Topic 2 Mechanics Flashcards
scalars
no direction only size
- mass
- energy
- temperature
- distance
- power
- speed
- time
vectors
both direction and size
- pressure
- force
- acceleration
- magnetic/electric/gravitational field strength
- displacement
- momentum
- velocity
distance definition
how far you have travelled (m)
displacement definition
the distance revelled in a particular direction (m)
speed definiiton
the rate of change of distance (distance/t)
velocity definition
the rate of change of displacement (displacement/t)
instantaneous speed definition
the rate of change of distance at one particular time (ms^-1)
average speed definition
the speed over a period of time (ms^-1)
acceleration definition
rate of change of velocity (v/t) ms^-2
when should you use equations of motion eg. v=u+at
if an object is accelerating at a constant rate ie. undergoing uniform acceleration
gradient of displacement-time graph represents
velocity
gradient of velocity-time graph represents
acceleration
area below line of velocity-time graph represents
displacement
area below line of acceleration-time graph represents
change in velocity (*still speeding up but the increase of velocity is getting less)
area below line of acceleration-time graph represents
change in velocity (*still speeding up but the increase of velocity is getting less)
area above x-axis of displacement graph represents
displacement in a positive direction, eg to the right of the origin
area below x-axis of displacement graph represents
displacement in a negative direction, eg. to the left of the origin
area above x-axis of velocity graph represents
motion in a positive direction (regardless of position)
area below x-axis of velocity graph represents
motion in a negative direction
area above x-axis of acceleration graph represents
acceleration in a positive direction (shows the direction of the net force)
area below x-axis of velocity graph represents
acceleration in a negative direction
relative velocity definition
the apparent or relative velocity of an object depends on the situation of the observer (called their frame of reference)
projectile motion definition
a projectile is an object moving through the air under the influence of only one force - gravity
why does the trajectory (path) of a projectile motion follows a parabola?
- VERTICAL component of velocity is constantly CHANGING (due to the force of gravity)
- HORIZONTAL component of the velocity is CONSTANT (assuming no air resistance)
- at the top vertical velocity = 0
how does the trajectory of a projectile motion object change with air resistance?
- shorter range
- lower maximum height that is reacher earlier
- gradient steeper on way down
see pg 16 topic 2 book for picture
projectile problems: horizontally, eg fired of cliff
what is the initial vertical velocity and horizontal velocity
initial vertical velocity = 0
horizontal velocity = launch velocity, v
projectile problems: at an angle above the horizontal
what is the initial vertical velocity and horizontal velocity
initial vertical velocity = v sinθ
horizontal velocity = v cosθ
projectile problems: at an angle below the horizontal
what is the initial vertical velocity and horizontal velocity
initial vertical velocity = v sinθ
horizontal velocity = v cosθ
projectile motion:
vertical motion is considered when calculating ________________
horizontal motion is considered when calculating ________________
VERTICAL motion is considered when calculating maximum HEIGHT reached or TIME of flight
HORIZONTAL motion is considered when calculating RANGE
points to remember for projectile problems
- at the highest point vertical velocity is zero
- acceleration is always downwards at 9.81ms^–2
- horizontal velocity is constant
- range = horizontal velocity x time of flight
- problems may be solved using kinematic equations of conservation of energy
what equation is used for VERTICAL motion
(max height, time in air)
s = ut + 1/2 at^2
v^2 = u^2 + 2as
v = u + at
what does fluid resistance (drag) depend on
- the velocity of the object relative to the fluid
- the size and shape of the object
- the fluid used (its viscosity)
fluid resistance definition
when an object moves through a fluid (liquid or a gas), there will be a frictional fluid resistance (drag) acting against the object’s motion
free-fall definition
when an object falls through under the influence of gravity it will initially accelerate at 9.81ms^–2. in certain situations (eg. very dense objects falling), air resistance can be ignored and the object is said to be in free-fall.
terminal velocity definition
- In reality, air resistance will affect the motion of falling objects. The magnitude of the air resistance will increase as the object’s velocity increases.
- The object will continue to accelerate but at less than 9.81ms-2.
- Once the magnitude of the upwards air resistance is equal to the downwards gravitational force (weight), the object will no longer accelerate as the forces on the object are now balanced.
- Its speed will remain constant and is called terminal velocity.
On graph what will terminal velocity look like?
straight line near the end, see pg 19 topic 2 book
mass (m) definition
the amount of matter in an object (kg). it does not change with a change in position
weight (W) definition
the force of gravity actin got an object (N). it will change if an object changes its position
gravity causes every…..
object in the universe to attract every other object with a force because of gravity.
size of gravity depends on
- the mass of the objects (more mass, more force)
- the distance between the objects (more distance, less force)
on Earth the strength of gravity is 9.81 Nkg^–1
weight =
mass x gravity (W=mg)
force defintion
basically a push or a pull (N)
free body diagram definition
a drawing that shows all the forces acting on an object
free body diagram weight direction
always straight down
free body diagram normal reaction
When an object pushes down on a surface, the surface pushes back on the object. The normal reaction force is always at 90º to the surface the object is on
free body diagram thrust
A push force, e.g. from an engine
free body diagram friction
A force that is always in the opposite direction to any motion. If it is due to air then it is called air resistance
free body diagram buoyancy
A support force from a fluid e.g. water or air
free body diagram lift
The upwards push on the wings of an aircraft due to its motion through air
free body diagram tension
The pull force due to a rope or chain
is the velocity changing when forces are balanced
not changing
is the velocity changing when forces are balanced
not changing
if upwards forces = downwards forces, and left forces = right forces, then forces are balanced. object will remain at rest of continue to move in straight line at constant speed, net force = 0 and there is no acceleration
is the velocity changing when forces are unbalanced
changing
forces are greater in one direction than the other, forces are unbalanced and object will accelerate in the direction of the greater force. The acceleration is produced by the overall (or net) force on the object
Newton’s first law
“an object continues in uniform motion in a straight line or at rest unless a resultant external force acts”
which basically means that a resultant force causes acceleration, and no resultant force
means no acceleration. The acceleration is in the direction of the resultant force.
If the resultant force on an object is zero, it will not be accelerating and is then said to be
in translational equilibrium (moving in a straight line at a constant speed).
Newton’s second law
“the resultant force on an object is proportional to the acceleration providing the mass of the object remains constant”
F = ma
Newton’s third law
“for every action on one object there is an equal but opposite reaction on another object”
eg. I am attracted towards the Earth by gravity: the earth is attracted towards me by gravity
eg. two cars collide and both exert the same size force on each other
normal reaction force’s impact on how heavy we feel eg. lift
- stationary or moving at constant velocity, r=w=mg, feel usual weight
- accelerating upwards, r–w = ma r = w+ma = m(g+a), feel heavier
- accelerating downwards, w–r=ma r= w–ma = mg–ma r = m(g–a), feel lighter
- cable cut and falling, w–r = mg r = w–mg = mg–mg r=0, feel weightless
resolving forces definition
break down a force into two components see pg 29 topic 2 book for example
resultant forces
the sum of all the forces acting on an object
F = √F1^2 + F2^2
θ = arctan(F1/F2)
resultant forces in equilibrium
all forces acting on it will add up to zero so Fnet = 0
If Fnet = 0, then a closed triangle can be drawn
pg 30 topic 2 book
object on incline, weight force’s two forces
- parallel to the slope (mg sinθ)
(= friction if speed constant) - perpendicular to the slope (mg cosθ)
(= normal reaction)
static friction definition
when a force is applied to an object resting on a surface, but the force is not large enough to move the object
occurs when there is no relative motion between the surfaces
(cannot be negative value)
dynamic friction definition
once the object exceeds the maximum value of static friction force Ff the object will begin to slide.
occurs when there is relative motion between the surfaces
coefficients of friction: bigger number, _____________ force required ie. ____________ _________ friction
bigger number, bigger force required ie. more static friction
measuring coefficient of static friction on slope:
Ff (friction force) =
R =
Ff = mg sinθ
R = mg cosθ
energy definition
“the capacity to do work”
means you need energy for things to happen or for work to occur
Hooke’s law
the extension of a spring is proportional to the applied force
F = KΔX
K = spring constant (Nm^–1)
ΔX = extension or compression (m)
conservation of energy
energy can be converted from one type to another. it cannot be created or destroyed.
work (J) definition
work is said to be done when a force (F) moves an object through a displacement (s). Work is therefore a transfer of energy
formulas for work
work done (J) = force (N) x displacement (m)
W(J) = F(N) s(m) cosθ
how to work out work done from graph
if the force being used is not constant then the work done can be calculated by either using the average force value or by calculating the area below a force-displacement graph
how to work out work done where an object is being lifted
the force required to life the object is equal to the object’s weight
power definition
the rate which work is done or energy is transferred (W or Js^–1)
efficiency definiton
a ratio of how much work, energy or power we get out of a system compared to how much is put in
linear momentum definiton
the product of mass and velocity
what is the conservation of momentum
“the total linear momentum of a system remains constant provided no resultant external force acts (eg. friction)”
elastic collision
no kinetic energy is lose during the collision. in reality it only occurs between sub-atomic particles
inelastic collision
kinetic energy is lost
impulse definition
the change in momentum (Δp or FΔt)
units: kgms^–1 or Ns
impulse = FΔt = Δp
*if F is changing, use the average force
how to find impulse on a _____-_____ graph
on a force-time graph, impulse is the area below the graph
explain explosions
- an explosion can be treated as a collision. typically a stationary system will break apart into two pieces.
- as with collisions, total momentum is conserved and will be equal to zero
- kinetic energy will always increase in an explosion (as it was 0 before)
- the kinetic energy comes from the energy source of the explosion
- see pg 48 topic 2 booklet for example
example of explosions
- recoil of gun
- ice skaters
- “zero-g” situations
