Section 6 part 2 - Further mechanics and thermal physics (circular and sh motion) Flashcards
Define uniform circular motion
An object rotating at a steady speed
Why do you accelerate when moving at a constant speed around a circle?
Velocity is continually changing due to change in direction. If velocity changes, there must be an acceleration as acceleration is rate of change of velocity
Which direction does the velocity act on an object in uniform circular motion?
Along the tangent to the circle at that point
Which direction does the centripetal force act on an object in uniform circular motion?
Towards the centre
Why does the acceleration of an object in uniform circular motion act towards the centre of the circle?
Because the change in direction of the velocity is towards the centre of the circle, so the acceleration acts towards the centre of the circle
What causes an object to move around on a circular path?
Being acted upon by a resultant force
What is a centripetal force?
The resultant force causing an object to move around a circle at constant speed
What is the centripetal force acting on a car when going around a roundabout?
The sideways friction between the vehicle’s tyres and the road surface
State the difference in friction for a car going around a banked track vs non banked track
Banked track has no sideways friction
Non-banked track has sideways friction
Why do banked tracks not have sideways friction for a vehicle going around it?
Centripetal force is given by the support forces of the tyre
How can you get a velocity time graph from a displacement time graph?
Differentiate the graph
How can you get an acceleration time graph from a velocity time graph?
Differentiate the graph
Define SHM
A type of periodic motion where the acceleration is directly proportional to the displacement and acts in the opposite direction to that of the displacement
For a mass spring system oscillating vertically, if you have A at the peak. B at equilibrium and C at the bottom, describe the acceleration and velocity
A - Max acceleration down, 0 velocity
B - 0 acceleration, max velocity
C - max acceleration up, 0 velocity
What is the amplitude of an oscillating object?
The maximum displacement of the oscillating object from equilibrium
What do you call the oscillations if the amplitude is constant?
Free vibrations
What is the time period
Time taken for one complete cycle of oscillation
What is the frequency
Number of cycles that pass a point per second
What can you conclude when comparing the acceleration time graph against the displacement time graph?
Acceleration is always in the opposite direction to the displacement
Which direction does the resultant force act on an oscillating object?
Towards equilibrium
What is the restoring force?
The resultant force that acts towards equilibrium
What determines the frequency of oscillation of a loaded spring?
Adding extra mass -> increases inertia which means each cycle of oscillation takes longer
Using weaker springs -> Restoring force would be less, acceleration would be less, each cycle of oscillation takes longer
What is the restoring force for a spring oscillating vertically that obeys Hooke’s law?
-kx
What is another equation for acceleration?
-kx/m
How does the tension in the spring vary?
Max when stretched downwards (maximum displacement downwards) = mg + kA
Min when compressed upwards (maximum displacement upwards) = mg - kA
What are free oscillations?
When an object oscillates with a constant amplitude because there is no friction acting on it
What is the equation for SHM energy stored in a spring?
1/2kx^2
What is the equation for SHM the total energy of the system?
1/2kA^2
What is the equation for SHM kinetic energy
1/2k(A^2-x^2)
What are dissipative forces?
Forces causing the amplitude to decrease by dissipating energy of the system to the surroundings as thermal energy
When is a motion of an object said to be damped?
When dissipative forces are present
When does light damping occur?
When the time period is independent of the amplitude so each cycle takes the same length of time as the oscillations die away
What is critical damping?
When the oscillation returns to equilibrium in the shortest possible time without overshooting
What is heavy damping?
When the object returns to equilibrium much more slowly than critical damping. No oscillating motion occurs e.g mass spring system in thick oil
Describe the 3 types of damping on a displacement time graph
Light - Same time period, but amplitude gradually decreases
Critical - Quater of a cos wave
Heavy - Negative gradient graph
ALL START FROM SAME MAX DISPLACEMENT
What is a periodic force?
A force applied at regular intervals
What are forced vibrations?
A system undergoes forced vibrations when a periodic force is applied to it
When does a system oscillate at maximum amplitude
When the phase difference between displacement and periodic force is 1/2 pi
When does a system resonate?
When the periodic force is exactly in phase with the velocity of the oscillating system
What is the frequency at maximum amplitude called?
Resonant frequency
What happens to the resonance of a system when the damping is lighter?
The maximum amplitude becomes larger at resonance
the closer the resonant frequency is to the natural frequency of the system
What is true for a an oscillating system with little or no damping at resonance in terms of frequencies?
Applied frequency of periodic force = natural frequency of system
