Find speed with tension, mass given. thanks

Click For Summary

Homework Help Overview

The problem involves a child on a swing, where the tension in the chains and the child's mass are given. Participants are tasked with finding the child's speed at the lowest point of the swing and the force of the seat on the child, while neglecting the mass of the seat.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the application of circular motion principles and the forces acting on the child, including tension and gravitational force. There are attempts to rearrange equations to find speed and questions about the correctness of their approaches.

Discussion Status

Several participants have provided guidance on the forces involved and the need to consider the normal force. There is ongoing exploration of the relationships between tension, gravitational force, and centripetal force. Some participants express uncertainty about their calculations and seek confirmation of their reasoning.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information they can use or the methods they can apply. There is a noted confusion regarding the application of forces and the setup of the problem.

MissPenguins
Messages
58
Reaction score
0

Homework Statement


009 (part 1 of 2) 10.0 points
A 33 kg child sits in a swing supported by two chains, each 3.7 m long. If the tension in each chain at the lowest point is 259 N, find the child’s speed at the lowest point. (Neglect the mass of the seat.) Answer in unit m/s.
b. Find the force of the seat on the child at the
lowest point. Answer in unit N


Homework Equations


F = mv2/r


The Attempt at a Solution


Rearrange equation, and I got sqrt(Fr/m) = sqrt(259N * 3.7m)/33 kg) = 5.3888...
I got it wrong. What did I do wrong? Thanks.
 
Physics news on Phys.org
MissPenguins said:

Homework Statement


009 (part 1 of 2) 10.0 points
A 33 kg child sits in a swing supported by two chains, each 3.7 m long. If the tension in each chain at the lowest point is 259 N, find the child’s speed at the lowest point. (Neglect the mass of the seat.) Answer in unit m/s.
b. Find the force of the seat on the child at the
lowest point. Answer in unit N


Homework Equations


F = mv2/r


The Attempt at a Solution


Rearrange equation, and I got sqrt(Fr/m) = sqrt(259N * 3.7m)/33 kg) = 5.3888...
I got it wrong. What did I do wrong? Thanks.


Good Evening Miss Penguins,

You're correct that this is a circular motion problem, but you neglected some of the forces. First you need to draw a picture, and determine the free body diagram for the isolated child.

The isolated child experiences a gravitational attraction, or weight, and a contact force from the seat, the normal force N. Here's a picture to get you started:

http://img109.imageshack.us/img109/6937/swingw.jpg

Hint: The normal force N has a magnitude of 2 times the tension, since the seat is massless.
 
Last edited by a moderator:
I don't know if this is the same as already suggested, but my thought is that the forces add up to the centripetal force:
Fc = 2T - mg
 
dr_k said:
Good Evening Miss Penguins,

You're correct that this is a circular motion problem, but you neglected some of the forces. First you need to draw a picture, and determine the free body diagram for the isolated child.

The isolated child experiences a gravitational attraction, or weight, and a contact force from the seat, the normal force N. Here's a picture to get you started:

http://img109.imageshack.us/img109/6937/swingw.jpg

Hint: The normal force N has a magnitude of 2 times the tension, since the seat is massless.

so I used the wrong equation for it? Thanks.
 
Last edited by a moderator:
Delphi51 said:
I don't know if this is the same as already suggested, but my thought is that the forces add up to the centripetal force:
Fc = 2T - mg

I don't know, but for the first answer, they are asking for speed though, and the equation u suggested doesn't have speed.
 
You will of course replace Fc with mv²/R as you did in the first go.
The difference is that mg is involved.
 
Delphi51 said:
You will of course replace Fc with mv²/R as you did in the first go.
The difference is that mg is involved.

I rearranged the equation and ended up with v = sqrt [(2T-mg)r]/m = sqrt [(2(259 N) - (33kg)(9.8m/s2))(3.7)] / (33 kg) = 21.818787...
Does that look reasonable?
 
MissPenguins said:
I rearranged the equation and ended up with v = sqrt [(2T-mg)r]/m = sqrt [(2(259 N) - (33kg)(9.8m/s2))(3.7)] / (33 kg) = 21.818787...
Does that look reasonable?

That m should be inside the square root, I believe.
 
dr_k said:
That m should be inside the square root, I believe.

Yeah it is inside. I submitted the answer, and it is wrong. :(
 
  • #10
MissPenguins said:
Yeah it is inside. I submitted the answer, and it is wrong. :(


It doesn't look that way, according to the equation you wrote.

Can you prove that the magnitude of the Normal force is equal to twice the tension?
 
  • #11
dr_k said:
It doesn't look that way, according to the equation you wrote.

Can you prove that the magnitude of the Normal force is equal to twice the tension?

What do you mean by it doesn't look that way? lol...well, I put it in the calculator that way. So v = sqrt of [((2T-mg)r) / m] = sqrt of [((2(259 N)-(33kg)(9.8m/s2)(3.7m) / 33 kg] = 21.8187878787 WRONG. :(
 
  • #12
MissPenguins said:
What do you mean by it doesn't look that way? lol...well, I put it in the calculator that way. So v = sqrt of [((2T-mg)r) / m] = sqrt of [((2(259 N)-(33kg)(9.8m/s2)(3.7m) / 33 kg] = 21.8187878787 WRONG. :(

Alright, I was able to get a right thanks to a friend. I use ((259*2) - (33)(9.8)) = 194.6, and then v = sqrt of (Fr/m) => sqrt of ((194.6)(3.7)/33 = 4.671058539.



Then in part 2, it says find the force of the seat on the child at the lowest point. I have no clue how to do it. :( Please help, thanks.
 
  • #13
Two ways to look at part 2.
First, there is no mass other than the child so the tension forces are entirely applied to the child via the seat.
Second, apply "sum of Forces = ma" to the child. The only forces are gravity and the seat force.
 
  • #14
Delphi51 said:
Two ways to look at part 2.
First, there is no mass other than the child so the tension forces are entirely applied to the child via the seat.
Second, apply "sum of Forces = ma" to the child. The only forces are gravity and the seat force.

Nvm, I got it, thanks a lot anyway. :)
 
Last edited:

Similar threads

Finding the frequency of a string based on Mass and Tension
  • · Replies 1 ·
Replies
1
Views
4K
Finding out the rotational speed of a mass
  • · Replies 59 ·
2
Replies
59
Views
5K
Find Mass and Speed given Tension and Angle
  • · Replies 2 ·
Replies
2
Views
3K
with this tension problem -- Mass on an accelerating cable
  • · Replies 3 ·
Replies
3
Views
2K
Tension in a deformed cylindrical bag on a surface
  • · Replies 18 ·
Replies
18
Views
2K
Uniform Circular Motion, Find the Tension
  • · Replies 8 ·
Replies
8
Views
3K
How Is the Force on a Child in a Roller Coaster Calculated?
  • · Replies 3 ·
Replies
3
Views
7K
Finding max velocity for a kart on a circular, banked track
  • · Replies 2 ·
Replies
2
Views
2K
Find the height to which a mass rises
  • · Replies 3 ·
Replies
3
Views
1K
Finding friction and tension for a train
  • · Replies 1 ·
Replies
1
Views
2K