Solved: Help with a block sliding down an Inclined platform

[member 744828]

Homework Statement

A 5 kg block slides down a 30° incline with an acceleration of 3.0 m/s. a) How much of a resistive force up the ramp must there be? b) What is the kinetic coefficient of friction between the block and the ramp

Relevant Equations

Fy = 50cos30 = 43.3N
Fgx= 50 sin 30 = 25 N

a=Fnet/m

Fy = 50cos30 = 43.3N
Fgx= 50 sin 30 = 25 N

a=Fnet/m
3.0 = Fnet/5
Fnet=15

Ff = m Fn
15=m (43.3)
m=0.35

 

[Orodruin]

What is your question?

 

[member 744828]

What is your question?

Is part b right? .35 for kinetic coefficient of friction.
Or should it be
Ff = m Fn
(25-15) = m (43.3)
m=0.23

 

[haruspex]

Is part b right? .35 for kinetic coefficient of friction.
Or should it be
Ff = m Fn
(25-15) = m (43.3)
m=0.23

Which do you think, and why?
Think about the relationship between $F_{net}, F_f$ and $F_{gx}$.

Please don't use the same symbol (m) to mean different things. Write mu, or, better, use LaTeX.

 

[member 744828]

Which do you think, and why?
Think about the relationship between $F_{net}, F_f$ and $F_{gx}$.

Please don't use the same symbol (m) to mean different things. Write mu, or, better, use LaTeX.

I am trying to help my daughter. I think it is:
Ff = mu *Fn
15 = mu * 43.3
mu=0.35

She believes it is the other answer.
Ff = mu*Fn
(25-15) = mu* 43.3
mu=0.23

 

[Orodruin]

That doesn’t really answer the question of why you think your answer is correct.

 

[PeroK]

I am trying to help my daughter. I think it is:
Ff = mu *Fn
15 = mu * 43.3
mu=0.35

She believes it is the other answer.
Ff = mu*Fn
(25-15) = mu* 43.3
mu=0.23

Your daughter should be helping you!

 

[member 744828]

That doesn’t really answer the question of why you think your answer is correct.

I don’t know, I haven’t done physics in many years.

 

[member 744828]

Your daughter should be helping you!

So she is right? I have not done this in a long time and trying to relearn with her notes.

 

[Orodruin]

I don’t know, I haven’t done physics in many years.

If in need of guidance, your daughter is very welcome here. It is better to have a direct connection to helpers than having a go-between that may misinterpret or misrepresent.

 

[Orodruin]

So she is right? I have not done this in a long time and trying to relearn with her notes.

Yes, she is right. Fnet is the net force, not the friction force. The relation is Fnet = 25 N - Ff. (Always use units in equations!!)

 

[member 744828]

If in need of guidance, your daughter is very welcome here. It is better to have a direct connection to helpers than having a go-between that may misinterpret or misrepresent.

I won’t misrepresent. By reading on the internet and her notes I am not sure if it should be 25-15 or 15. I would generally have her brother help but he is away at college.

 

[PeroK]

So she is right? I have not done this in a long time and trying to relearn with her notes.

Yes, she's right.

 

[member 744828]

Yes, she is right. Fnet is the net force, not the friction force. The relation is Fnet = 25 N - Ff. (Always use units in equations!!)

Thank you, that was the piece I was unsure of. Fnet = 25N- Ff
I copied and pasted from the picture and some units didn’t come over.

 

[Orodruin]

I won’t misrepresent.

Not consciously obviously. However, if the underlying understanding of the concepts is lacking, there is always a risk of misrepresentation. For example, you misrepresented the material from the book by assuming Ff = 15 N. I am sure that was not your intention, but it occurred.

We are here to help, but it is always easier to help the student directly.

 

[brotherbobby]

I don't see any drawing of a free-body diagram (FBD). You may copy and paste your own sketch on pen and paper. Better still, there are excellent drawing softwares.
I think an FBD is essential for problems of this sort, specially if you're a beginner.

 

[brotherbobby]

There you go @galibe, I draw for you the diagram of the problem using MSWord. You need to find the resistive force $\color{red}{f_R}$. It took a little more than ten minutes.

Please note that I haven't shown all the forces. Can you complete the diagram by showing them?

 

[haruspex]

I am trying to help my daughter. I think it is:
Ff = mu *Fn
15 = mu * 43.3
mu=0.35

She believes it is the other answer.
Ff = mu*Fn
(25-15) = mu* 43.3
mu=0.23

If you have some reasoning behind your answer, please state it. If you don't, why are you hampering your daughter?