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Eddie_boy · Oct 21, 2009, 07:37 PM

Okay, so I have an object and they tell me to first find the net force, I know net force means the sum of all forces, they say that the mass of the object is 1.0kg, and it travells ant 5.0m/s and that it enters a region of ice where the coefficient of kinetic friction is 0.10(that part confuses me:confused:) they tell me to find the distance the object travels before coming to a halt. So I drew a diagram with all the forces Fg-gravity Fa-acceleration Ff-friction Fn-natural how do I find the Net Force? What is the natural force(if there is any)

I added the Fa+Ff and got 4.9 do I include Fg and Fn?

Please and thank-you

Nhatkiem · Oct 21, 2009, 07:55 PM

Originally Posted by Eddie_boy

okay, so i have an object and they tell me to first find the net force, i know net force means the sum of all forces, they say that the mass of the object is 1.0kg, and it travells ant 5.0m/s and that it enters a region of ice where the coefficient of kinetic friction is 0.10(that part confuses me:confused:) they tell me to find the distance the object travels before coming to a halt. so i drew a diagram with all the forces Fg-gravity Fa-acceleration Ff-friction Fn-natural how do i find the Net Force? what is the natural force(if there is any)

i added the Fa+Ff and got 4.9 do i include Fg and Fn?

please and thank-you

Lets try to tackle this one question at a time.

Its good you know the net force is just the sum of all the forces, that's great! It seems though you are confused as to the DIRECTION of these forces.

We know it is traveling at a speed of 5m/s as soon as it enters the platform of ice. The coefficient of friction that is told is what ultimately gives us the information of kenetic friction, which will stop this 1 kg object :D.

On your diagram, you should have $F_g$ pointed down, $F_k$ pointed in the opposite direction of the velocity vector! (This implies the negative acceleration where $F_k$ is the force of friction). And its not the "Natural" force, it is the Normal force, usually this is the force that is opposite of gravity (This is what allows an object like a ball to bounce). $F_n$ should point in the opposite direction of $F_g$ .

Now using $F_k=\mu_kF_n$ to find the decelerating force where $\mu_k$ is the co efficiency of kinetic friction.

Then you can use $F_k=ma$ to figure out what the acceleration is, and use the your kinetics formulas to find out the stopping distance!

When you have finished the above, you should know $a,v_o,v_f$ , just find $(x_f-x_o)$ (This is your displacement or stopping distance).

If you need more help, just post a reply and I'll see what I can do.

Eddie_boy · Oct 21, 2009, 08:05 PM

Okay, so you say Fn (normal force, my bad) equals -9.8?
And to find the net force I add all the forces together, right?

Nhatkiem · Oct 21, 2009, 08:10 PM

Originally Posted by Eddie_boy

okay, so you say Fn (normal force, my bad) equals -9.8?
and to find the net force i add all the forces together, right?

Acceleration is not a force ;). If $F_g=mg$ , then $F_n=-F_g$
So we get $F_n=-mg$ (Note that g is negative, so -g is positive!)

And yes you add $F_n+F_g+F_k$

Eddie_boy · Oct 21, 2009, 08:18 PM

Originally Posted by Nhatkiem

Acceleration is not a force ;). If $F_g=mg$ , then $F_n=-F_g$
So we get $F_n=-mg$ (Note that g is negative, so -g is positive!)

And yes you add $F_n+F_g+F_k$

oh okay thanks!

Nhatkiem · Oct 21, 2009, 08:27 PM

Originally Posted by Eddie_boy

oh okay thanks!

Great! Hope you found what you were looking for :cool: