Mechanics Flashcards
(25 cards)
Total momentum is conserved if ___.
There are no external forces acting on the system.
An elastic collision is when the ___ is conserved and the ___ is conserved.
Total momentum. Total kinetic energy.
An inelastic collision is when the total kinetic energy is ___, and the total momentum
___.
Not conserved. Conserved.
What is the inertia equation for a hollow disc or solid object rotating in a circle?
I = mr^2
What is the inertia equation for a solid disc?
I = 1/2mr^2
What happens when we increase rotational inertia?
When we increase rotational inertia, the angular velocity increases because angular momentum is constant. L = Iw.
Total kinetic energy equals
Linear kinetic energy + rotational kinetic energy
What is simple harmonic motion? What are the two requirements?
A repetitive motion of an object, back and forth in a straight line from an equilibrium position.
Displacement is equal the restoring force towards equilibrium.
Restoring force is always in the opposite direction to displacement.
At maximum displacement in SHM, there is ___.
Maximum restoring force and acceleration opposite displacement towards equilibrium. Velocity is 0.
At equilibrium in SHM, there is ___.
No restoring force or acceleration. Velocity is maximum.
How to know which sin/cos equation to use?
Simply draw out the graph for either displacement, velocity or acceleration. Then use whatever equation that fits the graph.
In SHM, the total energy =
Total kinetic energy + total potential energy
Vmax =
Aw
How do you know if angular momentum is conserved?
If no external torques act about the centre of mass of an object or system.
µ
10^-6
M
10^6
m
10^-3
k
10^3
n
10^-9
A satellite is rotating, it loses a piece of itself. What happens to its angular velocity?
It stays the same. L = Iw. When the piece is lost, the satellite loses rotational inertia as well as angular momentum as the piece carries angular momentum with it. The decrease in L and I cancel out so angular velocity remains constant.
A ball has 2 kgms^-1 falling down. It bounces back up with 1 kgms^-1, what is the momentum change of the ball?
3 kgms^-1
A person rolls two cylinders of the same mass and shape down a hill. Cylinder A is hollow and cylinder B is solid. Which cylinder gets to the bottom of the hill first?
Cylinder B gets to the bottom of the hill first. Cylinder B is solid so its effective radius is less than cylinder A. This means it also has less rotational inertia. Since they both have the same mass and shape they both receive the same torque. T = Ia. Since cylinder B’s rotational inertia is less, its angular acceleration is greater, so it reaches the bottom of the hill first.
Resonance
Where the driving frequency matches the natural frequency so it results in maximum amplitude.
A car is at the top of the hill, position A. It moves down the hill and begins to go around in a vertical circle, position B. Compare the size of the force of the ground acting on the car at position A and B.
At position A, only weight force is acting on the car. The force from the ground is equal to this weight force. At position B, the car is accelerating upwards. The force from the ground is equal to the car’s weight force PLUS the centripetal force required to accelerate the car upwards to keep it in its circular motion. Thus the force from the ground is greater at position B.
