Calculating Kinetic Friction Coefficient for Sliding Soccer Player

[kdrobinson]

[note from mentor: this was originally posted in a non-homework forum, so it lacks the homework template.]

Hi guys,
I need your help with the last part of following problem please.
A soccer player with 70kg mass slides for 0.75 seconds on the grass to stop a soccer ball.

1) Acceleration (magnitude & direction) of the player during the slide if initial speed of the player is 5 m/s and final speed is 2.2 m/s? How far did the player slide in the 0.75 seconds?
My answer: a= -3.7 m/s^2 (sliding in the -x direction) for 2.7m.

2) What is the coefficient of kinetic friction between the player and the ground in these condition?
I can't figure this part out to save my soul. Can any of you help please?

 

[Orodruin]

a= -3.7 m/s^2 (sliding in the -x direction)

If you have a negative acceleration while traveling in the negative direction, then you are speeding up. However, your answer is only off by a sign.

2) What is the coefficient of kinetic friction between the player and the ground in these condition?
I can't figure this part out to save my soul. Can any of you help please?

What have your thoughts been? How can you relate the coefficient of friction to the acceleration of the player? (Suppose you were only given the coefficient of friction, how could you find the acceleration?)

 

[kdrobinson]

If you have a negative acceleration while traveling in the negative direction, then you are speeding up. However, your answer is only off by a sign.
What have your thoughts been? How can you relate the coefficient of friction to the acceleration of the player? (Suppose you were only given the coefficient of friction, how could you find the acceleration?)

I'm really not sure. I've looked at this so long, that I'm not seeing a relationship. Hint please?

 

[kdrobinson]

I'm really not sure. I've looked at this so long, that I'm not seeing a relationship. Hint please?

Good catch on the negative sign...thank you.

 

[Orodruin]

how do you relate the acceleration to the force acting on a body?

 

[kdrobinson]

how do you relate the acceleration to the force acting on a body?

Is it mu(mg)

 

[Orodruin]

That is the frictional force, but it is not what I asked. I asked for how that force is related to the acceleration of the player. In other words, given that the player has an acceleration 3.7 m/s^2, what force must be acting on the player?

 

[kdrobinson]

That is the frictional force, but it is not what I asked. I asked for how that force is related to the acceleration of the player. In other words, given that the player has an acceleration 3.7 m/s^2, what force must be acting on the player?

Unless it's gravity, I have no clue.

 

[Orodruin]

I am not asking for where the force comes from. I am asking for a simple mathematical relationship between the resultant force and the acceleration. It should have been mentioned in your very first mechanics class and is one of the fundamental assumptions in classical mechanics.

 

[kdrobinson]

I am not asking for where the force comes from. I am asking for a simple mathematical relationship between the resultant force and the acceleration. It should have been mentioned in your very first mechanics class and is one of the fundamental assumptions in classical mechanics.

I'm sorry. That fundamental class was 25 years ago. I'll keep digging for the relationship, but I honestly do not have a clue. I've been working on this one problem for 4.5 hours.

 

[Orodruin]

Then I would suggest that a refresher on classical mechanics is needed in order to be able to solve this type of problems. The relation you need is so fundamental that you will get absolutely nowhere in kinematics problems without it.

Let me ask you this: If you push a 3 kg mass with a force of 15 N, what is the acceleration of the mass? What law did you use to obtain your result?

 

[kdrobinson]

Then I would suggest that a refresher on classical mechanics is needed in order to be able to solve this type of problems. The relation you need is so fundamental that you will get absolutely nowhere in kinematics problems without it.

Let me ask you this: If you push a 3 kg mass with a force of 15 N, what is the acceleration of the mass? What law did you use to obtain your result?

Newton's 2nd Law

F = ma --> a = F/m
a = 15N/3kg = 5m/s^2; Correct?

 

[Orodruin]

Yes. So what prevents you from applying Newton’s second law to your problem?

 

[kdrobinson]

Yes. So what prevents you from applying Newton’s second law to your problem?

Ahhhh... got it.. thank you very much!