M3 Newton's Laws and Momentum Flashcards
how are acceleration and force related
acceleration is directly proportional to force
the bigger the force, the greater the acceleration
how do we represent force
use an arrow
the direction of the arrow shows the direction of the net force
the size of the arrow shows the magnitude of the force
define newton
the force that will give a 1kg mass an acceleration of 1 ms^-2
units = kgms^-2
what’s the equation for weight
mass X gravitational field strength
for objects of different masses what do they have the same?
acceleration
what is the difference between mass and weight
mass is the same everywhere
weight depends on the gravitational field strength
would a moon buggy be easier to push on the moon? would it be easier to lift?
pushing it would be the same because the mass and acceleration are the same and F = ma so the force would be the same regardless of where you were
lifting would be easier of the moon because g is smaller on the moon and W = mg so the weight would be smaller hence being easier to lift
draw a velocity-time graph for someone in free fall
increases, reaches terminal velocity, parachute released, decreases, terminal velocity again
What does the magnitude of drag depend on
Speed of object
Cross sectional area of object
Roughness or texture of object
Density of fluid the object is travelling through
What is the special theory of relativity
As you approach the speed of light, mass is no longer constant
So F=ma doesn’t apply
Define work
Force applied X distance moved in the direction of the force
Define joule
The energy transferred when a force of 1N is applied over a distance of 1m
What is the equation for work done when there’s an angle and when there isn’t an angle
When there’s an angle W = Fcos(x)s
No angle W = Fs
Is work done a scalar or vector
What are its units
Scalar
J or Nm
Derive the equation for kinetic energy
Ek = Fs
F = ma and s = 0.5(u+v)t and a = (v-u)/t
So Ek = m((v-u)/t) X 0.5(u+v)t = 0.5m(v-u)(v+u) =0.5m(v^2 - u^2) But at the initial velocity there is no kinetic energy so it doesn't need to be there so Ek=0.5mv^2
What is the conservation of energy
Energy cannot be created or destroyed
It can be transferred from one form to another, however the total energy will remain constant (in a closed system)
Derive the equation for gravitational potential energy
Ep = Fs
S is the height
F = mg
Ep = mgh
what is the elastic potential energy of something
work done by the deforming force
derive an equation for elastic potential energy
PE = average force X extension
Hooke’s law: F = kx
Average force = 0.5kx
PE = 0.5kx X x
= 0.5kx^2
in a free fall with no air resistance what happens to the kinetic energy and the potential energy at the start and end
START
Ek = 0
E = Ep + Ek
= mgh + 0
END Ep = 0 Ek = 0.5mv^2 apply v^2 = u^2 + 2as u = 0 v^2 = 2as Ek = 0.5m(2as) =0.5m(2ah) a=g Ek = 0.5m(2gh) = mgh
so Ep at start = Ek at end
what is the equation for
Ep?
Ek?
Ep = mgh Ek = 0.5mv^2
A pendulum has a mass of 5kg and is held at a height of 0.15m
how fast will it move at its lowest point
GPE loss = mgh = 5 X 9.81 X 0.15 = 7.36J At the bottom the energy is transferred to kinetic energy so Ek = 7.36 = 0.5mv^2 0.5 X 5X v^2 = 7.36 v = 1.7ms^-1
OR
mgh = 0.5mv^2 gh = 0.5v^2 v = square root of 2gh = square root of 2 X 9.81 X 0.15 = 1.7 ms^-1
if there is friction what does Ep equal
Ep = Ek + Wf
Wf is friction which is the force X displacement
define power
the rate at which work is done
what are the base units of power
Js^-1
are these scalars or vectors:
power, work done and time
all scalars
what is the equation for efficiency in terms of power
power output / power input X 100
what is power when the force and motion are in different directions
P = Fvcosx
what are the equations for kinetic energy
Ek = 0.5mv^2
Ek = mas
Ek = Ep - Wf
what do sankey diagrams represent
what are the features of them
represent the efficiency of an appliance
useful energy goes to the right and wasted downwards
the size of the arrow represents the fraction of energy transformed
what is newton’s first law of motion
a body will stay still or move in a straight line at a constant speed unless there is a resultant force acting on it
means when there is constant velocity, there is a zero resultant force acting on you
what is Newton’s second law of motion
(Resultant) force is (directly) proportional or equal to the rate of change of momentum
F = ma or F = change in p / change in t
define stopping distance
what is the equation
distance travelled in the time between a driver first spotting an obstacle and vehicle coming to a complete stop
thinking distance + braking distance
what is the equation for thinking distance in terms of SUVAT
s = ut
define thinking distance
what factors affect it
the distance travelled during the driver’s reaction time
age, tiredness, alcohol, drugs, illness, speed
define braking distance
what factors affect it
the distance travelled after the brakes are applied until the car comes to rest
road condition, tyre condition, speed, brake condition, mass of car
what is the equation for braking distance in terms of SUVAT
s = -(u^2) / 2a
acceleration will be negative also as the car will be decelerating so the displacement overall will be positive
derive F = p/t into F = ma
change in p = mv - mu F = mv - mu / t F = m(v-u) / t mass is constant so F = m (v-u)/t (v-u)/t = a F = ma
what is Newton’s third law of motion
When body A exerts a force on body B, body B exerts a force on body A that is equal, opposite in direction and of the same type
how would you describe an object with 300N going to the right and 300N going in the opposite direction
balanced
net force = 0N
how would you describe an object with 400N going to the right and 300N going to the left
unbalanced
object will move the right
400-300 = 100
resultant force in 100N to the right
a car which is accelerating then changes forces so 400N goes behind and 300N in front
what happens
decelerates
300-400 = -100
resultant force is -100N to the left
define resultant force
sum of all the forces acting on an object, taking relative directions into considerations
what is momentum
what does a higher momentum mean
the stop ability of an object
the more momentum an object has, the harder it is to stop
what does p=mv mean
what are the units for momentum
momentum = mass X velocity
kgms^-1
is momentum a vector or scalar quantity
vector
what is the equation for change in momentum
p = mv - mu
how can you relate the equation for momentum and newton’s 2nd law
F = p / t Ft = p Ft = mv - mu
A ball hits a wall at 4ms^-1 and bounces back at 3.5ms^1. the mass of the ball is 2kg, what is the change in momentum?
p = mv - mu
p = (2 X 4) - (2 X -3.5)
= 8 - - 7
= 15 kgms^-1
what is impulse
the product of the force and the time interval over which the forces act
what is the equation for impulse
F X change in t
relate impulse to Newton’s 2nd law
F = ma = m(v-u)/t =mv-mu / t Ft = mv - mu Ft = p so impulse = change in momentum
on a force-time graph, what represents the impulse
area under the graph
is impulse a vector or scalar quantity
vector
how would you reduce the force in a collision
increase the time
seat belts
crumple zones
what is the conservation of momentum
total linear momentum of two objects before they collide equals the total linear momentum after the collision
apply the conservation of momentum to an air rifle recoiling
the forward momentum gained by the pellet is equal in magnitude to the backward momentum of the rifle
what is an elastic collision
where momentum is conserved and kinetic energy is conserved
so no energy is dissipated into other forms
what does it mean if a collision is inelastic
some of the kinetic energy is converted into other forms during the collision eg. sound or heat
(most collisions are inelastic)
what are the steps for 2D momentum
A2
Step 1: Work out the initial horizontal momentum of ball A.
Step 2: Work out the horizontal and vertical components of A after the collision
Step 3: Use the horizontal component to work out the horizontal velocity of B after the collision
Step 4: Use the vertical component to work out the vertical velocity of B after the collision.
Step 5: Use Pythagoras to find the magnitude of the velocity and calculate the angle below the horizontal.
what is the conservation of momentum in terms of 2D momentum (A2)
the horizontal momentum before a collision = the horizontal momentum after a collision
the verticle momentum before a collision = the verticle momentum after a collision
describe how forces acting on a parachutist change as he falls
the skydiver leaves the plane and accelerates until the -air resistance and his weight equal
- he’s traveling at terminal velocity but this is too fast to land
- the parachute is opened which increases the air resistance making it larger than weight
- he slows down until air resistance equals weight again, reaching terminal velocity again but at a safer level to land
define terminal velocity
this occurs when the frictional forces and the driving force of an object equal
