forces Flashcards
scalar meaning with examples
they have magnitude (size) but no direction
speed, distance, mass
vector meaning with examples
have magnitude (size) and direction, shown with arrow
force, displacement, weight
what is a force
a push or a pull caused by an interaction
examples of contact force
friction, air resistance, tension in rope
examples of non contact force
magnetic force, gravitational force, electrostatic force
how to calculate resultant force using scalar drawing/ vector diagram
creating a parallelogram using given measurements then drawing vector/resultant force
how to resolve a force that acts at an awkward angle from scalar drawings/vector diagrams
use grid paper to draw given angle and force. draw horizontal and vertical.
how to know if something has been elastically deformed
returns to original shape when (more than one) forces removed
what is the relationship between the force pulling on a spring and how far the spring stretches, and where are the limits?
extension is directly proportional to force
at limit of proportionality,
how to investigate relationship of force and extension
• set up: weighted clamp stand with two clamps attached, one with meter ruler and one with spring, ensure top of spring is at 0m
• record unstretched length at eye level
• hang 1N mass and record new length
• repeat, ensure spring is at rest
• calculate extension (length - original)
• plot extension against weight graph
what is moment and unit for moment
the turning/rotating effect of a force
Nm
why does force applied have to be perpendicular to the pivot when caluclating moment?
distance will decrease
less distance = more force, because
moment = f x d
how do you balance an object on a pivot
make anticlockwise moment = clockwise moment
how do leavers make it easier to do work
they increase the distance from the pivot, so less force is needed. they are force multipliers
how do gears make it easier to do work
larger gears have a larger radius, meaning larger distance to pivot, meaning moment is greatrr
why do fluids create pressure
the particles can flow. they collide with walls of container at right angles, meaning they exert a force, as pressure is N/m^2
unit for pressure
pascal (Pa)
how does density and pressure change as you get higher in atmosphere
density decreases as there are less aur molecules above
pressure decreases as particles no longer collide with earth’s surface
how does pressure with depth and density change in a container of water
as depth and density increases, there are more water particles above, meaning there is a greater weight acting downwards, so pressure increases
why dies upthrust occur in water
pressure increases with depth, so
when an object is submerged, the bottom of the object has a larger pressure than the top.
this means the force acting on the bottom is larger than the top, meaning there is a resultant force upwards, upthrust.
what happens for something to float
if weight = upthrust
upthrust = weight of water displaced:
less dense object means it weighs less than water displaced
so object does not have to fully submerge for water displaced to equal weight
what are some typical daily speeds
walking 1.5m/s
running 3
cycling 6
car 25
train 50
plane 250
sounds 330
what is speed and velocity like when car drives in circle
speed is constant
velocity is constantly changing as direction changes
straight uphill line on distance time graph vs velocity time graph
DT: travelling at constant speed
VT: accelerating at constant speed
flat line on distance time graph vs velocity time graph
DT: stationary
VT: travelling at constant speed
curved line on distance time graph vs velocity time graph
DT: accelerating/decelerating
VT: increasing acceleration/deceleratio
how to find distance travelled (displacement) on velocity time graph
find area under graph
if curved, count squares
when should you use each type of acceleration equation
change in velocity/time
when an object accelerates
v^2 - u^s = 2as
(final - initial = 2 x accel x distance)
when an object accelerates at a constant/uniform rate
what happens when an object falls through a fluid, as example of skydiver in air
• when skydiver falls, weight is only acting force, due to gravity.
resultant force and acceleration: down
• as they fall, fricton with air molecules, air resistance, acts upwards.
weight > air resistance
resultant force and acceleration: down
• increase in velocity means increase in air resistance, until
air resistance = weight
resultant force: none
this means velocity stays constant. this is called terminal velocity
• parachute opens. surface area has increased, meaning so has air resistance
air resistance > weight
resultant force: upwards, skydiver decelerates
• decrease in velocity means decrease in air resistance, until
air resistance = weight
resultant force: none
this means velocity stays constant. skydiver is at lower terminal velocity.
factors that effect terminal velocity of an object
shape and area
larger objects have greater area for force of friction to act, with same amount of gravitational force as smaller object
what is drag
resistance you get in a fluid
e.g air resistance, friction
what is newtons first law of motion
if the resultant force acting on a stationary object is zero, it will remain stationary
if the resultant force acting on a moving object is zero, it will remain moving at the same velocity
also known as inertia
what do forces look like when something moves at constant velocity
driving force = drag force
what is newtons second law
F = ma
acceleration of an object is:
* proportional to the resultant force
(larger force = larger acceleration)
* inversely proportional to the mass of the object
(larger mass = smaller acceleration)
give speed, acceleration and force of family car going from road to motorway
15m/s to 30m/s
requires 2m/s^2 acceleration
require 2000N force
what is inertial mass and how do you find it
measure of how difficult it is to change velocity of an object
larger inertial mass = larger force for given acceleration
it is the ratio of force / acceleration
what is newtons third law
when two objects interact, the forces they exert on each other are equal and opposite
give example of caneo of newtons third law of motion
man paddling in canoe
paddle pushes on water, water pushes back on paddle. the forces are equal in magnitude, but opposite in direction
what does the velocity time graph look like for a skydiver
investigating acceleration practical
• attach trolley of known mass to a string that loops over a pulley and place 100g mass on other end, which will provide the force. place a light gate halfway, connected to data logger
• place piece of card with gap in middle on the trolley. this will be detected by the light gate twice
• as mass will accelerate trolley, string, and masses, so the acceleration of the system is being investigated
• draw a starting line and place front wheels just before it
• release trolley
• record measurements from light gate through data logger
• repeat twice to get average acceleration of the whole system
investigate the effect of mass on acceleration practical
keep force at end of string constant, but attach mass to trolley to increase the mass of the system
mass has increased, force stays the same
how does changing the mass in the acceleration practical prove newtons 2nd law
mass of system has increased, force remains the same
F = ma
so acceleration should decrease as
a = F/m
how to investigate effect of force on acceleration practical
• start with three masses on the trolley and record acceleration
• move one mass from trolley to end of string and record
• mass of the system still remains the same, as everything is attached, however force is changing
• repeat, adding 1 mass
how does changing the force of the acceleration practical prove newtons second law
accelerating force is increasing, mass f system stays the same
F = ma
acceleration should increase
what is stopping distance with formula
distance it takes to perform emergency stop after seeing obstruction
thinking distance + braking distance
what is thinking distance and what is it affected by
distance travelled during reaction time
• speed
• reaction time - tiredness, drugs, alcohol, distractions
what is braking distance and what is it affected by
distance travelled after braking force
• speed
• weather
• road surface
• tyre conditions
• brake conditions
example of stopping distance
typical family car at 30mph:
thinking: 9m
braking: 14m
how to measure someones reaction time
• A rests hand on fixed surface
• B holds ruler so 0 hangs between thumb and forefinger
• B drops without warning, A catches quick as possible
• record measurement on ruler
• longer measurement = longer reaction time. research table to find reaction time
relationship between speed and thinking distance versus speed and braking distance and why
speed & thinking: linear
speed & braking: exponential because:
kinetic energy = 1/2mass x velocity^2
meaning if velocity doubles, kinetic energy quadruples
why is it dangerous for a car to decelerate
brake presses against wheel -> friction -> thermal energy
high speed requires large braking force, overheating
what is momentum measured in and what affects it
kg m/s
mass and velocity
what is the momentum of a stationary object
no momentum
what is conservation of momentum
in a closed system, momentum before an even = momentum after event
in terms of momentum, what happens when a moving car hits a parked car
both move at same momentum as initial momentum of moving car.
momentum is conserved and mass has increased, meaning they move in same direction at lower velocity
how can you use conservation of momentum to calculate velocity or mass
momentum before = momentum after
find total momentum
so do calculations on each side 🤷♀️ (practice questions)
relationship between force and momentum
force increases velocity
larger force = larger change in momentum
how to reduce danger of change in momentum when stopping in a car
• crumple zone
• seat belt
• air bag
• helmet
• crash mat
all makes change of momentum slower, meaning smaller force
