Magnitude of the Force of Friction

[Helpme7534]

Homework Statement

A wooden crate with a mass of 20.0 kg is dragged across a floor at a constant speed by a force of 85.0N. The magnitude of the force of friction here is ____ N.

Homework Equations

The Attempt at a Solution

 

[rl.bhat]

According to Newton's first law, when a body moves with constant velocity?

 

[Helpme7534]

? no acceleration?

 

[rl.bhat]

Can you state the Newton's first law?

 

[jegues]

no acceleration

Correct, now expand on this using Newtons 2nd law.

 

[jegues]

No. Can you state the Newton's first law?

I'm confused, constant velocity should mean that there is 0 acceleration?

 

[Helpme7534]

Newtons first law is pretty much intertia, an object in motion tends to stay in motion, its 11:00pm my time and I've been struggling with it for a while, kinda frustrated so i no its rude but if u don't mind i need a lil fast help

 

[jegues]

In the absence of a net force, a body either is at rest or moves in a straight line with constant speed.

Apply this knowledge with Newton's 2nd law to draw your conclusion.

$$\sum F = ma$$

 

[Helpme7534]

k here's what i got

F=ma
85N=20kg(a)
a=4.25 m/s2

I don't no how to get force of friction

 

[jegues]

k here's what i got

F=ma
85N=20kg(a)
a=4.25 m/s2

I don't no how to get force of friction

You told me before that with constant velocity, acceleration was 0. You should be able to conclude that,

$$\sum F = 0$$

Now simply choose a coordinate system for your x-axis (which way is positive which way is negative) and assign each horizontal force the proper sign and solve for the force of friction.

You need to review your understanding of Newtons laws.

 

[stanton]

And mind you that friction force acts the opposite direction from the force acting on the object. i.e., if you move a block to the right, friction force act to the left. So, friction force would have minus sign.

 

[Helpme7534]

well could the answer be a negative number then? I need a 3 digit answer

the second part of the question is

The coefficient of friction here is a.bc x 10 ^-d
The values of a, b, c, and d respectivly are ____

 

[stanton]

well could the answer be a negative number then? I need a 3 digit answer

the second part of the question is

The coefficient of friction here is a.bc x 10 ^-d
The values of a, b, c, and d respectivly are ____

Sorry, I might have confused you. What I want to say is that find all force act on the object, use net force equation(f=ma=0, which I told you in another post). net force will be
total force-friction force because friction acts the opposite. That is why I mentioned minus sign. I hope I am going right. Could someone check this for me?
Net force=total force-friction force = 0

 

[Helpme7534]

wow so confused so my final answer for the first part will be zero? and for the seconds part it will be zero??

 

[stanton]

No. Now take a closer and careful look at the equation I gave it to you.
total force-force of friction=0.
This does not imply force of friction is zero.
Equate this equation we get
force of friction=total force.
now find WHat force acts on the object as I told you before?
we find gravity and pulling force.
so, mg+pulling force=total force
(weight of the object)(9.8)+85n=total force.

And I am so sorry. Here I am staying right now is 12:30 in the evening. And visit this website. It has specific information that will help you to find answer. There it tells you how to find the total force too.
http://www.physics247.com/physics-homework-help/friction-and-weight.php

 

[Helpme7534]

so 281.2 is the total force but where do i go from there?

 

[jegues]

so, mg+pulling force=total force
(weight of the object)(9.8)+85n=total force.

This is incorrect. We're applying Newton's 2nd law in a 1 dimensional motion.

 

[Helpme7534]

lets clarify i only have physics 20-1, and i really need help with this questions, can someone please show me the formulas to use and how to fill them in? it would be greatly appreciated