Mechanics key point Flashcards
(9 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)
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
