Seesaws Flashcards
2 Physical Quantities of Rotational Motion?
1) Angular Position (Rotation)
2) Angular Velocity
Vector quantities
Newtons First Law Rotational Motion
A rigid object that’s not wobbling & free of external torques, rotates at constant angular velocity.
i) Rotation to your right
ii) Rotation to your left
i) Downward rotation
ii) Upward rotation
3 ways measure Angular Position
1) As a degree ex. 90*
2) Rotationally ex. quarter, half turn
3) Radius ex. Pi/ 2 radius down
Angular Velocity def & tells (3)?
Rate objects angular position change w/ time
Tells:
1) speed object going through angles
2) axis object’s spinning
3) (right hand rule) specific direction about axis
Torque
Twisting force causing rotation.
-changes pivoting objects angular velocity & violates N’s 1st Law of Rotational Motion (has external torque/influence & .•. not rotate constant angular velocity)
Seesaws Rotational Inertia affected every time_____?
Seesaws ends hit ground (exerting external torque)
Angular Acceleration
Rate at which angular velocity change w/ time
- vector quantity of rotational motion
Net Torque
1/more total torques acting upon object
- causes angular acceleration
Angular Acceleration causes (2)?
- Net Torque
2. Rotational Inertia (mass): resistance to angular acceleration
A larger net torque has a ____ acceleration & a smaller net torque has a _____ acceleration
- Larger
2. Smaller
Angular Acceleration proportional (keeps constant ratio)?
Net Torque acting upon object
Angular Acceleration inversely proportional?
Seesaws rotational mass
Newton’s 2nd Law of Rotational Motion
Angular Acceleration = net torque/ rotational mass
Torques produce x & x produces Torques
Forces
Can’t produce torque in what direction from objects centre of rotation?
Toward/away from/ on objects’ centre of rotation
Can produce torque in what direction from objects center of rotation?
Exerting force perpendicular to lever arms vector (amount & direction: determine 1 point in space relative another) direction.
Objects vector extends from?
Objects centre of rotation to point where force exerted (extension b/w points is vector)
Torque =?
Lever arm • force (only force perpendicular to lever arm incl.)
Torque you exert on bolt (w/wrench) about its centre of rotation=?
Your force• (wrenches) lever arm extension
Lever
A rigid bar resting on a pivot.
Forces are at ____ to lever arms
Right angles
Torques are located?
90• horizontal to the downward force & perpendicular to the lever arm.
2 ways seesaw balanced?
1) sum of torques = 0 (cancel out)
2) riders & seesaw together have Center of Gravity vertically above pivot & .•. produce no torque.
The two torques produced by the objects on the seesaw are____ each other.
Opposite.
Moving object (& force) position lower down lever arm (closer to pivot) increases/decreases torque it produces.
A shorter lever arm decreases the torque it produces & is closer to the COR.
To produce 0 torques (balance) objects w/ same weight, must be equidistant from ____ & .•. have same _______.
The center of rotation & .•. have same lever arm distances from pivot.
The lever arms are in the same direction so the objects____ are in the same direction.
Torques
Center of Gravity
The effective location of an objects weight (downward acceleration)
- all about forces (gravity)
- point over which object can be perfectly balanced; net torque due to gravity about that point = zero
Center of Mass
Effective location objects mass, it’s natural pivot
- all about inertia & motion
- the average location of the mass distribution
- objects Center of rotation: Point about which all physical quantities of rotational motion are defined.
- point that stays put as object spins (the pivot)
For (small) objects…..
G
COG proportional ____
Centre of mass
Riders @ ends of board higher/lower rotational mass?
Higher & .•. riders closer to pivot have lower rotational mass.
Every piece of____ contributes to an objects rotational mass & depends on (2)?
Mass:
1) where portion ordinary mass is
2) how far portion ordinary mass from pivot (axis of rotation)
Rotational mass contributed by ordinary mass=?
Ordinary mass • distance b/w ordinary mass & axis of rotation
Rotational Mass depends on____?
The location of your axis of rotation
Tennis Racket has 3 rotational masses (ordered by rotation)
Smallest: rotation about the vertical axis (most of mass close to axis)
Larger: rotation about the horizontal
axis.
Largest: when COR located at either end of racket & pivot about such point.
Two objects dropped simultaneously w/ same force & at same distance from ground but different masses, which falls first?
Object w/ more angular acceleration & .•. more angular velocity. Object w/ < mass.
You fall of a cliff, what’s the cause (i) & resistance (ii) angular acceleration of your body?
(i) C: Net Torque on you (due to gravity)
(ii) R: Your rotational mass
Rotational Mass=?
Mass• distance^2
You & child 1/2 your height fall @ same angle, same distance swimming pool
Who hits water 1st?
The child has more angular acceleration & more angular velocity.
- You have 2x weight • 2x lever arm (2x as far your COG from axis of rotation- feet) = 4x Net torque
- You have 2x mass • 2x distance (on average) from AxofRot
RotMass= mass• distance^2
.•. 2x mass • 2x distance^2 =
8x rotational mass of child.
Have 4x net torque but 8x rotational mass.
Angular Acceleration = Net torque/mass
4/8=
1/2 Angular Acceleration vs child.
Unsupported object thrown straight up in air doing 2 types motion?
1) Translational- it’s center of mass falling in arc of falling objects
2) Rotational- rotating about its center of mass (it’s natural pivot)
Find objects center of mass method?
Place object on flat surface & spin it: object naturally spins about its center of mass (point stays put as object spins)
The farther riders mass from the pivot
the greater seesaw’s _____ _______ ____ & the slower it’s _____ _______.
1) overall rotational mass
2) angular acceleration
How do seesaw riders riders affect each other?
They support each other (cancel weights- keep from falling) & exchange energy (through seesaw & torques) as seesaw rotates back & forth
- 1 exerts torque on 2 about the pivot
- 2 exerts torque on 1 about the pivot
N’s 3rd Law of Rotation
For every torque object (i) exerts on (ii), object (ii) exerts an equal but oppositely directed torque on object (i).
Torque Units
S.I: Nmeter (m away from pivot)
Eng: FootPound (1 pound/foot away from pivot)
When lowering (unbalancing) seesaw lowered & raised object transfer energy, what occurs w/ distance
Work= force•distance
- Lowered object has > (downward) force but < distance
- Raised object has < (upward) force but > distance
- Both work cancel each other out & obey N’s 3rd law rotation
To increase work on screwed nail with hammer (plucker)…
Increase the lever arm distance from pivot.
Work You’re doing on hammer:
Small force & large distance.
Work hammer doing on nail:
Large force & small distance
When the gravitational field across an object is uniform _____ & ______are identical.
COG & COM
