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Old 01-24-2023, 01:34 PM   #5 (permalink)
redpoint5
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Quote:
Originally Posted by Cd View Post
What explains the continued momentum of objects inside the truck as brakes are applied ? ( the example of my relative with the van )
The objects within the vehicle are slowed by the application of the brakes, since they are contained in the body being slowed.
But yet they continue to fly forward..
That's Newton's 2nd law of motion, that an object in motion stays in motion unless acted upon by a force. All of the individual parts of the truck that are securely attached can't move, so when the tires start slowing the truck as the brakes are applied (the tires push forward on the road surface), all of the firmly attached parts push on the the part they are attached to.

As the vehicle slows, the rear bumper pushes on the brackets that support it, which pushes on the frame, which pushes on all the other material of the truck. All of the objects want to continue on their original trajectory, but now that the tires are pushing forward on the road, the vehicle pushes backwards in proportion.

Quote:
I'm guessing the movement of mass from gravity might give a slight boost in coasting ? ?
Think of a semi with a trailer full of loose cargo.
If it suddely coasted downhill, and the load slid to the front of the truck, would it do anything due to ( gravity ? )
This is harder for me to conceptualize and explain.

If the cargo started out in the front of the truck as it started up the hill, it would all slide back and slightly slow the truck as it hit the back wall. Essentially the truck didn't "know" there was mass in it until it collided with the wall and started increasing the objects potential energy by hauling it up the hill.

When the truck heads downhill, the weight slides forward, hitting the forward wall of the truck and transfers that potential energy into a tiny bit of speed absorbed by the truck.

The net result of the slowing down and speeding up is exactly the same. There simply is no passive way to gain energy, as that would be a perpetual motion machine.

Changing the velocity of an object (either the speed or direction of travel, or both) requires an expenditure of energy.

If that's not confusing enough, the really interesting thing is that if a truck hits a brick wall, the rear of the vehicle doesn't "know" that until the speed of sound in the materials of the truck reach it.

Here's an example of a slinky being dropped. The bottom of the slinky doesn't "know" it's been dropped until all of the molecules above it have begun to move.


I used to think maybe we could communicate faster than light using physical objects. Imagine a steel rod extending from Earth to the moon. I tell the person on the moon that I will signal them by pushing on my end of the rod, and they see it move on their end. Turns out the speed of sound in steel is 5,100 m/s. It would take almost 21 hours from the time I pushed until they saw their end move. Light on the other hand only takes 1.25 seconds.
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Last edited by redpoint5; 01-24-2023 at 02:25 PM..
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