Finding final velocity from a fall

  • Thread starter lzh
  • Start date

lzh

111
0
I'm having trouble with this problem:
"A 79kg person escapes from a burning building by jumping from a window 25m above a net.
The acceleration of gravity is 9.81 m/s^2
Assuming that air resistance is a constant 88 N force on the person during the fall, find the person's velocity just before hitting the net"
So for this problem i used F=ma:
79g-88=79a
a=8.6861m/s^2
this is the correct acceleration i believe and to find final velocity i used:
Vf^2=Vi^2+2ad where d is displacement
Vf^2=2(8.6861)(25)
vf=20.85m/s
I'm using utexas's webhw service, and the answer above was not correct, can someone tell me what i'm doing wrong?
 
Last edited:
not 100% sure on this but i think:
the 79g-88N=79a is wrong the units dont cancel out properly, you know the 88N upward on the person what's the force he excerts downard?
with those two figures you can use F=ma knowing F and m to determine the actual a, and plug that into the Vf^2=Vi^2+2ad equation
 
I solved the problem by calculating the amount of downward force one the person; mainly his weight minus the resistance. Then I used that F in the work formula; W=FD. D would be the 25m. I got the W in Jules so I let that W = KE, which is kinetic Energy. so, W = MV^2. and solved for the desired quantity.
Same ans. So I suppose you’re ans is right.
 

lzh

111
0
not 100% sure on this but i think:
the 79g-88N=79a is wrong the units dont cancel out properly, you know the 88N upward on the person what's the force he excerts downard?
with those two figures you can use F=ma knowing F and m to determine the actual a, and plug that into the Vf^2=Vi^2+2ad equation
79g is his weight, which is in N, which cancels out
I solved the problem by calculating the amount of downward force one the person; mainly his weight minus the resistance. Then I used that F in the work formula; W=FD. D would be the 25m. I got the W in Jules so I let that W = KE, which is kinetic Energy. so, W = MV^2. and solved for the desired quantity.
Same ans. So I suppose you’re ans is right.
but it isn't right...
 
not 100% sure on this but i think:
the 79g-88N=79a is wrong the units dont cancel out properly, you know the 88N upward on the person what's the force he excerts downard?
with those two figures you can use F=ma knowing F and m to determine the actual a, and plug that into the Vf^2=Vi^2+2ad equation
I'm not sure wat u mean. But the unites do cancel out propely:
(790N-88N)/79 = a.
 
well N is kg*m/s^2 79kg is just the weight you're missing the acceleration (gravity)
 
Typo:
*79kg.
 

lzh

111
0
yeah g as in acceleration due to gravity
 
sry was distracted >.< i get the same answer as you.
are you providing the required number of significant digits?
 

lzh

111
0
rounding and sig figs doesn;t matter
 
out of curiosity; what kind of deceleration is applied form falling on the net for a safe landing. Too much would cause the person to recoil up. too little would cause he/she to hit the ground.
 

Doc Al

Mentor
44,660
985
I'm having trouble with this problem:

So for this problem i used F=ma:
79g-88=79a
a=8.6861m/s^2
this is the correct acceleration i believe and to find final velocity i used:
Vf^2=Vi^2+2ad where d is displacement
Vf^2=2(8.6861)(25)
vf=20.85m/s
Your method and answer look correct to me.
 

Want to reply to this thread?

"Finding final velocity from a fall" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top