Download Bottle Flip/ Angular Momentum

Angular Momentum & the
Bottle Flip Challenge
Bohunt School, Tuesday 13th December
How can you tell the difference between a hardboiled egg and a raw egg?
(without cracking them open)
What is momentum?
A moving object has momentum.
• Momentum = the tendency of the object to keep moving in the
same direction
• Momentum = mass (kg) x velocity (ms-1)
• Force = Rate of change of momentum
• Forces in car crashes are dissipated by special features.
• Seat belts
Seat belts stop you tumbling around inside the car if there is a
collision. However, they are designed to stretch a bit in a
collision. This increases the time taken for the body's
momentum to reach zero, so reduces the forces on it.
• Air bags
Air bags increase the time taken for the head's momentum to
reach zero, so reduce the forces on it. They also act a soft
cushion and prevent cuts.
• Crumple zones
Crumple zones are areas of a vehicle that are designed to
crush in a controlled way in a collision. They increase the time
taken to change the momentum in a crash, which reduces the
force involved
Angular momentum (‘L’)
• Spinning objects have angular momentum.
• Angular momentum is always conserved, but
can be transferred from one object to another.
• If a spinning object expands, its rate of rotation
will slow down.
Examples of Angular Momentum
Investigating angular momentum
• L=angular momentum=I ω
• I - moment of inertia - a resistance to angular
acceleration
• Depends on shape of the object
• Straight line motion – mass
• Spinning = mr2 (depends on r)
• ω – angular velocity
• Angular force = Torque
• Pull the string on a top -> applying a torque to
make it speed up -> angular momentum
increases.
• It slows down after being released due to
frictional torque
As the water spreads out it slows
the rate of spin
If you push the ruler back evenly, in what order
will the cartons topple over?
Stability
• An object will fall ‘back’ over if the centre of
mass of the object is above the base (this is
why tall buses are more likely to fall over).
• Having a small amount of orange juice lowers
the centre of mass.
Metre Rule
• Sometimes the centre of mass acts through the geometric
centre
• It is not possible to balance the ruler unless its centre of
gravity is over your finger >> find the centre of gravity of the
ruler by sliding your fingers from the ends towards the
middle.
• Observe carefully and you’ll notice that only one finger
moves at a time.
• As you slide your fingers, the force of friction pushes back.
The more weight on your finger, the greater the force of
friction.
• One of your fingers is supporting slightly more of the ruler’s
weight than the other; that finger gets “stuck.” The other
finger will move until it is the one supporting the most
weight, then it will get stuck instead. Friction makes sure that
when your fingers meet they are both supporting the same
amount of weight.
Centre of Mass
Investigate which bottle falls over
last
How can we change the height the ball will bounce
to?
Coefficient of restitution
Some materials are bouncier than others. How
well two objects will bounce together is called the
coefficient of restitution.
A coefficient of restitution of 1 means it will
bounce back to the same height.
A coefficient of restitution of 0 means it will not
bounce at all.
So how does this help our
flipping bottles?
• As the water ‘sloshes out’ it
reduces the spin. Angular
momentum is conserved.
• A low centre of mass makes
the bottle more stable.
• Not very bouncy. Low
coefficient of restitution.
What would the perfect bottle
look like?
Watching bottle flips:
• https://www.youtube.com/watch?v=9XOcUAY9q
vw
• https://www.youtube.com/watch?v=mOzH63wa
uAc
Try flipping the bottles with
different amounts of water:
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•
•
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•
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0%
20%
40%
60%
80%
100%