P1: Motion

  1. Measuring length and volume

  • Measuring volumes

    • Regular shaped solids: volume = cross-section x length

    • Liquids: measuring cylinders

    • Irregular shaped solids: Displacement

      • Partially fill a measuring cylinder with water to cover the object

      • Immerse object. Final volume - original volume = volume of the object

  • Units of length and volume:

    • 1 litre = 1dm³

    • 1 ml = 1 cm³

  1. Density

  • Mass (kg): Quantity of matter an object is made of

  • Equation: density = mass/volume (P=m/v)

  • Unit: g/cm³ or kg/m³

  • Values

    • Objects less dense than water will float

    • Density of water = 1000 kg/m³

  1. Measuring time

  • Longer periods: analogue clock, digital clock

  • Short periods: plumb bob

  • Scalar quantity: only magnitude (size). Eg. time, temperature…

  • Vector quantity: magnitude and direction Eg. velocity, acceleration..

  1. Understanding speed

  • Distance, time and speed

    • average speed = total distance travelled/total time taken

    • units depend on what it is being timed/measure by. eg: m/s, km/h

  • Distance-time graphs

      • steeper line → faster speed

    • Finding speed from a distance-time graph:

      • Speed = gradient of graph

  1. Acceleration

  • An increase in speed

  • Equation: acceleration = change in velocity/change in time

  • Units depends on velocity/time unit. Eg: km/h per second

  • Speed time graphs

      • steeper slope = greater acceleration

      • negative slope = deceleration

      • slope = 0 → constant speed

    • Distance travelled from speed-time graph:

      • distance = area under the speed-time graph

  1. Mass, weight, density

  • Mass vs weight:

    • Mass: how much matter an object has in kg

    • Weight: gravitational force that acts on the object in newtons (N)

  • Gravity and weight

    • Gravitational field strength: 9.81 N/kg

    • Weight = mass x gravitational field strength

      • W = mg

  • Free fall: acceleration due to the earth’s gravity

    • Free fall = gravitational field strength (9.8N/kg)

  1. Forces

  • Forces can change the shape and size of an object by stretching, squashing, bending, twisting

  • Unbalanced forces can change the motion of an object

  • Friction:

    • force between 2 objects moving against each other

    • always acts in the opposite direction to movement

    • Effects:

      • Slows objects down

      • Heats objects up

  • Resultant forces: Combined effects of 2 forces on one object

    • resultant force = forward force - backward force

    • no resultant force → no acceleration, remains at rest and at a constant speed in a straight line

  1. Force, mass and acceleration

  • Greater mass = less acceleration

    • Double mass → half the force

  • force = mass x acceleration

    • f = ma

    • unit: m/s²

  1. Stretching strings

  • Extension of a string:

    • length of stretched string = original length + extension

    • Double N that is stretching the string → extension doubles

  • Limit of proportionality: the point at which the graph is no longer a straight line

  • spring constant = force/unit extension

  1. Turning forces

  • Moment of a force: the turning effect of a force on a pivot

    • Pivot: fixed point at which the object will turn

    • Moment is bigger if force is bigger

    • moment is bigger with acts further from the pivot

    • moment is greatest at 90 degrees to the object

    • moment = force x perpendicular distance from pivot

      • Unit depends on question: eg. N/m

  • Stability and centre of gravity

    • Centre of gravity: the point where all mass is centered

      • To be stable an object should have a low centre and a wide base

    • Finding the centre of gravity:

      • Hang the object with a plumb bob → centre of gravity lies on the line going down the plumb bob

      • Repeat 2 more times → centre of gravity is cross-section

  1. Pressure

  • A large force pressing on an area

    • small area → high pressure

    • large area → low pressure

    • unit: Pa