in desperate need of help for physics... PLEASE help!!?

Question

A 89 kg fullback, running at 8.2 m/s, collides in midair with a 129 kg defensive tackle moving in the opposite direction. Both players end up with zero speed.

(a) What was the fullback's momentum before the collision?

(b) What was the change in the fullback's momentum?

(c) What was the change in the tackle's momentum?

(d) What was the tackle's original momentum?

(e) How fast was the tackle moving originally?

Answer 1

Mass of the fullback, m1=89 kg
Speed of the fullback, v1=8.2m/s
Mass of the tackle, m2=129 kg
Speed of the tackle, v2=?

(a) Fullback's momentum before the collision=m1*v1
=(89*8.2)
=729.8 kg m/s.

(b) Change in fullback's momentum = (Final momentum) -
(Initial momentum)
=m1*0 - m1*v1
=(0-729.8)kg m/s
= -729.8 kg m/s

(c) From the Law of Conservation of Momentum,
(momentum of the system before the collision)=(momentum of the system after the collision).

That is, m1*v1 + m2*v2 = m1*0 + m2*0
i.e., 729.8 + 129*v2 =0+0
i.e., 129*v2=-729.8
i.e., v2=(-729.8 / 129) m/s

Change in tackle's momentum = (Tackle's final momentum) -
(Tackle's initial momentum)
=m2*0 - m2*v2
=0 - 129*(-729.8 / 129)
729.8 kg m/s

(d) Tackle's original momentum = m2*v2
=129*(-729.8 / 129)
= -729.8 kg m/s

(e) Tackle's original (initial) speed = v2
= (-729.8 / 129) m/s
= 5.657 m/s

Wishing you all the best in this wonderful Science of Physics. Best of luck !

Answer 2

Momentum = mass x velocity, although it seems as if whoever stated this question is instead using it to mean mass x speed. The difference is that velocity has a direction, technically, and speed is just the magnitude of the velocity.

After the collision both have zero speed and hence zero velocity. Zero times anything is zero, so they both also have zero momentum.

Change in each guy's momentum is just his old momentum minus his new momentum.

Now, let's go back to the notion that momentum has a direction. The SUM of the momentums of the two guys after the collision is 0. Assuming what is called a "perfectly elastic" collision, conservation of momentum tells us that this sum was also zero before the collision. I.e., they had momentums of equal magnitude, but in opposite directions, which canceled out when they ran into each other.

You should be able to take it from here yourself!