Engineering Physics 1

Question 1

Define Inertia

Answer 1

A resistance to a change in velocity when subjected to a force

Question 2

Define Moment of Inertia, what is its unit

Answer 2

Measure of an object’s resistance to being rotationally accelerated about an axis

kgm²

Question 3

How to calculate moment of inertia (I)

For a point mass

An object

Answer 3

Point mass : I = mr²

Object = Σmr²

Sum of individual I’s of each point mass

r = Dist from axis(centre) of rotation

Question 4

How to calulate Moment of Inertia for

A Disc

A sphere

Answer 4

Disc: 0.5 mr²

Sphere: 0.4 mr²

Question 5

Explain 2 factors that affect an objects moment of inertia

Answer 5

• Total mass of object
• How its mass is distributed about the axis of rotation
  
  • More particles with a lower r, so will have a lower I, so the sum of them will be lower

Question 6

Explain why someone tucks in their legs when doing a back flip

Answer 6

Decrease their moment of inertia as more of their mass is at a smaller dist. from the axis of rotation

Question 7

What is the total rotational E_k of an object

How to calculate it

Answer 7

Sum of the rotational E_K of all the individual particles making up the object

E_K = O.5Iw²

Question 8

What is a flywheel

Answer 8

A heavy metal disc that spins on an axis and has a large moment of inertia

As it spins, it stores rotational E_K that can be transferred to other uses

So a lot of force is required to accelerate it, once it starts it is difficult to stop

Question 9

Name 4 factors affecting the amount of energy that can be stored in a flywheel

Answer 9

• Mass of the flywheel
• Angular speed of the flywheel
• Friction
• The way the mass is distributed
  
  • Concentrated further from axis or closer?

Question 10

Name 3 ways to decrease the friction on a flywheel

Answer 10

• Lubricating the bearings
• Using bearings with superconductors so the flywheel can levitate
• Using a vacuum/sealed container

Question 11

Explain 4 uses of flywheels

Answer 11

• Regenerative braking
  
  • When braking, energy lost will store up flywheel
• Wind Turbines
  
  • To store excess power on windy days, days with little demand
• Smoothing torque and angular velocity
  
  • In systems where power is produced in bursts, flywheel will charge up with burst and then output it smoothly
• Production Processes
  
  • Processes that use continuous, repeating actions where a lot of power must be output in a short time

Question 12

Define Angular Displacement (θ)

Answer 12

Angle turned through in any given direction

In radians

Question 13

Define Angular Speed / Velocity (ω), what is its unit

Answer 13

Angle an object moves through per unit time

Speed - Has only magnitude

Velocity - Magnitude and direction, can be anticlockwise/clockwise. Is always changing

rads^-1

Question 14

Define Angular Acceleration (α), what is its unit

Answer 14

The change in angular velocity over time taken

rads^-2

Question 15

How to convert from rpm to rads^-1

Answer 15

Multiply by 2π/60

Question 16

How do you calculate the dist an object is pulled up by 1 rev. of a pulley

Answer 16

Simply the circumference of the pulley

Question 17

Define Torque, what is its unit

Answer 17

The product of a force and its dist. from its axis of rotation

Nm

The rotational equivalent of linear force

Question 18

What is the law of conservation of angular momentum

Answer 18

When no external torque acts, the angular momentum of a system remains constant

Question 19

What is the unit of Angular momentum

How do you calculate it

Answer 19

kgm²s^-1

=Iω

Question 20

Why is it that when a skater moves her arms into her chest, she moves faster

Answer 20

Moving her arms in decreases her I

But by the law of conservation of angular momentum, since there is no resultant torque, her angular momentum won’t change

Si her angular velocity will increase

Question 21

What will happen if you make 2 rotating objects collide

Answer 21

By the law of conservation of Angular Momentum

The new object will have an angular momentum equal to the sum of their angular momentums

Question 22

Define Angular Impulse

How do you calculate it

Answer 22

The product of torque and its duration

So equal to the change in angular momentum

Area under a Torque, Time graph

T∆t = ∆ (Iω)