Forces and Motion

• Force


  The push or pull due to the interactions between objects
• Forces


  • Can change the direction of an object
  • Can change the speed of an object
  • Can change the size of an object
  • Can change the shape of an object
  • Cannot change the mass of an object
• Force


  Mass times acceleration, measured in Newtons
• Weight


  Mass times gravitational field strength, measured in Newtons
• Gravitational field strength


  10 Newtons per kg on Earth, 4 Newtons per kg on Mars
• Momentum


  An object or mass in motion
• Momentum


  Mass times velocity
• Law of conservation of momentum


  The total momentum before a collision equals the total momentum after a collision
• Impulse


  The effects of forces over time, also the change in momentum
• Calculating impulse


  1. Force times time

  2. Change in momentum
• Force


  Change in momentum over time
• Scalar quantity


  A magnitude. It can be described fully with a single numerical value
• Scalar quantities


  • distance, speed, time, mass, energy
• Vector quantity


  A quantity that has both a magnitude and a direction
• Vector quantities


  • Force, velocity, displacement, momentum, moment
• Distance-time graph


  Gradient (or Slope) can be used to calculate speed

  
• Distance-time graphs


  • A = constant speed
  • B = at rest
  • C = accelerating

  
• Average speed


  Distance(m) / Time(s)
• Acceleration


  The change in velocity per unit of time
• Acceleration unit


  m / s2
• Equation for acceleration


  Acceleration =(Change in velocity)/ time
• Velocity-time graph


  Gradient (slope) can be used to calculate acceleration

  
• Velocity-time graph


  Area under the graph can be used to calculate distance travelled

  
• Equation linking final speed, initial speed, acceleration and distance


  v^2 = u^2 + 2as

  
• Forces acting on objects


  • A = weight
  • B = Air Resistance (Drag)

  
• Forces that always oppose motion


  • Friction
  • Air Resistance (Drag)
• Force holding planets around the Sun

  Gravitational Force
• Force holding electrons around nucleus

  Electrostatic Force
• Ways a force can affect an object


  • Change the shape (extension/compression)
  • Change the speed
  • Change the direction
• Ways a force can affect the body it is applied on


  • Change the shape of the object (extension/compression)
  • Change the speed of the object
  • Change the direction the object is moving
• Resultant force for object A is 5N to the right

  
• Resultant force for object B is 0N (balanced)

  
• Resultant force for object C is 13N downwards

  
• Magnitude of resultant force is 45N
• Unbalanced force, mass and acceleration


  • Force (N) = Mass(kg) x Acceleration (m/s2)
  • F = m x a
• Mass, weight and gravitational acceleration


  • Weight(N) = Mass(kg) x g (m/s2)
  • W = mg
• Relationship between stopping distance, braking distance and thinking distance


  Stopping Dist. = Thinking Dist. + Braking Dist.
• Factors affecting stopping distance


  • Reaction time
  • Weather conditions
  • Initial speed
  • Driver's conditions
  • Road Conditions
  • Mass of the car
  • Tire Conditions
• Factors affecting air resistance on a falling object


  • Surface Area
  • Speed
• How a falling object reaches terminal velocity

  1. At first object falls under the effect of its weight accelerating with g. As it accelerates, air resistance opposing the motion increases therefore resultant force acting on the object decreases and since F=ma, acceleration decreases.

  2. When air resistance becomes equal to weight, forces are balanced so resultant force=0 therefore a=0 and object reaches to terminal velocity.