Angular Momentum & the Bottle Flip Challenge Bohunt School, Tuesday 13th December

How can you tell the difference between a hard-boiled 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 beltsSeat 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 bagsAir 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 zonesCrumple 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?

• 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.

Stability

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?

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.

Coefficient of restitution

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=9XOcUAY9qvw• https://www.youtube.com/watch?v=mOzH63wa

uAc

Try flipping the bottles with different amounts of water:

• 0%• 20%• 40%• 60%• 80%• 100%