dynamics

types of forces

contact forces

  • exists between objects that are in contact

  • e.g. frictional force, air resistance, normal force, tension

non-contact forces

  • do not require objects to be in contact to exist

  • e.g. gravitational force, magnetic force, electrostatic force

mass vs weight

mass

  • a measure of the amount of matter in a body

  • SI unit - kg

weight

  • the gravitational force acting on an object that has mass

  • SI unit - N

  • gravitational field: the gravitational force per unit ass placed at that point

    • it is a region in which a mass experiences a force due to gravitational attraction

    • SI unit - N kg-1

  • W = mg

relationship

  • the weight or gravitational field strength acting on an object is directly proportional to its mass

difference

mass

weight

the measure of the amount of matter in an object

the gravitational force acting on an object that has mass

scalar quantity

vector quantity

SI unit - kg

SI unit - N

independent of gravitational field strength

dependent on gravitational field strength

measured with a beam balance or calibrated electronic balance

measured with a spring balance

effects on forces

  • a body at rest moves

  • a moving body increases in speed

  • a moving body decreases in speed

  • a moving body changes direction

newton laws

1st law

  • newtons first law of motion states that every object will continue its state of rest or uniform motion in a straight line, unless a resultant (net) force acts on it

  • if absence of a net force

    • a stationary object will remain stationary

    • an object moving at constant velocity will continue to move at constant velocity

relation to inertia

  • inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to its mass

  • mass is the property that resists the change in motion (inertia)

    • an object with greater mass will have greater inertia

  • the larger the mass of an object, the harder it will be for the object to

    • start moving

    • slow down

    • speed up/move faster

    • change direction

  • inertia explains why people should wear seatbelts

    • if driver suddenly applies breaks, he will continue to move foward due to inertia

2nd law

  • newtons second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. the object accelerates in the direction of the net force

  • Fnet = ma

    • m - mass

    • a - acceleration

3rd law

  • newtons third law of motion states that if body A exerts a force FAB on body B, then body B will exert an equal and opposite force FBA on body A

  • same magnitude

  • same type of force

  • opposite direction

  • act on different bodies

friction

  • friction is the contact force that opposed or tends to oppose motion between surfaces in contact

  • depends on

    • materials in contact

    • surface roughness

    • proportional to the normal forces pressing the surfaces together

    • independent of the area of contact

positive effects

  • to walk

  • to slow down moving objects

    • using brakes

    • skydiver uses parachute in midair to vary air resistance and change speed

  • for grip

    • prevents skidding

    • rock climbers use chalk powder on their hands to absorb perspiration and improve their grip

    • to hold a pen/pencil

negative effects

  • wear and tear

    • tyres wear out on vehicles

    • soles of shoes wear out

  • unwanted slowing down or stopping of motion

  • generates unwanted heat

    • in machinery, friction reduces efficiency as large amount of energy is ‘wasted’ as heat

reducing negative effects of friction

  • use lubricants/grease between moving parts of machines

  • use smooth and polished surfaces

  • use wheels and rollers

    • e.g. conveyor belts

  • use ball bearings around axles in bicycles and cars

  • use air as cushion

    • e.g. magnetic levitation trains

  • adopt a streamlined shape

    • e.g. ships, submarines and fast moving fishes have streamlined bodies to reduce water resistance

terminal velocity

  • when the skydiver slides off from the plane, the only force acting on it is gravitational force. initial acceleration is uniform at 10m s-2

  • as he falls, air resistance opposing his motion starts to increase. net force downward is decreased, and he falls at decreasing acceleration

  • air resistance continues to increase until it is equal to his weight. there is zero acceleration and he falls at terminal velocity subsequently

  • the skydiver now opens his parachute. the large surface area of the parachute gives rise to a larger air resistance. net force is upwards and the skydiver decelerates

  • as the skydiver continues to slow down, the air resistance decreases until it becomes balanced with his weight. a lower terminal velocity is reached and the skydiver continues to the ground at constant velocity

factors

  • weight

    • the object that has larger weight needs a larger air resistance to balance its weight

      • accelerates for longer duration, larger terminal velocity

      • reach the ground at shorter time (same height) → due to larger average speed

  • surface area

    • the object that has a larger surface area traps moire air molecules underneath as it falls

      • air resistance increases at a faster rate

      • final air resistance is equal for both cases as weight is equal

      • accelerates for a shorter duration, smaller terminal velocity

      • reach the ground in a longer time (same height) → due to smaller average speed