# Why did the bike rotate in the physics experiment in the film 187?

• sharpnova
In summary, in the beginning scene of the film One Eight Seven, Samuel Jackson's character has one of his students hold a bicycle while sitting in a chair and pump the pedals until the wheel reaches around 180 rpm. When the student tilts the bike to the right, the chair starts rotating due to conservation of angular momentum. This is because when the wheel is spinning horizontally, it has angular momentum which must be balanced by the chair rotating in the opposite direction to keep the total angular momentum of the system at zero. Additionally, the force causing the chair to rotate is the application of a force on the wheel, which creates angular momentum and must be balanced by the chair rotating in the opposite direction. When the bike is held vertically and the wheel
sharpnova
In the beginning scene of the film One Eight Seven, Samuel Jackson's character has one of his students hold a bicycle while sitting in a chair, and pump the pedals until the wheel got up to around 180 rpm.

When he told the student to tilt the bike to the right, the chair started rotating.

Why? I can't think of any legitimate physics reason why this would happen.

It is due to conservation of angular momentum.
If he sits down on the chair with the wheel not spinning, the total angular momentum of the system is zero.
If he then starts spinning the wheel when it is horizontal, he gives the wheel angular momentum, and so the chair must spin in the opposite direction to balance the angular momentum and keep the total angular momentum of the system at zero.

(you can't create a net angular momentum out of nothing, just like you can't create a net momentum out of nothing)

I understand that it happens due to conservation of angular momentum, but I have two questions about it:

1. why doesn't the chair rotate when the bike is held vertical and its wheel is spun. Doesn't the wheel have angular momentum in this state as well?

2. aside from the conservation principle at work, what actual force is causing the chair to rotate. where is it coming from? what is the force explanation for this demonstration of conservation of angular momentum?

## 1. Why did the bike rotate in the experiment?

The bike rotated in the experiment due to the principle of conservation of angular momentum. When the student in the film pushed the handlebars to one side, they created a force that caused the bike's wheels to rotate in the opposite direction, according to Newton's third law of motion. This rotation of the wheels resulted in a change in the bike's angular momentum, causing it to rotate as well.

## 2. What factors can affect the rotation of the bike in the experiment?

Several factors can affect the rotation of the bike in the experiment, including the force applied to the handlebars, the mass and distribution of weight on the bike, and the friction between the bike's wheels and the ground. Additionally, the speed and direction of the bike before the handlebars are turned can also impact the resulting rotation.

## 3. How does the bike's design contribute to its rotation in the experiment?

The design of the bike plays a crucial role in its rotation in the experiment. The placement of the bike's wheels, the size and shape of the handlebars, and the overall weight distribution all affect the bike's ability to rotate. Bikes with a lower center of gravity and narrower handlebars are generally more stable and less likely to rotate in this type of experiment.

## 4. Is the rotation of the bike affected by the rider's weight?

Yes, the rider's weight can affect the rotation of the bike in the experiment. The heavier the rider, the more force is applied to the handlebars, resulting in a greater rotation. Additionally, the distribution of the rider's weight on the bike can also impact its rotation, as a heavier weight on one side of the bike can cause it to rotate in that direction.

## 5. Can the rotation of the bike be predicted using mathematical equations?

Yes, the rotation of the bike can be predicted using mathematical equations, specifically those related to angular momentum and torque. These equations take into account factors such as the force applied, the distance from the axis of rotation, and the moment of inertia of the bike to calculate its resulting rotation. However, these equations may not account for all variables and may only provide an estimate of the bike's rotation in the experiment.

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