Section 1 - Forces and Motion Flashcards
compare speed and velocity
BOTH:
- tell you how fast you’re going
- measured in m/s (or km/s or mph)
HOWEVER
speed is just how fast you’re going but velocity must also have direction specified
objects could be travelling at a constant speed with changing velocity (object is changing direction at same speed)
avg speed formula
avg speed = distance moved / time taken
acceleration
how quickly velocity is changing
(could be change in speed or direction or both)
unit acceleration
m/s^2 (NOT m/s that is velocity)
acceleration formula
acceleration = change in velocity/ time taken
(a negative value means deceleration - slowing down)
formula only works when acceleration is constant
final velocity formula
v² = u² + 2as
v = final velocity
u = initial velocity
s = distance travelled while accelerating
a = acceleration
formula only works when acceleration is constant
distance-time graphs
tell you how far something has travelled
how distance time graphs describe motion of object
gradient = speed of object
flat = stopped
steeper = faster
curves = represent acceleration
curve getting steeper = speeding up (increasing gradient)
levelling off curve = slowing down (decreasing gradient)
avg speed = total d/ time
how velocity time graphs describe motion of an object
gradient = acceleration
flat = steady speed
steeper = greater acceleration/deceleration
uphill (/) = acceleration
downhill () = deceleration
area under any part of graph = distance travelled in that time
curve = changing acceleration
speed = read value off axis
gravity and its effects
the force of attraction between all masses
effects:
1) on surface of planet it makes all things accelerate towards ground)
2) gives everything a weight
3) keeps planets moons and satellites in orbits
weight vs mass
mass = amount of stuff in an object, the same anywhere in the universe
weight = caused by pull of gravity and changes in different planets when the force of gravity pulling is more or less
weight = measured in newtons using spring balance or newton meter
mass = NOT A FORCE measured in kg with a mass balance
gravity/ weight force
close to planet acts straight downwards
reaction force
acts perpendicular to a surface and away from it (so if surface is horizontal, the reaction force acts straight upwards)
electrostatic force
force between two charged objects. direction depends on type of charge (like charges repel, opposites attract)
thrust
push or pull due to an engine or rocket speeding something up
drag/ air resistance / friction
slows something down
lift
occurs due to an aeroplane wing
tension
occurs in a rope or cable
balanced forces examples
weight and reaction force (prevents it from continuously falling)
what is friction
friction is the force that opposes motion
if an object has no force pushing it along it will always slow down and stop because of friction
to travel at a steady speed things always need a driving force to counteract the friction
how can friction occur
1) between solid surface that are gripping (static friction)
2) between solid surfaced gripping past each other (sliding friction)
- both of these can be reduced by adding a lubricant such as oil in between. this friction can cause wear.
3) resistance or drag from fluids
- to reduce friction keep shape of object streamlined
- for a given thrust the higher the drag the lower the top speed
- in a fluid friction always increases as the speed increases
The First Law of Motion - balanced forces
balanced forces mean no change in velocity, the object will stay still or carry on at a constant speed because there is zero resultant force
Second Law of Motion - resultant force
means acceleration in the direction of the resultant force. Acceleration can take forms of:
- starting
- stopping
- speeding up
- slowing down
- changing direction
and on a diagram arrows will be unequal
resultant force fromula
= mass x acceleration (F=ma)
- bigger force = greater acceleration/ deceleration
- bigger mass = smaller acceleration
- to get a big mass to accelerate you need a bigger force
- you must add or subtract the forces to get the overall resultant force
vector quantity
have a size and direction
such as: force, velocity, acceleration, momentum…
scalar quantity
have size but no direction
such as mass, temperature, time, length…
terminal velocity process
- object first starts to fall, it has much more force accelerating it towards the ground than resistance slowing it down
- as velocity increases resistance builds up
- resistance force gradually reduces 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 velocity - its terminal velocity
terminal velocity of an object depends on:
shape and area
- 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, which depends on the shape and size
hookes law
extension is proportional to force
natural length
the unscratched metal wire is called natural length (l)
hookes law stops working
- at first hookes law is obeyed and there is a straight line relationship between force and extension
- when force becomes great enough the graph starts to curve at the elastic limit
- if you increase the force past the elastic limit the material will be permanently streched.
- materials like rubber only obey hookes law for really small extensions
after an elastic deformation
a material can return to its original shape, and if it does it displays elastic behaviour
- they will do this as long as hookes law is obeyed
stopping distance
thinking distance + breaking distance
what is stopping distance
the distanced covered by a car in the time between a driver first spotting a hazard and the car coming to a complete stop
Thinking Distance
distance the car travels in the time between the driver noticing the hazard and applying the breaks
affected by
- how fast your going
- your reaction time (inexperience, alcohol, drugs, old age, tiredness…)
Breaking distance
distance the car travels during deceleration whilst the breaks are being applied
affected by:
- how fast your going
- the mass of your vehicle (larger mass takes longer to stop)
- quality of breaks
- quality of grip (which depends on road surface, weather and tyres)
