involving force and Newtons laws

In summary: She falls asleep and dreams that she is a 100 kg fireman being lowered from a floor 10 meters above the ground by a rope whose breaking strength is 800 N. In her dream, she knows that the 800 N limit must be adhered. When she starts from rest, the person loewring her knows that the 800 N limit must be adhered. But when she gets to the ground, she feels a force that is greater than the 800 N limit. In her dream, she finds out that the force is the work done by the rope to keep her from falling. When she wakes up, she realizes that this is her dream and she writes it down. When she finishes her work, she goes
  • #1
Cicima
20
0
I got this problem as part of a long homework packet, was able to figure out all the problems but the last 3, so here is one of them, but i don't have a book. I really have no idea on how to approach this problem, so i was hoping someone could lead me through it and help me figure it out...

A 100 kg fireman is being lowered from a floor 10 meters above the ground by a rope whose breaking strength is 800 N. The fireman starts from rest and the person loewring the fireman knows that the 800 N limit must be adhered.
Draw a free body diagram showing all of the forces extered on the fireman.
What is the fireman velocity when he touches the ground?
The fireman moves 20 cm during his stopping. What is the average normal force exerted on the fireman during the stop?
I don't really know where to start, not sure of the equation i need to find the fireman velocity and how to use the given numbers. As forthe diagram, i drew one showing 100kg of his wieght pushing down, the ropes strength which is 800N while he's getting lowered. So what now? Please help me through this problem, i don't have a book and have looked online for formulas and similar problems but can't find any.
 
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  • #2
Can someone please help me do this problem?

So far i got...
the fireman is accelerating such that ma=mg-800N
i think i have to find a, but i don't know how to, can you guys help me find it?
after i find a, i can find v by doing d=v^2(2a) where D is 10m, but i don't know how to find a so that's olding be back.
after i find v, the aveerage force of impact is mv^2/2=F*d, where d=20cm
So i guess my questions are how do i find acceleration and am i right with what my other work is? I am hoping someone can help so i can finally finish this and get some sleep. :zzz:
 
  • #3
Cicima said:
I got this problem as part of a long homework packet, was able to figure out all the problems but the last 3, so here is one of them, but i don't have a book. I really have no idea on how to approach this problem, so i was hoping someone could lead me through it and help me figure it out...

I don't really know where to start, not sure of the equation i need to find the fireman velocity and how to use the given numbers. As forthe diagram, i drew one showing 100kg of his wieght pushing down, the ropes strength which is 800N while he's getting lowered. So what now? Please help me through this problem, i don't have a book and have looked online for formulas and similar problems but can't find any.
100kg weight is about 1000N (Weight=mg, I used g =10 because I'm tired too.) 1000 down, and 800 up is the max, that's a net of 200 down, right? F_net=ma, per Newton2. 200 = 100a. a=2. Examine the forces and write the right equation. Now let's get some zzzz's...
 
  • #4
It looks like you are able to use energy methods, that's probably all you need to solve this problem.

If the fireman were in free fall, then all of the potential energy would be converted to kinetic energy, by the conservation law, so that

K_final - U_Start = 0

However, there is an external force here, which is the rope. The rope is doing work, it is actually bleeding energy out of the system that would have gone into kinetic energy. So the system is losing energy. So...

K_final - U_Start = ?

If you can figure that out, then you can solve for the velocity.

Speaking of sleep.. What a good idea. Hope this is enough to help you solve it now.

Good night,
Dorothy
 

1. What is force?

Force is a push or pull that is applied to an object. It can cause an object to move, change direction, or change shape.

2. What are Newton's laws of motion?

Newton's laws of motion are three fundamental principles that describe how objects behave when a force is applied to them. The first law states that an object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an external force. The second law states that the force applied to an object is equal to its mass multiplied by its acceleration. The third law states that every action has an equal and opposite reaction.

3. How do forces interact with each other?

Forces can interact with each other in a variety of ways. When two forces act in the same direction, they are added together. When two forces act in opposite directions, they are subtracted from each other. Forces can also cancel each other out if they are equal and opposite in direction.

4. How are forces measured?

Forces are measured in Newtons (N), named after Sir Isaac Newton. One Newton is equal to the force needed to accelerate one kilogram of mass at a rate of one meter per second squared.

5. How do Newton's laws apply to everyday life?

Newtons's laws of motion can be observed in everyday life. For example, the first law can explain why we wear seatbelts in a car to prevent us from continuing to move forward if the car suddenly stops. The second law can explain why it takes more force to push a heavy object than a lighter one. The third law can be seen when you push off the ground to jump, your body exerts a force on the ground and the ground exerts an equal and opposite force on your body, propelling you into the air.

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