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Physics momentum Question help please

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helai.j:
No one seams to answer my Questions when i post in the specific topics so, i guess no harm trying here :)

can anyone answer these 2 Questions for me please ? i dont need just the answers i want to know how they are done . thanks

1)   A movie stuntman with a mass of 90 kg stands on a stationary 1 kg skateboard. An actor shoots the stuntman with a 9 mm pistol. The 8-gram bullet leaves the pistol at 358 m s?1 and embeds completely in the stuntman’s bulletproof vest. At what speed will the stuntman roll away?


2)   In a stunt for an action movie, the 100 kg actor jumps from a train which is crossing a river bridge. On the river below, the heroine is tied to a raft floating towards a waterfall at 3 m s?1. The raft and heroine have a total mass of 200 kg.

a    If the hero times his jumps perfectly so as to land on the raft, and his velocity is 12 m s?1 at an angle of 80° to the river current, what will be the velocity of the raft immediately after he lands? Draw a vector diagram to show the momentum addition. (Ignore any vertical motion.)

b    If the waterfall is 100 m downstream, and the hero landed when the raft was 16 m from the bank, would they plummet over the fall? (Assume the velocity remains constant after the hero has landed.)

ashish:

--- Quote from: helai.j on October 09, 2010, 12:51:41 pm ---

1)   A movie stuntman with a mass of 90 kg stands on a stationary 1 kg skateboard. An actor shoots the stuntman with a 9 mm pistol. The 8-gram bullet leaves the pistol at 358 m s?1 and embeds completely in the stuntman’s bulletproof vest. At what speed will the stuntman roll away?




--- End quote ---

since momentum is always conserved

(8/1000)*358=(90+1)*V
V=0.03147m/s 

i find this speed weird !
is it right?

helai.j:
yes that one is correct, thank you :)

ashish:

--- Quote from: helai.j on October 09, 2010, 04:14:52 pm ---yes that one is correct, thank you :)

--- End quote ---
:D you are welcome

nid404:

--- Quote from: helai.j on October 09, 2010, 12:51:41 pm ---2)   In a stunt for an action movie, the 100 kg actor jumps from a train which is crossing a river bridge. On the river below, the heroine is tied to a raft floating towards a waterfall at 3 m s?1. The raft and heroine have a total mass of 200 kg.

a    If the hero times his jumps perfectly so as to land on the raft, and his velocity is 12 m s?1 at an angle of 80° to the river current, what will be the velocity of the raft immediately after he lands? Draw a vector diagram to show the momentum addition. (Ignore any vertical motion.)

b    If the waterfall is 100 m downstream, and the hero landed when the raft was 16 m from the bank, would they plummet over the fall? (Assume the velocity remains constant after the hero has landed.)

--- End quote ---

I was looking for astar's post but failed. I tried, I'm not sure but this took me 15 mins only to figure out the entire scene  ::)  :P

a) So perependicular to river's flow. Y component velocity

100X 12sin80+ 200X0= (100+200)v
v=3.94m/s
^^speed to the bank...sidestream
Parallel to river's flow. X component of velocity

100X12cos80+ 200X3= (100+200)v
v= 2.69m/s
^^speed to the waterfall...downstream
b) Time taken to reach the bank= 16/3.94 = approx 4s
    Time taken to reach the falls= 100/2.69=approx 37s

the raft will this reach the bank safely.

I'm not sure. -.-

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