Mechanics: On the Move Flashcards
When do we use the 4 suvat eqs?
When acceleration is constant!
Can use with any freefall where air resistance is negligible.
Any situation where object is thrown vertically upwards.
Any projectile situation.
What if a = 0?
Eqs all reduce to s=v x t i.e v=u
What did Galileo find out about freefall?
Any two objects must fall at the same rate, regardless of their relative weights.
Any object falling vertically must accelerate.
Acceleration due to freefall, g?
= acceleration when no external forces acting on an object apart from force of gravity.
Two-stage problems - object released from rest falling then hitting a bed of sand.
1 - falling motion due to gravity: acceleration = g
2 - deceleration in sand: initial velocity = velocity of object before impact
Therefore v at end of first stage = u at start of second stage.
What is a projectile?
A projectile is any object acted upon only by the force of gravity.
3 principles followed by projectiles?
- acceleration of object always equal to g and always down bc gravity.
- horizontal velocity is constant bc acceleration has no horizontal component.
- the motions in the horizontal and vertical directions are independent of each other.
Verticals vs horizontals ?
(maxime wisdom)
Vertical = use suvat
Horizontal = use s= v x t, because v is constant!
Horizontal projection: stone thrown from top of cliff follows a curved path (PARABOLIC PATH) before hitting water.
Comment on path shape, distance and time.
- Its path through the air becomes steeper and steeper as it drops.
- The faster its projected, the further away it’ll fall in the sea.
- Time taken to fall into the sea doesn’t depend on how fast it’s projected (Galileo).
Projectile speed at end just before contact with ground?
vy = velocity in y axis i.e. vertical velocity
vx = velocity in x axis i.e. horizontal velocity
sqrt (vx ^2 + vy ^2)
aka sqrt (u ^2 + (-at) ^2)
What is drag force?
A frictional force that drags on the projectile when it moves through air because of air resistance.
Shape of projectile affects drag force, and may also cause lift force. What is lift force?
Happens if the shape of the projectile causes the air to flow faster over the top of the object than underneath it. As a result, the pressure of the air on the top surface is less than that on the bottom surface. The pressure difference causes a lift force on the object.
(Spinning ball also experiences this force but can be in any direction depending on how it’s spun.)
If no gravity, how will the object projected move. What is its velocity (h+v)?
Move is straight line of constant velocity, U, at angle θ (to the horizontal). Initial velocity has horizontal component U cosθ and vertical component U sinθ.
What is its displacement (h+v)?
Velocity unchanged so displacement unchanged.
h: Ut cosθ
v: Ut sinθ
If we bring back gravity…
Horizontal unchanged.
Vertical displacement
= Ut sinθ - 0.5 g t^2
Vertical velocity
= U sinθ - g t
