Rotational Dynamics Flashcards

1
Q

An unbalanced torque causes a change in the

A

angular rotational motion of an object

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2
Q

definition of moment of inertia

A

moment of inertia of an object as a measure of its resistance to angular acceleration about a given axis

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3
Q

moment of inertia depends on

A

mass and the distribution of mass about a given axis of rotation

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4
Q

I = mr^2

point mass

A
I = moment of inertia (kg m^2)
m = mass
r = distance from centre of axis of rotation to point mass
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5
Q

Moment of inertia = I

A

measured in kg m^2

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6
Q

I = 1/12ml^2

rod about a centre
where the centre of the rod is at the centre of the axis of rotation

A
I = moment of inertia (kg m^2)
m = mass (kg)
l = length of rod (m)
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7
Q

I = 1/3ml^2

rod about end
where the end of the rod is at the centre of the axis of rotation

A
I = moment of inertia (kg m^2)
m = mass (kg)
l = length of rod (m)
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8
Q

I = 1/2mr^2

disc about centre
a 3d disc, unlike the point mass which is flat.

A
I = moment of inertia (kg m^2)
m = mass (kg)
r = distance from end of disc to centre of axis of rotation
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9
Q

I = 2/5mr^2

sphere about centre

A
I = moment of inertia (kg m^2)
m = mass (kg)
r = radius of the sphere, distance from centre of axis of rotation to edge of sphere
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10
Q

Torque

T

A

(N m)

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11
Q

Torque

T = Fr

A
T = Torque (N m)
F = Force (N)
r = perpendicular distance between direction of force and axis of rotation (m)
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12
Q

If the force is not applied perpendicular i.e an angle of 15 degrees then its perpendicular component would have to be calculated by

A
T = Fr
T = sin15 x r
T = 0.65r
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13
Q

An unbalanced Torque will produce an angular acceleration about an axis of rotation

Tᵤₙ = Iα

A
Tᵤₙ = Unbalanced Torque (N m)
I = Moment of inerti (kg m^2)
α = angular acceleration (rad s-2)
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14
Q

Angular Momentum

L

A

kg m^2 s-1

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15
Q

L = mvr
L= mr^2w
L=Iw

A
L = angular momentum (kg m^2 s-1)
m = mass (kg)
v = linear velocity (ms-1)
r = radius (m)
I = moment of inertia (kg m^2)
w = angular velocity (rad s-1)
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16
Q

The principle of conservation of angular momentum

A

Total angular momentum before = Total angular momentum after

17
Q

The conservation of angular momentum formula

Not in data book!

A

I₁w₁= I₂w₂

Just as linear momentum is conserved when two or more objects collide in absence of external forces

I = moment of inertia (kg m^2)
w = angular velocity (rad s-1)
18
Q

when combining different objects use this formula

I = Σmr^2

The sum of moment of inertia , add them together

A

Moment of inertia = the sum of the mr^2

find their I then add them together

19
Q

Rotational Kinetic Energy

Ek = 1/2Iw^2

A

Ek + kinetic energy of a rotating body (J)
I = moment of inertia (kg m^2)
w = angular velocity (kg m^2)

20
Q

Ep = Ek (linear) + Ek (rotational)

so

A

mgh = 1/2mv^2 +1/2lw^2

21
Q

worked example of rotational kinetic energy

A
Ep lost = Ek gained
mgh = 1/2mv^2 + 1/2lw^2
since w = v/r
mgh = 1/2mv^2 + 1/2l x (v^2/r^2)
take v^2 out both leaving as a fraction (1/r^2)
mgh = v^2 (1/2m + 1/2l x (1/r^2)

this is now solvable with numerical values.

22
Q

Ep lost =

23
Q

Why does an object no longer travel in a circular path when the speed is reduced

A

Tension will be reduced

Weight is bigger than the centripetal force

24
Q

An object at its highest point about its rotation of axis will have the equation

A

Fᵤₙ = Tension + Weight

Both weight and tension are in the downward direction towards the centre of the axis

25
An object at its lowest point about its rotation of axis will have the equation
Fᵤₙ = Tension - Weight | Tension is in the upward direction towards the centre of the axis and weight is in the downwards direction
26
the moment of inertia has decreased when a person pulls there legs in why?
the distribution of mass is closer to the AXIS of ROTATION!
27
a solid cylinder has the same moment of inertia as what
as a disc I = 1/2mr^2
28
why would there be a decrease in moment of inertia between two different moments of inertia experiments involving kinetic energy?
Energy is lost
29
a cube has the same moment inertia as what
as a disc I = 1/2mr^2
30
if an object is denser then the moment of inertia is
greater and thus the speed of the mass will be less