# Momentum and force vectors

## Homework Statement

Which of the following statements are true? (Select all that apply.)
1

a. If the net force points in the direction of motion, the object speeds up. If the change in

b. momentum vector points opposite the direction of motion, the object slows down.

c. The net force always points in the direction of the motion (i.e, the direction of the momentum vector).

d. If an object moves with constant momentum, there are no forces acting on the object.

e. If the change in momentum vector points opposite the momentum vector, the object speeds up.

f. The change in momentum vector always points in the direction of the net force.

dP = Fnet*dt

## The Attempt at a Solution

a, b, d, f

When i submitted this answer I was incorrect and I am not sure why.

collinsmark
Homework Helper
Gold Member
If there are no forces at all acting on an object, then the net force on that object is obviously zero.

But if there are multiple forces acting on an object, and the [vector] sum of all those forces is zero, then the net force is zero. But that doesn't necessarily mean that there are no individual forces acting on the object at all.

Go through the statements again, paying close attention to the specific wording, and keeping the above idea in mind. If there are no forces at all acting on an object, then the net force on that object is obviously zero.

But if there are multiple forces acting on an object, and the [vector] sum of all those forces is zero, then the net force is zero. But that doesn't necessarily mean that there are no individual forces acting on the object at all.

Go through the statements again, paying close attention to the specific wording, and keeping the above idea in mind. I tried a,b,d - but it was also incorrect - I don't understand what I am missing. The object has constant momentum, which indicates there are no forces acting on it.

Is it possible for the momentum vector to point opposite the direction of motion?

collinsmark
Homework Helper
Gold Member
I tried a,b,d - but it was also incorrect - I don't understand what I am missing.
The object has constant momentum, which indicates there are no forces acting on it.
Consider an object sitting on the ground (flat), completely at rest. There are a couple of forces acting on it as it sits there. There is the force of gravity, mg, and there is also the normal force in the opposite direction. So there are individual forces acting on the object. What is the object's change in momentum as it sits there motionless?

(Hint: when analyzing the statements in the original problem, it's important to pay close attention to the specific wording. )
Is it possible for the momentum vector to point opposite the direction of motion?
You should be able to answer this one yourself. =========================================

Beyond that, if you want specific help, tell us your thoughts and reasoning on each statement individually. Then I can offer help on individual statements (but you must show that you've put effort into each statement. This is per the forum rules).

How is my logic?

a. If the net force points in the direction of motion, the object speeds up.

True, if a force is applied in the direction of motion (F=ma) the mass stays the same, so the acceleration of the object must increase.

b. If the momentum vector points opposite the direction of motion, the object slows down.

False. This is impossible. The momentum vector always points in the direction of the velocity.

p=(ym)*v (v and p being vectors)

c. The net force always points in the direction of the motion (i.e, the direction of the momentum vector).

False, if an object is slowing down, it is possible that the net force is being applied in the opposite direction of the motion.

d. If an object moves with constant momentum, there are no forces acting on the object.

False, it is possible that there are forces, but they are canceling each other out.

e. If the change in momentum vector points opposite the momentum vector, the object speeds up.

False, if the change in momentum is opposite the motion, this means the object is losing momentum, which means it is slowing down.

f. The change in momentum vector always points in the direction of the net force.

True, if the object is losing momentum (- change in p) a net force is being applied in the direction of the loss. Likewise, if an object is gaining momentum (+ change in p) a net force is being applied in the direction of the gain.

collinsmark
Homework Helper
Gold Member
a. If the net force points in the direction of motion, the object speeds up.

True, if a force is applied in the direction of motion (F=ma) the mass stays the same, so the acceleration of the object must increase.
Yes. Well, I'd be careful how you say that though. The acceleration is in the same direction of the net force. So if a "positive" net force is applied, the acceleration is also "positive" (But not necessarily changing -- it's the velocity that does the changing after the force is applied).

But yes, I think you have the right idea in general. b. If the momentum vector points opposite the direction of motion, the object slows down.

False. This is impossible. The momentum vector always points in the direction of the velocity.

p=(ym)*v (v and p being vectors)
Okay, step back a little on this one, and be careful about the problem statement. It seems that the first part of the statement b is actually attached to the end of statement a. It's just confusing the way the the statements are separated. (Take a look at the very end of statement a.)

I see statement b. as,
b. If the change in momentum vector points opposite the direction of motion, the object slows down.​

So the question isn't about the momentum vector itself, but rather the change in momentum vector. At least that's how I interpret it.
c. The net force always points in the direction of the motion (i.e, the direction of the momentum vector).

False, if an object is slowing down, it is possible that the net force is being applied in the opposite direction of the motion.
Fair enough. d. If an object moves with constant momentum, there are no forces acting on the object.

False, it is possible that there are forces, but they are canceling each other out.
There ya go. That's the one I was talking about with the tricky wording. It's true, that if the net force on an object is zero, its momentum is constant. But the statement isn't addressing the net force here, but rather it addresses any force (regardless of whether forces might cancel). e. If the change in momentum vector points opposite the momentum vector, the object speeds up.

False, if the change in momentum is opposite the motion, this means the object is losing momentum, which means it is slowing down.
Very nice. f. The change in momentum vector always points in the direction of the net force.

True, if the object is losing momentum (- change in p) a net force is being applied in the direction of the loss. Likewise, if an object is gaining momentum (+ change in p) a net force is being applied in the direction of the gain.
Works for me! So now just take a look at statement b., realizing that the first part of statement b is somehow stuck at the end of statement a.

And welcome to Physics Forums!

b. If the change in momentum vector points opposite the direction of motion, the object slows down.

True. If the change in momentum vector points opposite the direction of motion, then the object is losing momentum, which means it is slowing down.

collinsmark
Homework Helper
Gold Member
b. If the change in momentum vector points opposite the direction of motion, the object slows down.

True. If the change in momentum vector points opposite the direction of motion, then the object is losing momentum, which means it is slowing down.
'Sounds just fine to me. So if I'm not mistaken, your present answer is: a, b, and f.

Does that give you the correct answer?