Gravity and Stuff Inside a Thing

Classical Mechanics Level 2

A truck is driving on a road on Earth. Inside the truck is a driver and a large crate of cargo. The cargo is secured to the truck bed. The driver is not wearing a seatbelt, and can move freely. Suddenly, the truck hits a large bump in the road, accelerating the whole truck upward, throwing the driver into the air, but still inside the truck.

Question: At the moment the driver is suspended in the air inside of the truck, does the truck's total mass still include the driver?

Yes. The driver is still enclosed inside of the truck, and so her mass is included. Impossible to determine. No. The driver is momentarily weightless, and her mass is not included.

Jim Bell by Jim Bell , 52, USA

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1 solution

Jim Bell
Dec 11, 2020

When astronauts returned to Earth with moon rocks, although weightless in space, the spacecrafts' mass increased due to the extra rocky cargo.

However, I do not know how this works. For example, think about how close a thing must be before its mass merges with another thing. Why does just being enclosed, but not in physical contact (as with the space rocks), add to the total mass of the containment vessel?

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You must use frames of reference to address this issue The inside of a truck is a frame of reference and the mass of everything inside it equals the total mass.

Vijay Simha - 6 months ago

I don't agree.

In the space case, from long period perspective, astronauts, rock and spacecraft always keep the same movement, so all get the same acceleration.

but the truck case can be different. at the moment when the driver in the air, if the truck gets another bump, then no impact on the driver as the force can't be transferred to her. Then how we say the total mass should include hers?

Hongqi Wang - 5 months, 4 weeks ago

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