  Inertia - the tendency to keep moving with the same velocity
• Newton's Third Law - when two objects interact, the forces they exert on each other are equal and opposite

  • an action force with a smaller mass will have greater acceleration than the normal force with a greater mass because of the conservation of momentum
• An object in equilibrium is in a state of balance where the resultant force acting on the object is zero as the action and normal forces are the same (e.g. book on a table)

  • book weight is down via gravity and also pulls back up
  • normal contact force is pushing up on book while normal contact force of the book pushes down
• Momentum - the product of an object's mass and velocity
• Conservation of Momentum - the total momentum of an object before and after an interaction is equal

  • momentum is conserved
  • if a white ball hits a stationary red ball of the same mass, white ball momentum decreases because of Newton's third law
  • force exerted on red ball makes it move and increases momentum
  • both balls add to original momentum
  • force = change in momentum/time
• Measuring Reaction Times:

  • Someone holds a ruler straight down and align your hand so it is at 0cm
  • The person will let go randomly and the cm the ruler is caught shows reaction time
  • Longer the distance the longer the reaction time
  • Repeat for accuracy
• Thinking Distance - the distance a driver travels during their reaction time

  • increases with distractions, tiredness, drugs and alcohol
  • increases with car speed
• Braking Distance is the distance travelled by a vehicle from the time it starts to brake until it completely stops:

  • car speed - faster speed means distance increases
  • car mass - heavier car needs more force to stop
  • brake conditions - worn or faulty brakes won't be as efficient
  • friction between road and tires - can skid on dirty, wet and icy roads as well as bald tires
• Large decelerations in cars cause serious injuries so crumple zones were designed to increase collision times which reduces the force of impact
• Vehicle brakes do work on the wheels which transfers energy from kinetic energy stores in wheels to thermal energy stores in brakes.

  This causes brakes to overheat and may cause vehicles to skid