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Physics > Forces, Movement, Shape and Momentum > Flashcards

Forces, Movement, Shape and Momentum Flashcards

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1
Q

what is a force?

A

simply a push or a pull

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2
Q

what is gravity/weight?

A

close to a planet this acts straight downwards

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3
Q

what is reaction force?

A

acts perpendicular to a surface and away from it (so if the surface is horizontal, the reaction force acts straight downwards.)

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4
Q

what is electrostatic force?

A

between two charged objects. the direction depends on the type of charge (like charges repel, opposite charges attract)

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5
Q

what is thrust?

A

e.g. push or pull due to an engine or rocket speeding something up

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6
Q

what is drag/air resistance/friction?

A

which is slowing the thing down

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7
Q

what is lift?

A

e.g. due to an aeroplane wing

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8
Q

what is tension?

A

in a rope or cable.

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9
Q

how can you draw forces acting on a body?

A
  1. chances are, there are loads of forces acting you right now that you don’t even know about. you don’t notice them because they all BALANCE OUT.
  2. any object with a weight feels a reaction force back from the surface its on. otherwise it would just keep falling.
  3. when an object moves in a fluid (air, water etc.) it feels drag in the opposite direction.
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10
Q

what is friction?

A
  • a force that opposes motion.
  • if an object has no force propelling it along, it will always slow down and stop because of friction (unless you’re in space)
  • to travel at a steady speed, things always need a driving force to counteract the friction.
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11
Q

in which 3 ways does friction occur?

A
  1. friction between solid surfaces which are gripping
  2. friction between solid surfaces which are sliding past each other
  3. resistance or “drag” from fluids (liquids or gases, e.g. air)
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12
Q

how does friction occur between solid surfaces which are gripping?

A
  • static friction
  • you can reduce this types of friction by putting a lubricant like oil or grass between the surfaces.
  • friction between solids can often cause wear of the two surfaces in contact.
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13
Q

how can you reduce friction that occurs between solid surfaces which are sliding past each other?

A
  • you can reduce this types of friction by putting a lubricant like oil or grease between the surfaces.
  • friction between solids can often cause wear of the two surfaces in contact.
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14
Q

how can you reduce friction that occurs as resistance or drag?

A
  • the most important factor by far in reducing drag in fluids is keeping the shape of the object streamlined, like sports cars or boat hulls.
  • lorries and caravans have “deflectors” on them to make them more streamlined and reduce drag.
  • roof boxes on cars spoil their streamlined shape and so slow them down.
  • for a given thrust, the HIGHER THE DRAG, the LOWER THE TOP SPEED of the car.
  • the opposite extreme to a sports car is a parachute which is about as high drag as you can get.
  • in a fluid, FRICTION ALWAYS INCREASES AS THE SPEED INCREASES.
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15
Q

how do vector quantities differ from scalar quantities?

A

vectors have size and direction, scalar quantities only have size.

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16
Q

give examples of vector quantities?

A

force, velocity, acceleration, momentum

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17
Q

give examples of scalar quantities?

A

mass, temperature, time, length

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18
Q

how do you work out resultant forces that act along a line?

A
  • to work out resultant force, you need to combine vectors.
  • e.g. whats the resultant force of a 220N force north, a 180N force south and a 90N force south?
    1. start by choosing a direction as the positive, lets say north. this means you add any forces in the north direction and subtract any forces in the south direction.
    2. resultant force = 220 - 180 - 90 = -50N, so 50 N south.
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19
Q

resultant force = mass x acceleration POINTS

A
  1. the bigger the force, the greater acceleration or deceleration.
  2. the bigger the mass, the smaller the acceleration
  3. to get a big mass to accelerate as fast as a small mass it needs a bigger force. just think about pushing heavy trolleys and it should all make sense.
  4. in a nutshell, any resultant force will produce acceleration (f=ma)
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20
Q

what is gravity?

A
  • gravity attracts all masses, but you only notice it when one of the masses is really really big, e.g. a planet. anything near a planet or a star is attracted to it very strongly.
  • this has three important effects:
    1. on the surface of the planet, it makes things accelerate towards the ground (all with the same acceleration, g, which is about 10m/s2 on earth)
    2. it gives everything a WEIGHT.
    3. it keeps planets, moons and satellites in their orbits. the orbit is a balance between the forward motion of the object and the force of gravity pulling it inwards.
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21
Q

why are weight and mass not the same?

A
  • mass is just the amount of “stuff” in an object. for any given object this will have the same value anywhere in the universe.
  • weight is caused by the PULL of gravity. in most questions the weight of an object is just the force of gravity pulling it towards the centre of the earth.
  • an object has the same mass, whether its on earth or on the moon, but its weight will be different. a 1kg mass will weigh less on the moon than it does on earth, simply because the force of gravity pulling on it is less.
  • weight is a force measured in newtons. its measured using a spring balance our newton meter. mass is not a force. its measured in kilograms with a mass balance.
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22
Q

what is stopping distance?

A
  • the stopping distance of a car is the distance covered in the time between the driver first spotting a hazard and the car coming to a complete stop.
  • thinking distance + braking distance
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23
Q

what is thinking distance and what is it affected by?

A
  • “the distance the car travels in the time between the driver noticing the hazard and applying the brakes”
    1. HOW FAST YOU’RE GOING:
  • whatever your reaction time, the faster your going, the further you’ll go.
    2. YOUR REACTION TIME:
  • affected by things like tiredness, drugs, alcohol, old age, and inexperience
24
Q

what is stopping distance and what is it affected by?

A
  • “the distance the car travels during its deceleration whilst the brakes are being applied”
    1. HOW FAST YOU’RE GOING:
  • the faster your going, the further it takes to stop
    2. THE MASS OF THE VEHICLE:
  • with the same brakes, the larger the mass of a vehicle, the longer it takes to stop
    3. HOW GOOD YOUR BRAKES ARE:
  • all brakes must be checked and maintained regularly. worn or faulty brakes will let you down catastrophically in an emergency.
    4. HOW GOOD THE GRIP IS:
  • depends on road surface, weather conditions and tyres.
25
Q

what do drivers need to do to avoid an accident?

A
  • need to leave enough space between their car and the one in front so that if they need to stop suddenly they can do so safely.
  • “enough space” means the stopping distance for whatever speed they’re going at.
  • speed limits are important because speed affects stopping distance so much.
26
Q

how can moving objects reach terminal velocity?

A
  • frictional forces increase with speed, but only up to a certain point.
  • when an object first starts to fall, it has much more force accelerating than resistance slowing it down.
  • as velocity increases, the resistance builds up.
  • this resistance force gradually reduces the acceleration until eventually the resistance force is equal to the accelerating force.
  • at this point, the object wont be able to accelerate any more. it will have reached its maximum/TERMINAL velocity.
27
Q

what does the terminal velocity of falling objects depend on?

A

THEIR FORCE AND AREA.

  • the accelerating force acting on all falling objects is gravity and it would make them all accelerate at the same rate, if it wasn’t for air resistance.
  • on earth, air resistance causes things to fall at different speeds, and the terminal velocity of any object is determined by its drag compared to its weight. the drag depends on its shape and area.
28
Q

example of terminal velocity?

A

HUMAN SKYDIVER.

  • without his parachute open he has quite a small area and a force equal to his weight pulling him down.
  • he reaches a terminal velocity of about 120 mph.
  • but with the parachute open, theres much more air resistance (at any given speed) and still only the same force pulling him down.
  • this means his terminal velocity comes right down to about 15mph, which is a safe speed to hit the ground at.
  • in both cases, once the skydiver has reached terminal velocity, RESISTANCE=WEIGHT
  • the difference is the SPEED at which that happens.
29
Q

what does hooke’s law say?

A

EXTENSION IS PROPORTIONAL TO FORCE

30
Q

what does hooke’s law say?

A
  1. the length of an unstretched metal wire is called its natural length, l
  2. if a metal wire is supported at the top and then a weight attached to the bottom, it stretches. they weight pulls down with a force F, producing an equal and opposite force at the support.
  3. this will also happen to helical springs and any object that will stretch WITHOUT immediate snapping or deforming.
  4. the extension of a stretched wire is PROPORTIONAL to the load/force. this relationship is called Hooke’s Law.
  5. a metal spring (or other object) will also obey Hooke’s law if a pair of opposite forces are applied to each end.
31
Q

PRACTICAL: you can investigate Hooke’s law with a spring.

A
  1. set up the apparatus. have plenty of extra masses and measure the weight of each with a balance.
  2. measure the length of the spring (e.g. with an accurate mm ruler) when no load is applied. ensure the ruler is vertical and measure the sprig at eye level.
  3. add one mass at a time and allow the spring to come to rest, then measure the new length of the spring. the extension is the change in length from the original length. repeat this until you have no fewer than 6 measurements.
  4. once done, repeat the experiment and calculate an average value for the spring length for each applied weight.
  5. plot on a graph (force on y and total extension on x)
32
Q

when does Hooke’s Law stop working?

A

WHEN THE FORCE IS GREAT ENOUGH.

  • theres a limit to the force you can apply for Hooke’s Law to stay true.
  • when the force is great enough the spring/rubber band reaches its ELASTIC LIMIT.
  • if you increase the force past the elastic limit, the material will be permanently stretched. when all the force is removed, the material will be longer than at the start.
  • some materials, like rubber, only obey Hooke’s law for really small extensions.
33
Q

when does a material display elastic behaviour?

A
  • if a material returns to its original shape once the forces are removed, it displays elastic behaviour.
  • metals display elastic behaviour as long as Hooke’s law is obeyed.
34
Q

what is the initial linear region of a force-extension graph associated with?

A

hooke’s law

35
Q

what is the relationship between momentum, mass and velocity?

A

momentum = mass x velocity

36
Q

how is momentum used in safety features?

A
  • a larger force means a faster change of momentum (and so a greater acceleration)
  • likewise, if someone’s momentum changes very quickly (like in a car crash) the forces on the body will be very large, and more likely to cause injury.
  • this is why cars are designed to slow people down over a longer time when they crash, the longer it takes for a change in momentum, the smaller the force (and the less severe the injuries are likely to be)
37
Q

how do crumple zones help with safety?

A

crumple zones crumple on impact, increasing the time taken for the car to stop.

38
Q

how do seat belts help with safety?

A
  • seat belts stretch slightly, increasing the time taken for the wearer to stop.
  • this reduces the forces acting on the chest
39
Q

how do air bags help with safety?

A

air bags also slow you down more gradually.

40
Q

momentum before =

A

= momentum after.

  • momentum is conserved when no external forces act
  • total momentum after is the same as it was before.
41
Q

what happens when a force acts on an object?

A
  • when a force acts on an object, it causes a change in momentum.
  • F = (mv - mu) / t
42
Q

what is newton’s third law?

A
  • IF OBJECT A EXERTS A FORCE ON OBJECT B, THEN OBJECT B EXERTS AN EQUAL AND OPPOSIT FORCE ON OBJECT A.
  • every action has an equal and opposite reaction.
  • that means if you push something, say a shopping trolley, the trolley will push back against, you just as hard.
    as soon as you stop pushing, so does the trolley.
  • e.g. when you go swimming. you push against the water with your arms and legs, and the water pushes you forwards with an equal-sized force in the opposite direction.
43
Q

what is newton’s first law?

A

so long as the forces on an object are all balanced, then it’ll just say still, or else if its already moving it’ll just carry on at the same velocity, so long as all the forces are balanced.

44
Q

what is newton’s second law?

A

if there is an unbalanced force, then the object will accelerate in that direction.

45
Q

what is a moment?

A

a moment is the turning effect of a force.

46
Q

what is the relationship between moment, force and perpendicular distance?

A

moment (Nm) = FORCE (N) x perpendicular DISTANCE (m) between line of action and pivot.

47
Q

example of moments

A
  • the force on the spanner causes a turning effect or moment on the nut. a larger force would mean a larger moment.
  • using a longer spanner, the same force can exert a larger moment because the distance from the pivot is greater.
  • to get the maximum moment (or turning effect) you need to push at right angles (perpendicular) to the spanner
  • pushing at any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot is smaller.
48
Q

where does the centre of gravity hang?

A
  • directly below the point of suspension
  • you can think of the centre of gravity of an object as the point through which the weight of a body acts.
  • a freely suspended object will swing until its centre of gravity is vertically below the point of suspension.
  • this means you can find the centre of gravity of any flat shape.
49
Q

why does a freely suspended object swing?

A
  • a freely suspended object will swing until its centre of gravity is vertically below the point of suspension.
  • the objects weight acts at a distance from the pivot, which creates a moment about the pivot.
  • this makes it swing until the centre of gravity is at its lowest (i.e directly under the pivot)
  • note: it rests in this position because theres no moment, the pivot is in line with the line of action of the force.
50
Q

how can you find the centre of gravity of any flat shape?

A
  1. suspend the shape and a plumb line from the same point, and wait until they stop moving.
  2. draw a line along the plumb line
  3. do the same thing again, but suspend the shape from a different pivot point,
  4. the centre of gravity is where your two lines cross.
51
Q

how can you find the centre of gravity for simple shapes?

A

look for lines of symmetry.

52
Q

where does the weight of a body act?

A

through its centre of gravity.

53
Q

what does the principle of moments say?

A

if an object is balanced then:

total ANTICLOCKWISE moments = total CLOCKWISE moments.

54
Q

how do the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam?

A
  • if a light rod is being supported at both ends, the upwards force provided by each support won’t always be the same
  • if a heavy object is placed on the rod, the support closest to the object will provide a larger force.
    (“light” means you can ignore the weight in your calculation. in general, if they don’t tell you the weight, you can ignore it)
55
Q

what happens if the moments are not equal?

A

if the total Anticlockwise moments do not equal the total clockwise moments, there will be a RESULTANT MOMENT…

…so the object will turn in the direction of largest force.

Physics (14 decks)

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