According to BBC News:

Dr Roland Ennos [from the University of Manchester in England] designed a machine which enabled him to measure the amount of friction generated by a fingerprint when it was in contact with the acrylic glass.

The machine was then strapped to the index finger of one of his students.

Dr Ennos expected the amount of friction to increase in proportion to the strength at which the acrylic glass was pushed against the finger.

This would have supported the theory that the fingerprint was helping to improve grip by ramping up friction levels.

However, the results showed that friction levels increased by a much smaller amount than had been anticipated.

Let’s take a close look at this. Firstly, what is friction and how does it work? Friction can be described as a force that resists movement between two objects that are touching. For a “normal” solid, friction is calculated by multiplying a constant called the coefficient of friction by the amount of force that’s being applied to keep the two surfaces touching. The coefficient of friction is a dimensionless constant that takes into account the two surfaces. The higher this coefficient, the greater the friction force. Using everyday experience, for example, the coefficient of friction between leather shoes on ice is much smaller than between sneakers on the concrete sidewalk. (There are tables we can use to looks these coefficients if we really wanted to. And it’s important to note that these coefficients are generally higher at higher temperatures for the same pair of surfaces.)

See if you can recall the following from high school physics:

Normal Friction

For normal solids, the friction force is strictly dependent on the force that’s pushing the two surfaces together. For instance, the friction force we encounter when we try to push a big aluminum cube on smooth wooden surface depends on how heavy that box is. Interestingly, and intuitively, a smaller cube with the same weight will have the same friction force. This is important to keep in mind!

What makes our fingers so interesting and this new research so surprising is that our fingerprints don’t act like normal solids. According to LiveScience, “the finger was not behaving like a normal solid; it was behaving like rubber. With rubber, friction is proportional to the contact area between two surfaces, not how hard they press together.” This makes things very interesting to consider. The harder we push or the tighter we grip something with our fingers, we are just increasing the surface area of contact. (In fact, because of the ridges on our fingers, there is naturally less contact area than if our fingers were perfectly smooth. So when press hard on something, we are increasing the contact area.) The fact that we increase the normal force (or the squeezing force) with our fingers isn’t directly related to the friction force as it is with normal solids (see above).

If fingerprints don’t help with grip and actually make it worse, why do we have them? ABC News summaries the possibilities:

A French team of researchers working with a mechanical hand loaded with tactile sensors found that fingerprint-like ridges improved the hand’s tactile sensitivity.

Another possibility is that fingerprints help wick water off of our hands, improving grip on wet surfaces, Ennos says. Or alternatively, they might work in coordination with soft finger and foot pads to help skin fit more snugly to abrasive surfaces, reducing shear stress. “We very rarely get blisters on the soles of our feet or our fingertips,” he [Ennos] says.

(Image from Wikipedia.)

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Forces on a Helicopter

Helicopters are fascinating creatures. One of their distinguishing features is their ability to hover in place. This is essentially accomplished through a torque and force balance. For the forces we must look at Newton’s second law of motion:

Newton's Second Law

Notice the arrows on top of the force and acceleration terms. These mean we’re dealing with force vectors, and an acceleration vector. These vectors have three components because helicopters live in three dimensions. For a helicopter to remain stationary the acceleration must equal zero, i.e., all three components of the acceleration vector must be zero. By setting the acceleration vector to zero in our equation, we see that the left side of the equation must be zero as well. Thus, the only way to achieve zero acceleration is if the sum of the forces add up to zero.

For the torques we turn to Euler’s (simplified) equation of motion:

Euler's Equation of Motion

It looks suspiciously similar to the first force equation above. In essence, torques are exactly the same as forces except that they deal with rotation instead of translation. If the sum of the torques aren’t zero it will cause the helicopter to spin around. Thus, for a helicopter to remain stationary both the sum of the forces, and the sum of the torques must be zero.

Gravity is the only force that acts on a helicopter when it’s hovering, so all we have to do is supply a force that counteracts gravity and we’re set. This is the job of the main blades. However, these spinning blades introduce a new torque on the helicopter, which throws off the torque balance and causes the helicopter to spin in the opposite direction of the main blade direction. This new torque is balanced with the tail rotor (on a conventional helicopter) that pushes the tail of the helicopter in the opposite direction of the spin. Thus the torques are balanced once again.  But we’re not finished yet. Similarly to how the main blades caused a torque as well as a force, by using a tail rotor to balance the torque, another force has been introduced that pushes the helicopter to one side or the other (depending on which way the tail rotor is facing). Add another propeller you say? Well it turns out that this force can be balanced simply by tilting the helicopter so that the force from the main blades acts slightly to one side. This balances the small force caused by the tail rotor. Finally all the forces and torques are balanced and we have a stationary helicopter hovering in the air. The next time you see a helicopter hovering stationary, see if you can see how it leans very slightly to one side to counteract the force from the tail rotor. This can be seen in the following video:

Image from R/C Airplane World.

Elevators come in handy when traveling many floors at a time.

Acceleration is a change in velocity over a period of time. Actually, this is specifically known as tangential acceleration (there are other forms of acceleration, namely that due to rotational motion, which may be discussed in a future post). The elevator’s motion can be broken down into three stages:

1. Start: the elevator accelerates from zero velocity (so you can step into the elevator safely) to its nominal travel speed.
2. Middle: the elevator travels at its nominal travel speed without speeding up nor slowing down.
3. End: the elevator decelerates (you can think of this as backwards acceleration) from its nominal travel speed to zero velocity (so you can step out of it and onto your destination floor safely).

The reason why you feel a force acting on your feet when the elevator starts up in the beginning and slows down at the end is because the elevator is accelerating during those stages. Newton’s second law states that force is directly proportional to acceleration. So, even if the elevator is moving very fast but at a constant speed, you will not feel an external force acting on you because there is no acceleration.

This same notion of not feeling anything different inside a moving vehicle (such as an elevator car) when there is no acceleration can also be applied to riding in a automobile on the road. When driving on the straight highway at a constant speed, you feel no external force acting on you and you sit comfortably. On the other hand, when you accelerate or decelerate your car, you feel a force pushing you back against the seat (during acceleration, or speeding up) or forward (during deceleration, or slowing down). Also, the force that you feel is directly proportional to the acceleration of the car, so if you speed up very quickly (which happens to not be very fuel-efficient) then you will feel a larger force.

Until next time, may your vehicular travels be smooth and comfortable without any jerky accelerations.