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Name:  Kiran
Status:  other
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Hi,

Does a moving body exert lesser weight on the ground ,than what it would have exerted when it was stationary? In other words does the weight of a moving body appear to be lesser than what it is when stationary?

EX: A wooden plank accross two walls would bear the weight a 40kg Bicycle if it moves accross that wooden plank very fast.But if the bicycle stands stationary on the plank,The plank breaks.. why is this?
            PLANK
         ---------------------------
            |                   |
            |                   |
            |                   |


(Hypothetically if there is a long weighing scale spanning for 10 miles,and a 1000 pound car is moving on the scale at 150 miles per hour, does the scale read lesser weight? --assuming that the weighing scale has no errors/limitations.)


Replies:
There is no reason for the moving bicycle to exert more force down on the plank when it is still compared to when it is moving (as long as it is not at the top of a curving trajectory).

More likely, the reason the plank does not break under the moving bike is a matter of time. The plank under the still bike probably does not break instantly; it will take a few seconds for it to bow to the breaking point, when the wood fibers separate from each other and break. If the bike is moving fast enough, it will not be applying the force to the center of the plank long enough for this to happen.

So on to your second question about the car on a scale: If the weighing pan of the scale really is ten miles long, it should weigh the moving car correctly. If the pan of the scale were a more reasonable length, it would depend on how the scale works. If it measures weights by compression of a spring or countering a reference weight, the car would have to be on the pan long enough for the springs to compress or for the weights to move to their equilibrium displacements.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


Assuming speeds much less than the speed of light, i.e. no relativistic effects the MASS of a body moving at constant speed is fixed. It is WEIGHT, which is the force exerted by gravity is the [mass * acceleration of gravity] which in the example you give is also essentially constant. The reason a speeding car might make it across the plank whereas a stopped car will not is caused by the fact that mechanical failure is often time dependent. That is it takes time for the structure to fail. So within limits the speeding car may beat the failure whereas a slow or stopped car will not.

Vince Calder


Ignoring the effects of lift (such as for an airplane) a moving object weighs the same as a stationary object. The plank breaks when the cyclist is stationary on it because it first has to deform to the breaking point and then it breaks. If you move across it very fast it doesn't have time to deform that far.

Greg Bradburn



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