# Question about bicycles and inertia

• Tabaristiio
In summary, the conversation discusses the possibility of pedaling a bicycle in a high gear that cannot be pedaled from a stationary position. It is mentioned that an electric motor or a stronger person can help start the bike in motion, but ultimately, the cyclist needs to generate enough power and torque to maintain the bike's movement and prevent it from stopping. The concept of power and torque is explained, and it is suggested that starting in a low gear allows for easier acceleration and transfer of energy from the cyclist's legs to the bike's kinetic energy. The use of a motor to reach higher speeds is also addressed, with the conclusion that it may not be effective for sustained high speeds.
Tabaristiio
In a bicycle, would I be able to pedal in a gear that I can’t pedal from a stationary position in if it was already started by another power source (engine, motor or by a stronger person) whilst keeping it rotating and preventing it from stopping?

So If an electric motor started moving a bicycle in a high gear that I can’t start to move it in, will I be able to move it after it’s already started moving and keep it moving whilst preventing it from stopping?

Sure, but you don't need an electric motor. Just start at the top of a hill and coast. When the speed gets high enough, you can pedal in high gear.

Tabaristiio said:
In a bicycle, would I be able to pedal in a gear that I can’t pedal from a stationary position in if it was already started by another power source (engine, motor or by a stronger person) whilst keeping it rotating and preventing it from stopping?

So If an electric motor started moving a bicycle in a high gear that I can’t start to move it in, will I be able to move it after it’s already started moving and keep it moving whilst preventing it from stopping?
If it is a tricycle, so no balancing is involved, and the machine is frictionless, without air resistance, it would start and gather speed no matter how high the gear.
Why should the torque applied by the rider be less when starting than when gong along? Our familiarity with IC engines makes us expect this feature to be universal, but muscle power and steam engines can apply full torque from zero speed.

In a high gear the pedal torque required(proportional to the force applied by legs) is higher. The bike has some rolling resistance from terrain and tyres. There is a point where the torque required only to overcome rolling resistance is higher than the maximum you can do.
A bicyclist should be able to deliver torque but also power is important. Power at pedal axle is torque times angular velocity. Where anyone can deliver reasonable torque even power for a limited time, competitive bicyclists can also maintain the power for a long time. That is they deliver enough torque at high axle angular velocity or enough torque at high speed. But I digress.
Basically at a high gear ratio you should be able to deliver enough torque to accelerate the bike slowly. But I suppose you want to keep it for long distances, that means you will be able to deliver a lower torque. The idea is the electric motor will help you get to a speed where you can keep your balance. But the higher your velocity the harder(power required, not torque) it it becomes to pedal because you will have to deliver more power then after 20 - 30kph the air resistance rises considerably and it does that with the square of velocity. The only thing is that at low speed the bike you can't keep balance or at least is difficult. Without air resistance the a man can go up to like 200kph on a finely tuned bicycle but after that rolling resistance and gear, chain, bearings losses cannot be overcome anymore even by a competitive cyclist.
A competitive cyclist can deliver more than 2000W during a short sprint where as an amateur can do like 550W. During long runs they do around 250W which is 8 times less than peak power and an average man around 100W which is 5.5 time less.
If the terrain is not perfect and also until you break the friction in the gears (also at high speed the gear and ball bearings friction is slightly higher) the bike will require a higher torque but that is not a problem in case you don't intend to make it extremely difficult to pedal.
Instead of using an electric motor only to set it in motion you can use helper wheels that can you retract after you reach like 5 kmph.
Slopes will be very difficult to climb therefore a motor would be much more helpful.

tech99 said:
Why should the torque applied by the rider be less when starting than when gong along?
The Torque is not the only important quantity involved. The cyclist can actually transfer very little Power to the wheels at zero speed. Power is the rate of doing work and you need speed in order to increase the Kinetic Energy of the bike.
(Power transferred = torque times rotational speed of pedals)
It's a sort of catch 22 because it will take you ages (assuming you don't fall off) to accelerate a stationary bike in a high gear. That's why you start in a low gear, which allows your legs to move faster and, thus, to transfer more Energy to the bike's KE per second. At high speed, your legs cannot supply enough torque whilst they are thrashing up and down so you then have to change up so that your legs are working at a decent rate and produce useful torque.

Last edited:
calinvass
If you plan to use the motor to take you at 25-30 kph its fine you can have some advantages like better acceleration more ground clearance (also less prone to break the pinion shifter) less likely for the chain to come off. But if you think the motor can take you to 40-50kph or more where you can coast, then it's not going to work.

## 1. How does inertia affect a bicycle?

Inertia is the resistance an object has to change its state of motion. In the case of a bicycle, the inertia of the wheels helps to keep the bike moving forward even when the rider stops pedaling. It also makes it easier for the bike to maintain its balance while in motion.

## 2. Why is it easier to maintain balance on a moving bicycle than a stationary one?

The rotating wheels of a bicycle create gyroscopic forces that help to stabilize the bike while in motion. This, combined with the bike's inertia, makes it easier to maintain balance and steer the bike in the intended direction.

## 3. How does the distribution of mass on a bicycle affect its inertia?

The distribution of mass on a bicycle can greatly affect its inertia. Bicycles with heavier wheels will have a greater rotational inertia, making it harder to change the direction of the bike. Bicycles with a lower center of mass will have a lower inertia, making it easier to maneuver and change direction.

## 4. Does the weight of the rider affect the inertia of a bicycle?

Yes, the weight of the rider can affect the inertia of a bicycle. A heavier rider will require more force to change the state of motion of the bike, while a lighter rider will require less force. This is why it is easier for smaller or lighter individuals to ride a bike than it is for larger or heavier individuals.

## 5. How does braking affect the inertia of a bicycle?

When brakes are applied to a moving bicycle, they create a force that acts in the opposite direction of the bike's motion, causing it to slow down. This also affects the bike's inertia as it requires more force to overcome the momentum and bring the bike to a complete stop.

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