Module 3 Forces and motion Flashcards
what is force used to indicate?
push or pull
what can resultant force acting on a object do it?
it can accelerate the object in a specific direction
what is displacement?
- the distance travelled in a particular direction- it is a vector with a magnitude and direction
what is speed? add units
how fast something is moving regardless of direction (ms-1)
what is instantaneous speed?
the speed at the moment it is measured- (speed over an infinitesimal interval of time)
what is the average speed?
the rate of change in distance calculated over a complete journey
what is velocity?
a vector quantity equal to the rate of change of displacement
what is acceleration?
the rate of change in velocity ( a vector quantity)
what is the eqaution for average speed?
average speed= distance travelled/ time taken
on a distance time graph on which axis is time on?
time is plotted on the x-axis
on a distance time graph which axis is distance plotted on?
the y-axis
how is a stationary object represented on a distance time graph?
the line is horizontal
on a distance time graph how is a object that is moving constant speed represented?
the line is straight and sloping
on a distance time graph what does the gradient equal?
the speed of the object
how can you find the instaneous speed on a distance time graph? (normally for a curved line on a distance time graph)
- draw a tangent to the distance time graph at the time
- use that tangent to determine the gradient
- the greater the gradient the greater the instaneous speed
answer these summary questions on distance and speed
-answers
what is the eqaution for average velocity? give the unit
average velocity= change in displacemnt/ time taken ms-1
what is a displacement- time graph used to determine?
the motion of an object
on a displacement time graph on which axis would displacement be on?
the y-axis
in a displacement time graph on which axis would time be on?
x-axis
how can you work out the velocity of an object from a distance - time graph? (of a straight line)
you work out the gradient (displacement/ time=velocity)
how do you work out the instaneous velocity of an object from a dispacement time graph? (from a curved line)
- draw a tangent from a certain point
- work out the gradient of that tangent
- this will give you the instantaneous velocity
answer these summary questions on displacement and velocity
ANSWERS
what is the eqaution for acceleration? give the unit
acceleration= change in rate of velocity/ change in time
ms-2
how do you work out the acceleration from a velocity time graph? (straight line)
-gradient of the straight line (change in velocity/ change in time)
how do you work out the acceleration from a velocity time graph (of a curved line)?
- choose a point (instantaneous acceleration) and draw a tangent from that point
- gradient= velocity/ time
- gradient= acceleration
how can you work out the displacement from the a velocity time graph?
- work out the area under the graph
- you can do this by dividing part under the grap into triangles and rectangles then work out the area of each shape and them all together.
look at the graphs 1,2,3,4 what can you say about the aceleration and/ or velocity
graph 1= a straight line of constant positive gradient: constant accceleration
graph 2= a straight line of zero gradient: constant velocity or zero accleration
Graph 3: a straight line of constant negative gradient: constant deceleration
Graph 4: a curve with chnaging graduent: acceleration is changing
answer these summary questions on acceleration
answers
in suvat what does each letter stand for?
s= displacment/distance
u= intial velocity
v= initial velcocity
a= acceleration
t= time taken for the change in velocity
when do you use suvat?
when there is uniform acceleration
what is uniform acceleration?
constant acceleration
what is the equation for final velocity when the accleration is uniform? (does not involve displacment s)
v=u + at
what is the equation for displacment when the accleration is uniform? (does not involve acceleration a)
s= 1/2 (v+u)t
or
s= (u + t)/ 2 x t
what is the equation for final velocity when the accleration is uniform? (does not involve time t)
v2= u2 + 2as
what is the equation for displacement when the accleration is uniform? (does not involve final velocity v)
s= ut + 1/2at2
1what is the equation for displacement when the accleration is uniform? (does not involve acceleration a)
s= vt - 1/2 at2
answer the summary questions on eqautions of motion
answers
when is acceleration = -9.81?
whent the direction is upwards or the direction of motion is against gravity
what is free fall?
the motion of an object undergoing an acceleration of ‘g’
what is the only force acting on an object in free fall?
the object’s weight
what procedures could you use to determine the acceleration of free fall, you will need to include;
- diagram of the apparatus
- method
- measurements you will make
- explain how ‘g’ is calculated
- be aware of sources of error
- experiment to measure the acceleration due to gravity
method: 1.) measure the height h from the bottom of the ball bearing to the trapdoor.
2. ) Flick the switch to simultaneously start the timer and disconnect the electromagnet releasing the ball bearing.
3. ) the ball bearing falls, knocking the trapdoor down and breaking the circuit- which stops the timer. - to calculate g use the equation h=1/2gt2
- errors that could occur; when measuring height you have an uncertainty of about 1mm, which can be resolved by using a set sqaure to make sure your eye is level with the ruler
what other eqautions can you use for objects in free fall (constant acceleration)? this is where acceleration is replaced with gravity
v=gt
v2=2gs
s=1/2gt2
s=vt/2
what is thinking distance?
the distance travelled between the moment when you first see a reason to stop, to the moment when you use the brake
what is braking distance?
the distance travelled from the time the brake is applied until the vehicle stops
what is the eqaution for finding stopping distance?
thinking distance + braking distance= stopping distance
what is the eqaution for thinking distance?
thinking distance= speed x reaction time
what factors affect reaction time?
- tiredness
- alcohol or other drug use
- illness
- distractions
what is braking force reduced by?
-reduced friction between the brakes and the wheels (worn of badly adjusted brakes)
-
what is friction reduced by?
wet, icy roads and worn out tyre treads
in projectile motion what unit stays the same
time
how do you work out the velocity of a projectile?
you use suvat
what eqaution do you use for the horizontal velocity of a projectile?
speed= distance/time
how to resolve a velocity of a projectile motion
you split the component with the resultant velocity in the diagonal and use
Vh/v= cos (pheta) for the horizontal
Vv/v= sin (pheta) for the vertical
how do you convert degrees to radians?
multiply by pie/180 degrees
or reverse
multiply by 180/pie
answer these sumary questions on projectile motions
what is the eqaution for net force from force and mass
net force= mass x acceleration
define the newton
the SI unit of force. It is equal to the force that would give a mass of one kilogram an acceleration of one metre per second per second
what is the weight of an object?
weight= mass x gravity
answer these summary questions on force, mass and weight
answer these summary questions of the centre of gravity
- the centre of mass will be at the 50cm mark (the middle) only if the ruler is uniform in both chape and density of the material. This may not have been the case
what is upthrust?
an upward buoyancy force acting on an object when it is in a fluid
what is tension?
the pulling force exterted by a string, cable or chain on an object
what is normal contat force (or reaction force)?
A force arising when one object rests against another object
what is drag?
The resistive force on an object travelling through a fluid (e.g aur and water)
(the same as friction)
what is friction?
the force that arises when two surfces rub against each other
as speed increases _______ friction increases.
as speed increases fluid friction increases.
what are the two types of friction?
- contact friction
- fluid friction (drag)
what energy transfer happens when there are frictional forces at work?
they convert kinectic energy into heat
how can forces be represented?
in a free body diagram;
where each force is represented by an arrow labelled with the force it represents
answer these summary questions on free body diagrams
- a two arrows coming off the middle of the ball; longer one going upwards (labelled drag), the other shorter one going downwards (labelled weight)
b. two arrows coming off the middle of the boat; longer one going upwards (labelled upthrust), the other shorter one going downwards (labelled weight)
when does a car reach terminal velocity?
-when the resistance forces are equal to the driving force so that there is no resultant force and no acceleration
as the velocity of a car increases why does the acceleration decrease?
because as the velocity increases, the resistant forces increase which reduces the resultant force which inturn reduces its acceleration
what would a velocity time graph showing terminal velocity look like?
-
what would an acceleration time graph showing terminal velocity look like?
-
explain why we can not use the eqaution v=u + at to predict the velocity of the car at a later time even when the forward force is constant
because the drag force may have changed (increased or decreased) affecting the acceleration of the car
how does a parachute work?
by lowering the terminal velocity of the object/person
a sky diver jumps from a stationary hot air ballon several kilometers above the ground
in terms of acceleration and forces, explain the motion of the skydiver;
immediately after jumping
at the time before terminal velocity is reached
at terminal velocity
immediately after jumping: the only force acting on the sky diver is gravity and his acceleration= 9.81, there is no air resistance
at the time before terminal velocity is reached: the air resistance has increased since the skydiver hits more air particles as speed increases. the sky diver accelerates at a slower rate
at terminal velocity= the air resistance equals his weight so he is no longer accelerating
how is the terminal velocity affected by a sky diver of a heavier size compared to one who is lighter but the same shape?
-the heavier skydiver will reach a higher terminal velocity at a later whlist the lighter one will reach a lower one at a quicker rate
how can you measure the terminal velocity of a ball bearing?
- equipment
- method
- errors
- fill a measuring cylinder with viscous liquid
- mark the position of a cylinder (towards its bottom end)
- meaure the seperation between these points with a metre rule
- gently drop a ball into the fluid. start the timer when it passes the upper mark and then stop the timer as it passes the lower mark. record this time of fal repeat the step above and record 2 or mor time of fall
- calulate the average time of fall
- the terminal velocity is determined by dividing the distance between the two marks by the average time of fall
why do heavier objects reach a faster terminal velocity?
because a greater drag force is needed to balance the extra weight
answer these questions on terminal velocity
what is meant by the moment of a force?
the turning effect of a force about some axis or point
what is the eqaution for moments?
moment = force x perpendicular distance from pivot (in m)
AKA
M= Fx
(big M= moment, F= force x= distance)
what is the principle of moments?
when a body is in rotational equilibrium, the sum of the anti-clockwise moment is equal to the sum of the clockwise moment.
what to look out for in moments
-make sure that when it says a the object is in equilibrium that you look for forces that may not be shown; because if there is a force acting downwards on the object then there must being exerted on the force to balance it out.
some forces act on the objects at an angle so you have to find the perpendicular distance by using pythagoras.
answer these summary questions on moments
what is a couple?
a coupe is a pair of forces of eqaul size which act parallel to each other, but in opposite direction
do couples cause any resultant force? and what do they produce?
a couple does not cause any resultant linear force, but dows produce a turning force (usually called a torque rather than a moment)
what is the eqaution for torque?
torque of a couple (Nm)= size of one of the forces (N) x perpendicular distance between the forces (m)
AKA
T= F x d
answer these summary questions on torque
what are coplanar forces?
coplanar forces are forces that lie in the same plane
answer these summary quesions triangle force
what is the density of a substance?
its mass per unit volume
what is the eqatuion for density and the unit?
(slanty) p= m/V
(m= mass and V=volume)
units for density is kgm-3
answer these summary questions on density
what is pressure?
what unit is it measured in?
pressure is the amount of force applied per unit area
measured in pascals (Pa)
also the same as Nm-2
what is the eqaution for pressure?
pressure= force/area
P=F/A
you can also have
pressure= object in fluid x density x the acceleration due to gravity
p= hpg
what is upthrust?
upthrust is an upward force that fluid exerts on objects that are completely or partially submerged in the fluid
answer these summary questions on pressure
answers
what is Archimedes’ principle?
archimedes’ priciple says that when body is completely or partially immersed in a fluid it experiences an upthrust equal to the weight of the fluid it has displaced
upthrust = weight of fluid displace
answer these summary questions on archimedes’ principle
answers
define the term work
when a force of 1J is moves it’s point of application 1m in the direction of the force
what is the eqaution for work done?
work done = force x distance (moved in the direction of the force)
what can work done be equal to?
work done= energy transferred
what eqaution can you use for work done at an angle to motion?
W= fx cosθ
answer these summary questions on work done
answers
what is power?
the rate of work done
it is the amount of energy transformed from one form to another per second
what eqaution can you use to work out the power from work done? include units
Power = Work done/ time
P= W/t
Power (Watts/W) Workdone (Joules/J) time (seconds/s)
define the Watt
the rate of energy transfer equal to 1 Joule per second
what eqaution can you use to work out power from velocity? include units
Power= Force x velocity
P= Fv
Power (Watts/W) Force (Newtons/N) Velocity(meters per second)
what is the eqaution used when you have to work out the power from velocity and the force and motion are in different direction?
P= Fvcosθ
answser these exam questions on work and power
answers
what is energy?
the capacity for doing work
what is the principle of the conservation of energy?
the principle of the conservation of energy states that the total energy of a close system remains constant
energy can never be created or destroyed, but it can be transferred from one form to another
give a description and example of kinectic energy
energy due to motion of an object with mass
e.g moving cars q
give a description and example of gravitational potential energy
energy of an object due to its position in a gavitational field
e.g water held in clouds
give a description and example of chemical energy
energy contained within the chemical bonds between atoms, it can be released when the atoms are rearranged
e.g energy stored in petrol and release when it is burnt
give a description and example of elastic potential energy
energy of electrical charges due to their position in an electric field
e.g electrical charges on a thundercloud
give a description and example of nuclear energy
energy within the nuclei of atoms, it can be released when the particle within the nucleus are rearranged
e.g energy from nuclear fission reactors
give a description and example of radiant (or electromagnetic energy)
energy associated with all electromagnetic waves, stored within the oscillating electric and magnetic fields
e.g energy from an LED
give a description and example of sound energy
energy of mechanical waves due to the movement of atoms
e.g energy emitted when you clap
give a description and example of internal (heat or thermal) energy
the sum of the random potential and kinetic energies of atoms in a system
e.g a hot cup of tea has more thermal energy than a cold oneq
what is the transfer of energy eqaul to?
transfer of energy is equal to work done
what is the eqaution for kinetic energy?
kinetic energy= 1/2 x mass x velocity2
Ke=1/2mv2
for objects that are travelling at the same speed, what is kinetic energy proportional to?
kinetic energy is proportional to the mass
finish of the sentence: GPE is gained when an object gets ________, and is lost when an object gets _________.
GPE is gained when an object gets higher, and is lost when an object gets lower
what is the eqaution for GPE?
GPE= mass x gravitational field x height
Ep=mgh
from the principle GPE=KE (mgh=1/2mv2)
show the eqaution for velocity when there are no resistive forces involved
V2=2gh
from the principle GPE=KE (mgh=1/2mv2)
show the eqaution for height when there are no resistive forces involved
h= v2/2g
state the eqaution for efficiency
efficiency= useful output energ/ total input energy x 100
answer these exam questions on conservation of energy and efficiency
answers
what do you call forces that produce extension?
tensile forces
what do you call forces that shorten an object (compression)?
compressive forces
what does hooke’s law state?
show the equation for hooke’s law
the extension of the spring is directly proportional to the force applied.
F=kx
(where k is a constant/stiffness constant that depends on the material being stretched and has units of Nm-1)
describe what is happening on this graph
- the forve-extention graph is a straight line from the origin up to the elastic limit (Point A) of the spring.
- In this linear region, the spring undergoes elastic deformation. This means that the spring will return to its orginal lenght when the force is removed.
- Beyond point A, the spring undergoes plastic deformation: permanent structural changes to the spromg occur and it does not return to its original lenght when the force is removed.
if a spring has a large constant does it mean it is easy or difficult to extend?
difficult to extend
show a eqaution where the force constants are combined in a series
1/k= 1/k1 + 1/k1
show an eqaution where the force constant is combined in springs that are arranged parallel to each other
k= k1 + k2
explain what happens to the atoms in a material that is at elastic deformation
- when the material is put under tension, the atoms of the material are pulled apart from one another
- the atoms can slightly relative to their equillibrium positions, without changing position in the material
- once the load is removed, the atoms return to their equillibrium distancr apart
explain what happens to the atoms in a material that is at plastic deformation
- some atoms in the materiak move position relative to one another
- when the load is removed, the atoms don’t return to their original positions
what does the gradient equal to in a force extention graph
k (stiffness constant)
describe an experiment in which you can investigate hooke’s law
how could you improve accuracy?
- attach the spring at one end using a clamb, boss and clamp stand secured to the bench using a G- clamp or large mass.
- set up a metre rule with a resolution of 1mm close to the spring.
- suspend slotted masses from the spring and, as you add each one, record the total mass added and the new lenght of the spring.
you could improve accuracy by:
- taking readings at eye level to reduce parallax errors
- measure the mass of each slotted mass using a digital balance
- to obtain reliable results, aim to take at least six different readings
answer these exam questions on hooke’s law
answers
answer these summary question on hooke’s law
answers
why is that when the material has gone through plastic deformation, not all of the worked done can not be fully recovered?
when the material has gone through plastic deformaton, the some of the work done on the material has gone into moving its atoms to new permanent positions. This energy is not recoverable.
what is the area under a force extention graph equal to?
the work done
what are the two equations you can use to work out the elastic potential energy
E= 1/2Fx
OR
E=1/2kx2
answer these summary questions on elastic potential energy
answers
what is tensile stress defined as?
what is eqaution for tensile stress?
what are the units?
the force applied per unit cross-sectional area of the wire.
tensile stress= force/ cross-sectional area
σ= F/A
units: Pa or Nm-2
what is tensile strain defined as?
what is the eqation for tensile strain?
tensile strain is defined as the fractional change in the orginal lenght of the wire
tensile strain= extension/original length
ε= x/L
(no units, its just a number since its a ratio of two lenght and may be written as a percentage)
what is a ductile material?
a ductile material can be easily drawn into a wire or hammered into thin sheets.
describe what is happening in this graph
- in this graph, the stress is directly proportionl to the strain from the origin to P, the limit of propotionality. The material obeys hooke’s law in this linear region.
- E represents the elastic limit. Elastic deformation occurs up to the elastic and plastic deformation beyond it
- Y1 and Y2 are upper and lower yield points, where the material extends rapidly
- the stress at the point labeled UTS represents the material’s ultimate tensile strenght which is the maximum stress that a material can withstand when being stretched before it breaks. (Beyond this point, the material may become longer and thinner at its weakest point, a process called necking)
- the material eventually snaps at its breaking point, labelled B. The stress value at the point of fracture is known as the breaking strenght of the material
what is the ultimate tensile strength?
this is the maximum stress that a material can withstand when being stretched before it breaks
what is necking?
a process where a material may become longer and thinner at its weakest point
what is breaking strength?
the stress value at the point of fracture
what does a strong material have?
a high ultimate tensile strength
what is meant by the Young’s Modulus of a material?
stress is directly proportional to strain
the ratio of stress to strain for a particular material is a constant which is known as Young’s Modulus
what is the eqaution for Young’s Modulus? give units
Young’s Modulus = tensile stress/ tensile strain
E= σ/ε
units for young modulus are Nm-2
describe an experiment you may do to find the Young Modulus?
Equipment: -
very long wire (the longer the thinner the wire, the more it extends for the same force which reduces uncertainty)
- clamp
- bench
- marker
- ruler with mm markings
- pulley
- weights
- balance
1. ) first you need to find the cross-sectional area of the wire. Use a mictometer to meausre the diamteter of the wire in several places and take an average of your measurements. you can use the formula area of circle= πr2
2. ) clamp the wire to the bench so weights can be hung of one end of it. Start with the smalled weight necessary to straighten the wire (dont include this weight in your final calculation)
3. ) measure the distance between the fixed end of the wire and the maker-this is your unstretched length. Then if you increase the weight, the wire stretches and the marker moves.
4. )increase the weight in steps, recording the reading each time (the extentions is the difference between this reading and the unstretched length. Use a balance to accurately find the weight you add at each step.
5. ) using the results from the experiment calculate the stress and strain on the wire and polt a stress-strain graph which should be a straight line if hooke’s law is obeyed
6. ) the gradient of the graph gives you the young modulus and the area under the graph gives you the elastic potential energy
how do you work out the Young Modulus from a stress strain graph?
you work out the gradient
(stress/strain= young modulus)
how do you work out the elastic potential energy from stress- strain graph?
you work out the area under the graph
how do you work out the energy per unit volume from a stress strain graph?
energy per unit volume= 1/2 σε
what does a stress-strain graph look like for a ductile material?
what does a stress strain graph look like for a brittle material?
what does a stress strain graph look like for polythene (a type polymeric material)
does the graph show that polythene behaves plastically or elastically?
polythene behaves plastically
what does a stress strain graph look like for rubber (a type polymeric material)
does the graph show that rubber behaves plastically or elastically?
rubber behaves elastically
answer these exam questions on young’s modulus
answers
answer these exam on stress strain graphs
answers
answer these summary questions on young modulus and stress strain graphs
answers
what is the eqaution for momentum? and unit
momentum= mass x velocity
kgms-1
equation for force from momentum
force= change in momentum/time
what is elastic collisions?
a collision between two bodies in which the total kinetic energy of the two bodies after the collision is eqaul to their total kinetic energy before the collision
what is inelastic collision?
a collision in which kinetic energy is not covered due to the action of internal friction
what is impulse?
the area under a force-time graph
the product of force and the time for which the force acts
how do you calculate impulse from a graph?
calculate the area under the graph
if two objects of equal mass collide what happens?
they move at 90 degrees from each other
what is newton’s first law of motion?
a body will remain at rest or continue to move with constant velocity unless acted upon by a resultant force
what is Newton’s 2nd law of motion?
the rate of change in momentum of an object is directly proportional to the resultant force and takes place in the direction of the force
what is momentum?
the tendancy of an object to keep moving in the same direction
what is Newton’s 3rd law of motion?
when two object interact, each exerts and equal but opposite force on the other during the intercation
how do you calculate te impulse of a graph?
impulse = force x change in time
what is the principle of conservation of momentum?
for a system of interacting objects, the total momentum in a specified direction remains constant as long as no external forces act on the system.
describe a case where there may be zero momentum
a gun recoils when a bullet is fired.
- the total momentum of this system remains the same and is equal to zero.
- the momentum of the gun and the momentum of the bullet have the same magnitude but in opposite direction
answer this worked question:
answer
answer these exam questions on momemtum and impulse
answer
answer these question on collisions in two dimensions
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