Help with Calculating Net Force, Resistance Forces and Work

[Jacko_065560]

Hi all, I am having difficulty in understanding this basic phyics relating to force, work,and movement, and I am struggling to do these questions. I would appreciate any help.

The following information was provided and then three questions followed.
A boy drags a box by a rope across a level floor. The box is moving at a constant velocity of 1.6ms-1 west. The boy applies a constant force of 120N along the rope. The rope makes an angle of 30 degrees to the horizontal.
Q1) What is the magnitude of the net force acting on the box?
Q2) Calculate the magnitude of the resistance forces acting on the box. Justify your answer.
Q3) How much work does the boy do against the resistance forces when he drags the box a distance of 5.0m?

I figure that I need to use "F = ma" somewhere, along with the "Work = Fs." I am unsure what else I would use because this is the main problem I am having, applying the equations and rules.

So far, I have managed to do the following workings in an attempt to complete the questions:

1) Using Trig, I calculated that the horizontal line of force acting on box in west direction is equal to 120NCos30 = 103.92N West. Is this the only working I need to do??

2) I think I need to connect the velocity with the 103.92N of horizontal force but I can't think of how to... I have looked through my notes and I guess that Newton's third law can be applied, but I am quite unsure...

3) Well "W = Fs" would be used, with W being unknown, the forces from questions 1 & 2 being substituted for the Net Force F and the displacement, s, would be 5m.

Homework Statement

Homework Equations

The Attempt at a Solution

 

[danago]

Unless I've missed something, Q1 ands Q2 seem to be the exact same questions.

Consider the acceleration of the box. Then relate this to the net force, using Newton's second law. (note that the question asks for the magnitude of the net force).

EDIT: Was question 1 supposed to be the force due to the rope?

 

[Jacko_065560]

My apologies, I have edited Q2 as above, "Calculate the magnitude of the resistance forces acting on the box. Justify your answer"

RE: EDIT, Question 1 is about the net force, so I take it to be the force in a westerly, horizontal direction and any resistance forces. The rope is at 30 degrees from horizontal so i calculated the horizontal force to be 103.9N (using 120cosine30).

So the acceleration is 0ms-2, because it is moving at a constant velocity of 1.6ms-1
F = ma
Net force = mass x 0...
Net force is 0?

 

[aamphys]

the answer to the second question is 0 N because its traveling at a constant speed

 

[C. Hollmann]

So far, I have managed to do the following workings in an attempt to complete the questions:

1) Using Trig, I calculated that the horizontal line of force acting on box in west direction is equal to 120NCos30 = 103.92N West. Is this the only working I need to do??

Yes, that is all the math you need for this one... But there is no acceleration, therefore no net force. The answer is zero!

2) I think I need to connect the velocity with the 103.92N of horizontal force but I can't think of how to... I have looked through my notes and I guess that Newton's third law can be applied, but I am quite unsure...

Again, you know that net force is zero, due to zero acceleration. This means the resistive force exactly balances the force applied through the rope. In order to arrive at zero net force, the resistive force must have the same magnitude, but opposite direction as your calculated force.

3) Well "W = Fs" would be used, with W being unknown, the forces from questions 1 & 2 being substituted for the Net Force F and the displacement, s, would be 5m.

Yup!

CH

 

[Jacko_065560]

thanks a lot to everyone who helped! i understand now!