# momentum question

I am doing some self study for physics and found some questions with answers from a secondary school textbook .

If a rebounding body has Force 7.5N after it hit s a wall , it has a velocity before collision of 30ms and ball weight of .15 kg  in an elastic collision, the kinetic energy of the ball due to the collision is .5mv^2?

The answer is decreases by 120j but .5mv^2? doesnt give this?
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Commented:
It sounds like something is missing from the question.
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Commented:
Need the question verbatim.
A body can have mass, energy, and momentum, but not force.
There is no change of energy in an elastic collision.
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Author Commented:
yes this is not homework from any  course .

what is given

magnitude of the momentum 7.5N
no external force acts on the ball during or after the collision
collision .08 sec

delta P=F  * delta t
mass .15kg
velocity before collision 30ms

so this is like hitting a tennis racquet shot
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Commented:
N (Newton) is not a unit of momentum.
Ns (Newton second) is a unit of momentum.
ms is not a unit of velocity
m/s is a unit of velocity.
Perhaps we are to assume a momentum change of 7.5N*.08 sec
But momentum and velocity have a direction component as well as a magnitude component,
so without knowing the relative direction of the  initial velocity and the momentum change,
we cannot know the final velocity (assuming that the final velocity is what is to be determined from the givens)
As there is no change of energy in an elastic collision, if the wall is stationary, so that its energy is constant, the magnitude of the final velocity of the ball after an elastic collision would have to be the same as the magnitude of its initial velocity.
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Author Commented:
momentum change area under a graph which is 300F X  0.5X .08=12

the object is hitting another moving object(no speed, mass given for the colliding object)

Here are the answers of which I dont get

The answer is magnitude of momentum is 7.5N/s

as a consequence of the collision of the kinetic energy of the ball in question has increased 120j

I have given all the information and an answer as nothing is missing
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Author Commented:
initial direction is not given but it says a horizontal motion of a ball which hits another moving towards it
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Commented:
N/s is not a unit of momentum, it is a unit of yank.  N*s is a unit of momentum
When you said "wall", did you mean a second "ball"?  If so, was the mass of the second ball the same as the mass of the first ball?
A change from .15kg at 30m/s to .15kg at 50m/s is an increase of 120j
.15kg can be accelerated from  30m/s to 50m/s by applying 3Ns

Where do the 300F, 0.5 and  .08 come from?
There is an F on the left of
300F X  0.5X .08=12
and no F on the right, so the equation is unbalanced.
Also, I don't know of an F that is a unit of momentum change,
so equation seems to be missing something.
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