Mechanics key point Flashcards
(21 cards)
What happens to a particle’s speed and velocity just before it hits the ground in free fall or projectile motion?
Speed = Maximum
→ Because the object has been accelerating due to gravity the entire way down.
Velocity = Minimum
→ Because velocity is a vector (has direction), and just before impact, it’s at its largest negative value (pointing downward).
Explanation:
Speed is the magnitude of velocity — always positive.
Velocity includes direction. Since the motion is downward, velocity becomes more negative.
So, numerically, velocity reaches its minimum value (most negative), while speed is at its peak.
At maximum vertical height, which variable in SUVAT is 0
Velocity at t
What do r and r0 represent
r is position vector (displacement as a vector)
r0 is initial position vector (Initial displacement as a vector)
r = r0 + ut +1/2at2
USE THIS FOR CONSTANT ACCELERATION ONLY
When is F≤μR and when is F=μR
Object at rest (ladders) 𝐹≤𝜇𝑅
Just about to move 𝐹=𝜇𝑅
(limiting) equilibrium
Already sliding 𝐹=𝜇𝑅
(kinetic)
F is never greater than 𝜇𝑅
How do we find the time which a velocity vector is equal to i+j
Set i and j of the velocity vector equal to each other
How do we find the time at which acceleration is only vertical (perpendicular to i)
Set the i component of the acceleration vector = 0 (because not moving up or down only left or right)
How do we find the time at which acceleration is only vertical (perpendicular to j)
Set the j component of the acceleration vector = 0
(because not moving left or right only up or down)
Why does friction act to the right on a ladder resting on a vertical wall like this /
Because the reaction force at the wall (top of the ladder) is facing left so friction must act in the opposite direction to oppose this
If S is the reaction force at a vertical wall with a ladder resting on it like this /
How would S differ if its centre of mass of the ladder acted closer to the top than the bottom, compared to if the ladder was uniform
New value of S would be larger as the moment of the weight about A would be larger
When we say a particle is accelerating in the direction of a vector, what do we do to find the value of t
One of the components will not contain t, find the multiplier (value of lamda) that will make the numerical value of that component equal the component of the other vector.
What does a = λj mean
That the i vector of a = 0
When resolving vertically or horizontally, which forces do we leave alone
The one which are already vertical or horizontal, you dont resolve them into components regardless of whether resolving vertically or horizontally
IF ITS ALREADY VERTICAL/HORIZONTAL LEAVE IT (When R(↑)/R(→)
If a string is not modelled as light, how does this affect tension
The tension will not be constant
The tension will increase as you go up the rope since it is supporting more rope
What’s the scenario where we think of the arrow as the pivot when resolving forces
r = s + c
Use this when there’s t in Q and need position vector
Where r is position vector, s is displacement, and c is constant (can be found through subbing in known values following integration)
Dont use SUVAT for any question which has T, use integration/differentiation
SUVAT IS FOR CONSTANT ACCELERATRION ONLY
What would happen if a string isnt modelled as light
The tension wouldn’t be constant throughout the length of the string
(would be greater at the point where the string is holding the most weight)
If something is smoothly HINGED to a vertical wall, explain the nature of the reaction force exerted by the wall
There are 2 reaction forces. 1 which acts vertically and 1 horizontally (have to figure out the orientation in the most appropriate way to make vertical and horizontal components appear equal at first glance. e.g. if you have many vertical components upwards and the beam is in equilibrium, its likely that the vertical component of the reaction by the wall acts downwards)
Kind of like when something is on a rough wall there’s F and there’s reaction
If something is up against a rough vertical wall, which way does F act and which way does the reaction force act
Technically theyre both part of the reaction force, but this is an easier way to remember it
The friction is easily worked out from the nature of motion of a beam is unsupported (friction opposes that direction)
The ‘reaction force’ always acts perpendicular to the frictional component (of the total reaction force)
If a car starts after another but accelerates at a higher rate, the point at which the faster accelerating car overtakes the other is always AFTER the point when their speeds are equal
If a question says find the length of time for which speed is less than a certain value what to do
Make an equaion for velocity at t=T
This is always (xi + (y-9.8t)j)
Find speed
set this less than whatever value is
find t values from inequality
take larger away from smaller
