Hi, need some help with friction

In summary, the conversation discusses finding the force of friction acting on a curling rock. Through calculations, it is determined that the friction force is 3.8N, the normal force is 186N, and the coefficient of friction is 0.02. The conversation also clarifies that the only forces acting on the rock are weight, normal force, and friction force.
  • #1
I_BeLeaf87
4
0
I have this assignment to do and I am stuck on finding the force of friction acting on a curling rock.

The mass is 19Kg and the acceleration is -0.2m/s^2

I calculated the force applied using F = ma

This gave me -3.8N

I let negative rep force applied
positive rep force of friction

Therefore I have 3.8N [forward]

I am not sure if I am right from here on. I think I calculated the normal force or so.

I used

Fn = ma + mg

My answer for the force of gravity was 190N so then I added the 2 together and got 186.2 since the Force applied was negative. What I find awkward is that 186.2 N is the actual Force of gravity but due to sig digits, I rounded to 2 sig digits. It is here I am lost. How will I be able to calculate the force of friction?

I don't know the co-effecient of friction either. We also need to solve for that but I think I know how to once I solve for friction. Any help will be appreciated.
 
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  • #2
You didn't say this explicity, but it seems like the rock is sliding and no other forces are acting on it. If that's the case, then the only force on the body is friction: Fnet = ma = f. The negative acceleration implies force opposing the motion, so the magnitude of the friction force is 3.8 N, the force you calculated.

The normal force in this problem is equal to the weight; check a free body diagram if you don't see this: Fnet = N - mg = ma = 0 (it doesn't move up or down). (Looking over what you wrote, your equation is right, you just have to recognize that a = 0 in the vertical direction.)

Modeling this as kinetic friction, use the fact that f = μN to solve for μ, the coefficient of friction.
 
  • #3
OK I don't get the part where you are explaining

Fnet = N - mg = ma = 0

N signifies Newtons, right? As in 3.8N, right?

I did this then

Fnet = 3.8N - 186.2 = -182.4 = 0

How does it equal zero?

Or is it

Fnet = 3.8N - 190 = -186.2 = 0 and that means that the 186.2 will cancel each other out.

I also tried to figure out the coeffecient of friction, I came up with 0.02, I am not sure if I am correct. I am not too good at this stuff. Dynamics is my weakness.

BTW, thank you very much for your help
 
  • #4
Originally posted by I_BeLeaf87
OK I don't get the part where you are explaining

Fnet = N - mg = ma = 0

N signifies Newtons, right? As in 3.8N, right?

My apologies for the ambiguity. N is the normal force. You called it FN and I should have followed suit (or at least explained the notation). Does that clear it up?
 
  • #5
Oh ok, yes it does help. I can see where you get the acceleration to be equal to zero.

So

186.2 - 186.2 = ma = 0

So how do I solve for friction?


OK hold on. To calculate the coeffecient of friction I will have to divide 3.8N/186.2N to give me 0.02 as the coeffecient. It's the friction part that's confusing me.

I'm sorry if I seem like a pain.
 
  • #6
No problem.

Your whole last paragraph is right. Summarizing the milestones of the problem:

Friction force = f = (19 kg)*(0.2 m/s/s) = 3.8 N

Normal force = N = mg = (19 kg)*(9.8 m/s/s) = 186 N

coefficient of friction = μ = f/N = 3.8/186 = 0.02

How's that?

There are 3 forces acting on the body: the weight, the normal force (that balances the weight) and the friction, opposing the motion (decelerating).
 
  • #7
Oh I was not even thinking that the friction force was 3.8 N. I was thinking that is was the force applied or something. I get it now. Thank you very much for your help, I really really appreciate it
 

1. What is friction?

Friction is a force that resists the relative motion between two surfaces that are in contact. It is caused by the microscopic irregularities on the surfaces of objects.

2. How does friction affect motion?

Friction can slow down or stop the motion of an object by converting its kinetic energy into heat. It also helps to provide traction and stability for objects to move on surfaces.

3. What are the different types of friction?

There are four types of friction: static, sliding, rolling, and fluid. Static friction occurs when two surfaces are not moving relative to each other, sliding friction occurs when two surfaces are sliding against each other, rolling friction occurs when an object is rolling on a surface, and fluid friction occurs when an object moves through a fluid.

4. How can friction be reduced?

Friction can be reduced by using lubricants, such as oil or grease, between two surfaces to decrease the amount of contact and reduce the friction force. Smooth surfaces and streamlining can also help to reduce friction.

5. What are some real-life examples of friction?

Examples of friction in daily life include walking on the ground, rubbing your hands together to generate heat, using brakes to stop a vehicle, and writing with a pen or pencil. Friction is also important in sports, such as skiing, where it helps to slow down the movement of the skis on the snow.

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