P5 - Forces

• What is a vector?

  a quantity that has both magnitude and direction
• What are examples of a vector?

  • force

  • velocity

  • displacement

  • acceleration

  • momentum
• What is a scalar?

  a quantity that has magnitude but no direction
• What are examples of a scalar?

  • speed

  • distance

  • mass

  • temperature

  • time
• What are examples of contact forces?

  • friction

  • air resistance

  • tension in ropes

  • normal contact force
• What are examples of non-contact forces?

  • magnestic force

  • electrostatic force

  • gravitational force
• What effects foes gravitational force have?

  • makes things fall to the ground

  • gives everything weight
• What is mass?

  the amount of 'stuff' in an object, has the same value everywhere in universe
• What is weight?

  force acting on an object due to gravity, changes depending depending on gravitational pull (gravity)
• What is a resultant force?

  overall force acting on an object, taking into account both magnitude and direction
• What happens to a springs energy stores if its elastically deformed?

  all of the work done is transferred into objects elastic potential energy store
• What is a moment?

  the turning point of a force
• What do levers do?

  increase the distance from the pivot at which force is applied, meaning less force is needed to get same moment
• Gears:

  • circular discs with 'teeth' around edges

  • teeth interlock so turning one causes another to turn in opposite direction

  • used to transmit rotational effect of a force from one place to another
• How does a larger gear effect the moment and speed?

  • moment: larger gear mean larger moment - distance from pivot is larger

  • speed: larger gear will turn slower than smaller gear
• What is pressure?

  the force per unit area
• What is density?

  measure of 'compactness' of substance, how close the particles are to each other
• How does upthrust work?

  • when object is submerged in fluid, the pressure of liquid exerts force on it from every direction

  • pressure increases with depth, so force exerted on bottom is larger than force acting on top of object

  • causes resultant force upwards called upthrust
• What is upthrust equal to?

  weight of fluid displaced by object
• What happens to an object if its weight is more than the upthrust?

  sinks
• How does density effect whether an object floats or sinks?

  object that is denser to liquid surrounding is unable to displace enough fluid to equal its weight
• What creates atmospheric pressure?

  air molecules colliding with surface
• Why does atmospheric presure decrease with altitude?

  atmosphere become less dense - fewer air molecules able to collide with surface
• What is displacement?

  measurement of distance and direction (vector) in a straight line from an object's starting point to finishing point
• Speeds:

  • walking - 1.5 m/s

  • running - 3 m/s

  • cycling - 6 m/s

  • car - 25 m/s

  • train - 30 m/s

  • plane - 250 m/s
• What is acceleration?

  change velocity in certain amount of time
• What is drag?

  resistance you get in a fluid (a gas or liquid)
• What is terminal velocity?

  where the frictional force is equal to acceleration force (resultant force is zero)
• What is Newton's first law?

  • if resultant force on stationary object is zero, object will remain stationary

  • if resultant force on moving object is zero, object will remain at the same velocity (direction and speed)
• What is Newton's second law?

  • larger the resultant force acting on object, more the object accelerates - force and acceleration are directly proportional

  • acceleration is inversely proportional to the mass of object - object with larger mass will accelerate les than an object with smaller mass
• What is inertia?


  the tendency for an object to continue in the same state of motion
• What is Newton's third law?

  when two objects interact, the forces they exert on each other are equal and opposite
• stopping distance = thinking distance + braking distance
• What affects thinking distance?

  • speed - the faster your going the futher you travel during the time you take to react

  • reaction time
• What affects breaking distance?

  • speed - the faster a vehicle travels the longer it takes to stop

  • weather/road surface - if it is icy or wet, or if there is leaves or oil on road, there is less grip (so less friction) causing tyres to skid

  • tyre condition - bald tyres (no thread left) they cannot get rid of water in wet conditions which leads to skidding

  • brake condition - if worn or faulty, you will not be able to apply as much force as well-maintained brakes
• How do brakes work?

  when brake pedel is pushed it causes brake pads to be pressed on wheels, contact causes friction causing work to be done - work done between the brakes and wheel transfers energy from kinetic energy stores of wheels to thermal energy stores of brakes, increasing temperature
• What is a closed system?

  no external forces act on it
• What is the consvervation of momentum?

  total momentum before an event is the same as after the event
• What safety features do cars have?

  • crumple zones - crumple on impact, increasing time taken for car to stop

  • seat belts - stretch slightly, increasing time taken for wearer to stop

  • air bags - inflate before you hit dashboard, compressing air inside it slows you down gradually