Forces Flashcards
a vector has…
a scalar has…
magnitude and direction
just magnitude
… cannot be negative but… can
scalars
vectors
examples of scalar
speed
distance
time
mass
energy
examples of vector
velocity
displacement
acceleration
force
momentum
what is scalar
can be measured with a number (magnitude)
what is a vector quantity
- measured with a number
- BUT ALSO
- has a direction
gravity- the larger the… the stronger the… the greater the…
mass
field
attraction
weight calculation
weight = mass x gravitational field strength
weight is measured using a… by…
force meter / calibrated spring-balance
by the weighing scales measuring the force you exert then divides by 10 to get the mass
gravitational field strength on earth
9.8
what happens to mass and weight of the same person on two different planets
mass stays the same
wieght is different because gravitational field strength is different
skydiver example of resultant forces
- initially, there is no air reisstance so only force acting on him is weight = accelerates
- air resistance increases, resultant force from weight decreases= acceleration decreases
- eventually equal and balance, so no resultant force = no acceleration
work done =
work done = force x distance
one joule of work is done when…
a force of one newton causes a displacement of one metre
1 joule = 1 newton metre
to stretch, bend of compress an object… need to be applied otherwise…
multiple forces need to be applied otherwise it will just move in one direction
elastic deformation
the object returns to its original shape when the load has been removed
plastic deformation
the object does not reutrn to its original shape when the load has been removed, this can be a spring when pulled too far
Hooke’s law
the extension of an elastic object, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
hooke’s law calculation
F = kx
Force = spring constant x extension
N Nm-1 m
hooke’s law : why does the graph stop being linear
- the limit of proportionality has been reached, no longer obeys hooke’s law
- turns to plastic behaviour (object pulled too far)
hooke’s law: if the graph is just linear with no non-linear end seciton…
… the material is brittle, so snaps instead of stretching afte tthe elastic limit
work done (spring) calc
work done = 1/2 x spring constant x extension squared
the work done on the spring =
= the elastic potential energy stored
moment of a force =
moment of a force = force x perpendicular distance
M = Fd
(roations and moments)equilibrium is when…
sum of anticlockwise moments = sum of clockwise moments
Gears: if a gear (1) is connected to another gear (2)with fewer teeth (smaller)
- second gear with turn faster
- but with less force
- in opposite direction to first gear
if a gear (1) is connected to a gear (2) with more teeth (larger)
- turns slower
- more force
- in opposite direction
pressure calc
pressure = force / area
pressure in fluids: an object floats if its weight is…
less than the weight of the water it displaces
buoyancy is the…
upwards force that counteracts the weight of the floating object
objects that float have a lower…
density than the water so the weight of the equivalent amount of water is greater than the weight of the floating object
pressure due to a column of liquuid =
pressure due to a column of liquid = height of column x density of liquid x gravitational field strength
p = hpg
upthrust (liquid)
- a partially or totally submerged object experiences a greater pressure on the bottom surfacr than the top surface
- this creates a resultant force upwards (upthrust)
earth’s atmosphere
thin layer on air around the earth which gets less dense with increasing altitude
explain why there the atmosphere gets less dense with increasing altitude
- the weight of the air is the force which causes the pressure
- so higher up, there are fewer air molecules than lower down
- so smaller weight so less pressure
assumptions made for a simple modle of the atmosphere
- isothermal (all at the same temp)
- transparent to solar radiation
- opaque to terrestrial radiation
if an object is moving in a circular motion, the acceleration… because…
increases because changing velocity is defines as acceleration so the change in direction means it accelerates
speed equation
speed = distance/ time
displacement time graphs:
- sharper gadient means…
- negative gradient means…
- horizontal line means…
- curved line means…
sharper gradient = faster speed
negative gradient = returning back to starting point
horizontal line = stationary
curved line = velocity is changing
velocity-time graphs:
gradient is…
sharper gradient means…
negative gradient means…
horizontal line means…
0 velocity means…
curved line means…
gradient is acceleration
sharper gradient is greater acceleration
negative gradient is deceleration
horizontal line is constant speed
0 velocity is stationary
curved line means acceleration is changing
falling in a fluid
- initially object will fall under gravity
- but then the drag forces will act and the object will move at terminal velocity
Netwtons first law
an object has a constant velocity unless acted on by a resultant force
Newton’s second law
the acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.
equation linking force, acceleration and mass
force = mass x acceleration
inertia
the measure of how difficult it is to change the velocity of an object
inertial mass =
inertial mass = force/ acceleration
newton’s third law
whenever two objects interact, the forces they exert on each other are equal and opposite
vehicle stopping distances
- after seeing a hazard, there is reaction time during which you travel x metres (thinking distance)
- then you react, causing the car to slow down and stop over y metres (braking distance)
to find reaction time… (practical)
ruler drop test
when a force is applied to the brakes of a vehicle…
- energy is transferred (by friction) to the wheel
- so the vehicle’s kinetic energy decreases
- and temp on the brakes increase
momentum =
momentum = mass x velocity
p = mv
momentum is always conserved in a…
collision or explosion (where there ar eno external forces like air resistance etc)
equation linking force, time and change in momentum
force = change in momentum / time
Safety features in cars - seatbelts
- seatbelts - stretch under large forces, stretching increases distance move slightly but extends the time taken for passengers to stop, decreases rate of change of momentum and therefore reduces force
safety features in cars - crumple zones
- without them cards would be a solid metal block which stops immediately
- softer areas at the front of the car which crumple upon a crash
- absorbs energy to deform and compact
- increases time taken for car to stop therefore reduced acceleration and force on passengers
safety features in cars - air bags
- without these your head would whip forward hitting the steering wheel
- inflates insantly upon a crash
- your head hitd this and slows down
- increases time taken for head to stop moving so reduces force on neck
