Section 6- Further Mechanics And Thermal Physics

Question 1

What is uniform motion in a circle?

Answer 1

An object rotating at a steady rate.

Question 2

Describe what you need to measure to find the speed of an object moving in uniform circular motion.

Answer 2

Circumference of the wheel: 2(pi)r
The frequency of rotation: f=1/T

v=2(pi)r= 2(pi)rf
T

Question 3

Define angular displacement and angular speed.

Answer 3

Angular displacement: the angle in radians of the object in time t.
2(pi)t= 2(pi)ft
T

Angular speed (w): angular displacement per second

Question 4

Why is velocity not constant when an object is travelling uniformly in a circle?

Answer 4

The object is accelerating.

Question 5

What is the directions of the acceleration?

Answer 5

The direction changes continually as the object moves and the acceleration is always towards the centre of the circle. It is called centripetal acceleration.

Question 6

What is the equation for centripetal force?

Answer 6

F=(mv)^2= mw^2r

r

Question 7

Explain why a passenger is thrown outwards if the car rounds a bend too quickly.

Answer 7

The centripetal force is provided by the sideways force of friction between the vehicle and the road.
For no skidding the force of friction between the tired and road must be less than that the limiting value F which is proportional to the vehicles weight.

Question 8

Describe what happens between an passenger and the seat when travelling over a curved bridge.

Answer 8

At the top of the hill the support force from the road is directly up and mg is directly down. The resultant force acts towards the centre of the hill.

Question 9

What forces provide the centripetal force on a banked track?

Answer 9

There is no sideways friction if the speed v is such that:
v^2=grtan0
Most tight bends on race tracks are banked to enable the car to drive at higher speeds.

Question 10

When is the contact force on a passenger on a Big Dipper ride is the greatest?

Answer 10

At the bottom because
S-mg=mv^2
r

Question 11

What is the condition that applies when a passenger fails to keep in contact with their seat?

Answer 11

v^2=gr so R=0 so there would be no force on the person

Question 12

What is meant by one complete oscillation?

Answer 12

T=1/f

Question 13

Define amplitude, frequency, period and free vibrations.

Answer 13

Amplitude: maximum displacement of object from equilibrium
Frequency: number of cycles per second
Period: time for one cycle of oscillation
Free vibration: constant amplitude with no frictional forces

Question 14

Describe the phase difference between two oscillators that are out of step.

Answer 14

Their phase difference (rad) stays the same=
2(pi)🔺t
T

Question 15

State two fundamental conditions about acceleration that apply to simple harmonic motion.

Answer 15

Acceleration is proportional to the displacement.

The acceleration is in the opposite direction to the displacement.

Question 16

Describe how displacement, velocity and acceleration vary with time.

Answer 16

The displacement starts at maximum displacement and one oscillation will be at T.
The velocity is 1/4 cycle out with displacement.
The acceleration is 1/2 cycle out with displacement (the opposite direction).

Question 17

What equation relates displacement to time for a body moving with simple harmonic motion?

Answer 17

2(pi)t=wt
T

Applies from equilibrium.

Question 18

What conditions music be satisfied for a mass-spring system or simple pendulum to oscillate with simple harmonic motion?

Answer 18

The acceleration must be proportional to displacement and always acts toward the equilibrium.

Question 19

How does the period of a mass-spring system depend on mass?

Answer 19

The extra mass increases the interia. The trolley would be slower so each oscillation would take longer.

Question 20

Describe how the period of a simple pendulum depends on its length.

Answer 20

The longer the spring the longer it will take to complete an oscillation.

Question 21

How does, in simple harmonic motion, kinetic energy vary with displacement?

Answer 21

At maximum displacement the energy is all potential energy.

At 0 displacement the energy is all kinetic energy.

Question 22

Describe the effects of damping on the characteristics of oscillations.

Answer 22

Light damping: each cycle takes the same length but the amplitude slowly decreases.
Critical damping: stops the object oscillating and return the object to equilibrium in the shortest time possible.
Heavy damping: object returns to equilibrium slowly without oscillations.

Question 23

What circumstances do resonance occur in?

Answer 23

The periodic force is exactly in phase with the velocity of the oscillating system.

Question 24

What is the difference between free vibrations and forced vibrations?

Answer 24

Forced vibrations have a periodic driving force applied to the oscillating system. The free vibrations do not.

Question 25

Why does a resonant system reach a maximum amplitude of vibrations?

Answer 25

The maximum amplitude becomes bigger at resonance with lighter damping. The maximum amplitude is the increased the closer to the natural frequency it gets.

Question 26

Define internal energy.

Answer 26

The internal energy of an object due to its temperature.

Question 27

What is the lowest temperature possible?

Answer 27

Absolute zero 0K / -273•C

Question 28

Which material heats up and cools down the fastest?

Answer 28

Lead

Question 29

What is specific heat capacity?

Answer 29

The energy needed to raise the temperature of unit mass of the substance by 1K without change of state.
🔺Q= mc(T2-T1)

Question 30

Define latent heat.

Answer 30

The energy needed to melt a solid at its melting point.

Question 31

Why does the temperature of a substance stay steady when it changes state?

Answer 31

The molecules are vibrating at the same average speed as before so have the same kinetic energy and stay the same temperature.

Question 32

What are the experimental gas laws?

Answer 32

Boyles law: pressure is proportional to 1/volume
Charles law: volume is proportional to temperature
Pressure law: pressure is proportional to temperature

Question 33

What is meant by isothermal change?

Answer 33

Change of system in which the temperature remains constant.

Question 34

What is an ideal gas?

Answer 34

A gas that obeys boyles law.
pV= constant
T

Question 35

Why is there an increase of pressure of a gas when compressed or heated?

Answer 35

The average speed of the molecules are faster so the impact on the walls are harder and more frequent.

Question 36

What does the mean kinetic energy of a gas molecule depend on?

Answer 36

The volume of each molecule is negligible compared to the gas.
They do not attract each other.
They move about in continual random motion.
The collisions between particles and the container are elastic.
Each collision is shorter than the time between impacts.