help with momentum

Question

if you wanted to increase the momentum of an object how could you go about doing this?

 

a.  increase its mass    (I say yes to answer a)

b.  excert a greater force on the object in the direction in which it is moving

c.  increase the time that a force acts on the object in the direction which it is moving

d.  any of these

 

 

if you wanted to decrease the momentum of an object how could you go about doing this

 

a.  decrease its mass (I say yes to answer a)

b.  exert a force on the object in the direction opposite to which it is moving

c.  increase the time that a force acts on the object in the direction opposite to which it is moving

d.  any of these

 

thanks

Answer 1

Hey L,

 

Looks like some conceptual questions on momentum, or the change in momentum! Let's look at this Newtonian equation:

 

FΔt = Δp

 

Let's keep in mind that force and momentum are both vectors, so they have magnitude and direction. Let's rewrite this equation as:

 

FΔt = pf - pi

 

pf = FΔt + pi

 

pf = FΔt + mvi

 

For the first part of this question, to increase the momentum (this means "pf" goes up), we want to see how changing either the force, time interval, or mass will affect the final momentum. Since force and momentum are vectors, if we apply a force in the same direction as the momentum, we add the vectors together, which will increase the momentum. Also increasing the time interval in which the force is applied also makes the final momentum greater, so any of the following for the first part will increase the momentum.

 

For the second part of the question, we want to decrease the momentum. If we look at the equation for final momentum and understand that the vectors for force and initial momentum at this point are in opposite directions, they will have opposite signs in the equation. This means if the force is the opposite direction of momentum, that the force will work to decrease the momentum. Since the term is FΔt, we notice also that if we increase the time that this force acts in the opposite direction, that this term will become more negative and also decrease the momentum. And lastly, we also know that decreasing the mass will also decrease the momentum, so the answer is also "any of these" for the second part.

 

I hope this helps.

 

Alex