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Will your new MacBook crash to the ground without MagSafe? (Yes.)

Glenn Fleishman | March 13, 2015
Perhaps we should have read it as an omen that the MagSafe 2 adapter for older, original MagSafe connectors was listed as discontinued on the Apple Store in the U.S. and Canada last week. It's back in stock this week, but MagSafe's future in new Mac laptops is uncertain with the revelation of the single-port MacBook. With just a USB-C connection for power, data, and display, MagSafe may be on its way to sing with the choir invisible.

In that scenario, we know you need to apply an initial force of between 2N and 50N depending on materials and assumptions to cause the MacBook to reach an acceleration rate of the magical 1m/s2, but then less force thereafter. Since we need to pull at 8N to 20N to remove the connector, this is clearly ambiguous. Will the plug have pulled out of the jack by the time static friction has been overcome?

And there's a wrinkle. If you accelerate faster, you exercise greater force. That is, take 1 kilogram to 2m/s2, and the force is 2N. Thus, take the action noted above of someone's leg intersecting with the cable, and assume the mass of the leg involved in the force is 10kg. That's a tenfold multiplier against any acceleration produced.

The cable's tip and socket are supposed to measure just 6mm. Assuming the USB extraction force's upper end, 20N, is the static friction and 8N is the kinetic friction, it's possible that the cable pops free in a fraction of a second before the laptop moves much, if at all.

Dr. Drang explained to me further:

What happens to the MacBook when you pull on the cable has less to do with the force you apply than the rapidity with which you apply it. If you pull on the cable with constant speed, or with any acceleration less than 4.8 m/s2, the connector will never come out, and you'll dump the MacBook on the floor. If you pull on the cable with an acceleration greater than 17.8 m/s2, the connector will come out and you'll heave a sigh of relief.

Because the USB Implementors Forum describes a wide range in the spec, until and if Apple's specific USB-C adapter is tested across multiple computers for extraction force, it's impossible to know the necessary acceleration.

Let's also look at cases with shear, where the cord is tripped over or yanked at any angle or orientation. Mack noted, "Pulling out the cable cleanly would be very difficult to do by accident. If you're pulling at any angle other than straight on, it's quite difficult, as the force is now directed in a direction that would increase friction a lot."

Dr. Drang and Mack both said that if the laptop was free enough to be dragged off a surface, that shear could be reduced by the laptop rotating first to a less oblique horizontal angle. However, there's still the matter of the angle from the MacBook to the point on the cable where force is being applied. That's likely vertical, a drop from the plug's slot to the floor where someone's foot or leg catches on the cable.

 

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