♡ Topic 2_Motion and forces ♡

• Scalar quantity:

  • A quantity that only has a magnitude
  • A quantity that isn’t direction dependent
• Vector quantity:

  • A quantity that has both a magnitude and an associated direction
• Vector quantity can be represented using vector arrows
• Vector arrows represent vectors by:

  • The length of the arrow represents the magnitude
  • The arrow points in the associated direction
• Examples of vector quantities:

  1. Velocity

  2. Displacement

  3. Force
• Examples of scalar quantities:

  • Temperature
  • Time
  • Mass
  • Speed
  • Distance
  • Energy
• Velocity is the speed of an object in a specific direction
• Equation for average speed:

  average speed = distance / time
• On a distance/time graph, the gradient of the line represents the speed
• On a displacement/time graph, the gradient of the line represents the velocity
• To calculate speed at a given time from a distance-time graph for an accelerating object:

  • Draw a tangent to the curve at the required time
  • Calculate the gradient of the tangent
• Equation for average acceleration:

  Acceleration = (Change in Velocity)/(Time Taken)

  Units: Acceleration (m/s²), Velocity (m/s), Time (s)
• Equation relating final velocity with initial velocity, displacement, and acceleration:

  v^2 = u^2 + 2as
• Distance travelled by an object from a velocity-time graph is equal to the area under the graph
• On a velocity/time graph, the gradient of the graph represents the acceleration
• Typical value for the speed of sound:

  330 m/s
• Typical value for human walking speed:

  1.5 m/s
• Typical value for human running speed:

  3 m/s
• Typical value for human cycling speed:

  6 m/s
• Approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface:

  10 m/s²
• The name given to the single force that is equivalent to all the other forces acting on a given object is the resultant force
• Newton’s first law for a stationary object:

  • If the resultant force on a stationary object is zero, the object will remain at rest
• Newton’s first law for a moving object:

  • If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)
• The defining equation for Newton’s Second Law:

  • Resultant force = Mass x Acceleration
  • F = ma
• Newton’s Second Law in words:

  • An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
• Weight is the force that acts on an object due to gravity and the object’s mass
• Weight depends on:

  • The object’s mass
  • The gravitational field strength at the given position in the field
  • Weight = mass x gravitational field strength
• The unit used for weight is the Newton (N)
• The unit used for gravitational field strength is N/kg
• A calibrated spring-balance or newton-meter can be used to measure an object’s weight
• An object travelling at a constant speed may not have a constant velocity when the object is changing direction, for example, moving in a circle
• An object travelling at a constant speed in a circle does not have a constant velocity because:

  • Speed is a scalar quantity
  • Velocity is a vector quantity which can only be constant if the direction is constant
  • In circular motion, the direction is continuously changing
  • Velocity is constantly changing
• The resultant force that acts on an object moving in a circle is called the centripetal force and acts towards the centre of the circle
• Inertial mass is:

  • A measure of how difficult it is to change a given object’s velocity
  • The ratio of force over acceleration
• Newton’s Third Law states that whenever two objects interact, the forces they exert on each other are always equal and opposite
• The equation used to calculate an object’s momentum is Momentum = Mass x Velocity
• The unit used for momentum is kgm/s (kilogram metres per second)
• In a closed system, the total momentum before and after a collision are equal
• An equation linking change in momentum, force and time is Force x Time = Change in Momentum (F Δt = mΔv)
• Human reaction times can be measured using the ruler drop test where Person A drops the ruler and Person B catches it, and the distance travelled corresponds to their reaction time