How Is Impulse Calculated in Collision Scenarios?

[matadorqk]

Homework Statement

Before a collision, a 25-kg object is moving at +12m/s. Find the impulse that acted on the object if, after the collision, it moves at:
a. 8.0m/s
b.-8.0m/s

Homework Equations

(c)=changes in:
To find impulse, we would use the F=ma, so F=m*(c)v/(c)t
So, F(c)t = m(c)v
Therefore, F(c)t=p2-p1 (impulse-momentum theorem).

The Attempt at a Solution

Ok, so here's what I did. Let's solve A first.
GIVEN:
m=25kg
v1=12m/s
v2=8m/s
F=25*(8.0-12)/(c)t
F= -100/(c)t
So, we take the difference in time to the other side.
F(c)t=-100
Therefore, its impulse is -100. At first I got confused because I had left two variables, but I think that's the impulse because F(c)t=p2-p1, so:
p2-p1=-100
25(8)-(25)(12)=-100
-100=-100
Im assuming that's the impulse, so its -100 N*s.

So, let's do this for B, which is -8.
GIVEN:
m=25kg
v1=12m/s
v2=-8.0m/s
F=?
F(c)t=m(v2-v1)
Impulse=25(-8-12)
Impulse=25(-20)
Impulse=-500
Seems to make sense, because to make the object that's moving go on its opposite direction it must be really strong. And also, both answers are negative as it moves in the opposite direction.

I would appreciate if you could check over my work to see if it is correct.

 

[Chi Meson]

The answer is 400 Ns.

While you are correct, initially, in looking at the problem, the impulse you can see by looking at the initial and final velocities, that the (c)v [to use your symbols] is -16 m/s.

Impluse IS F(c)t, so the answer is 25 kg * (-16m/s)

m

 

[matadorqk]

The answer is 400 Ns.

While you are correct, initially, in looking at the problem, the impulse you can see by looking at the initial and final velocities, that the (c)v [to use your symbols] is -16 m/s.

Impluse IS F(c)t, so the answer is 25 kg * (-16m/s)

m

For Problem A, the c(v) is V2-V1, so 8-12 would equal -4.
For Problem B, the c(v) is V2-V1, so -8-12 would equal -20. Where did you get -16? Did you use -20-(-4)? Wouldn't that be the change of the change in velocity?

 

[Chi Meson]

For Problem A, the c(v) is V2-V1, so 8-12 would equal -4.
For Problem B, the c(v) is V2-V1, so -8-12 would equal -20. Where did you get -16? Did you use -20-(-4)? Wouldn't that be the change of the change in velocity?

Sorry, I misread the question. I had thought it was "8m/s to -8m/s." Your answers are correct, but still you did far more work than necessary.

 

[matadorqk]

Hehe

Im in my first year of physics so I am really careful step by step, hehe! I handed in the homework today so thank god the answer was right hehehe! thanks soo much for your help! :)

'better safe than sorry'