Help with Calculating Net Force, Resistance Forces and Work

In summary, the conversation revolved around a question about basic physics concepts of force, work, and movement. The main problem was applying equations and rules to solve three given questions. The questions involved a boy dragging a box with a rope, the box moving at a constant velocity, and a constant force being applied at an angle to the horizontal. Through the use of trigonometry and Newton's laws, it was determined that the net force on the box was zero and the resistive force had the same magnitude but opposite direction as the applied force. The final question involved using the equation for work to calculate the work done against the resistance forces. The conversation ended with the understanding of the concept and gratitude for the help provided.
  • #1
Jacko_065560
3
0
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

 
Last edited:
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  • #2


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?
 
  • #3


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?
 
  • #4


the answer to the second question is 0 N because its traveling at a constant speed
 
  • #5


Jacko_065560 said:
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!

Jacko_065560 said:
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.

Jacko_065560 said:
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
 
  • #6


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

1. What is net force?

Net force is the overall force acting on an object, taking into account all the individual forces acting on it.

2. How do you calculate net force?

To calculate net force, you need to add all the individual forces acting on an object. If the forces are acting in the same direction, you simply add them together. If the forces are acting in opposite directions, you subtract the smaller force from the larger one.

3. What is resistance force?

Resistance force is the force that opposes the motion of an object. It can be caused by friction, air resistance, or other factors.

4. How do you calculate resistance force?

The calculation of resistance force depends on the specific situation. For example, you can calculate frictional resistance force by multiplying the coefficient of friction by the normal force. Air resistance force can be calculated using the formula 0.5 x density of air x velocity squared x frontal area of the object.

5. What is work in physics?

In physics, work is defined as the product of the force applied on an object and the distance it moves in the direction of the force. It is measured in joules (J).

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