Tension in cables pulling a box

In summary, the question asks about the tension in two cables lifting a 400-kg box with constant velocity. After drawing a free-body diagram and considering the forces, the net force equation is set up to find the tension. However, the inclusion of the normal force and weight in the equation is incorrect as they cancel out. Removing these forces gives the correct answer of 2560 N. The confusion about the normal force is cleared up by understanding that it is a reaction force and only exists when there is a surface of contact.
  • #1
oceanflavored
44
0

Homework Statement


A 400-kg box is lifted vertically upward with constant velocity by means of two cables pulling at 40.0o on either side of the vertical direction. What is the tension in each cable?

Homework Equations


Force = mass(acceleration)
Weight = mass(gravity)

The Attempt at a Solution


My free-body diagram had the two forces: one west of north, the other east of north; the normal force pointing in the upward vertical direction; and the weight pointing in the downward vertical direction.
Then, I reasoned that since the box was moving with constant velocity, the acceleration would be zero, meaning the net force would be zero:
So, net force = 2(F cos 40.0o) + N - W = 0
The problem with this equation of mine is that my normal force and weight cancel out, leaving me with 2(F cos 40.0o) = 0, which doesn't exactly give me an answer.

The book says the answer is 2560 N. Please tell me what I'm doing wrong :|
Thank you so much!

PS: All the people on this site are LIFESAVERS :) I'm pretty sure if I get a 100 on my test next Friday, it's because of all the people who keep answering my questions, so THANK YOU! :D
 
Physics news on Phys.org
  • #2


You added a force that's not there. Since the box is being pulled by the strings, there is gravity downwards and the tensions on the strings that cancel it. So just get that extra force out and see if it works.
 
  • #3


Thank you so much :)

That did work, but I'm still confused, as to why there wouldn't be a normal force pointing upwards? I always thought that if weight is pointing downwards, there would always be the normal force (the reaction force) pointing upwards?? :|
 
  • #4


Well the normal force is a force perpendicular (normal) to the surface of contact. For example if the box was just sitting on the ground, then you would have a normal force equal to the weight of the box acting up.
In your question the box was being lifted up, so it's not touching the ground.
 
  • #5


GOT IT :)))))))))

Thank you SO SO SO SO much! :D
 

Related to Tension in cables pulling a box

1. What is tension in cables pulling a box?

Tension in cables pulling a box refers to the force applied by the cables to the box in order to move it. This force is typically measured in Newtons (N) or pounds (lbs) and is dependent on factors such as the weight of the box, the length and angle of the cables, and the strength of the cables themselves.

2. How is tension calculated in cables pulling a box?

Tension in cables pulling a box can be calculated using the formula T = (m x g) / cosθ, where T is the tension, m is the mass of the box, g is the acceleration due to gravity, and θ is the angle of the cables with respect to the horizontal. This formula assumes ideal conditions and does not account for factors such as friction.

3. Why is tension important in cables pulling a box?

Tension is important in cables pulling a box because it determines the amount of force that is being applied to the box. If the tension is too low, the box may not move at all. If the tension is too high, it can cause damage to the box or the cables. It is important to carefully calculate and monitor tension to ensure safe and efficient movement of the box.

4. How does the weight of the box affect tension in cables pulling it?

The weight of the box directly affects the tension in the cables pulling it. As the weight of the box increases, the tension required to move it also increases. This is because a heavier object requires more force to overcome its inertia and move it. Therefore, it is important to consider the weight of the box when calculating the tension in the cables.

5. What factors can affect tension in cables pulling a box?

Aside from the weight of the box, other factors that can affect tension in cables pulling a box include the length and angle of the cables, the strength and elasticity of the cables, and any external forces such as wind or friction. It is important to carefully consider and account for these factors to ensure safe and efficient movement of the box.

Similar threads

Problem involving space elevators
    • Introductory Physics Homework Help
Replies
19
Views
866
Why are tension and friction in the same direction?
    • Introductory Physics Homework Help
Replies
6
Views
2K
Equilibrium of Forces and Torques on Sawhorses Supporting a Person on a Board
    • Introductory Physics Homework Help
Replies
6
Views
914
Maximum tension in a cable holding a drawbridge
    • Introductory Physics Homework Help
Replies
5
Views
2K
Work Pulley Problem with constant speed
    • Introductory Physics Homework Help
Replies
2
Views
255
Seemingly simple equilibrium problem - force balance
    • Introductory Physics Homework Help
Replies
6
Views
1K
Find the tension in the cable and reactive forces at ground
    • Introductory Physics Homework Help
Replies
4
Views
4K
For which case would the applied force be greater?
    • Introductory Physics Homework Help
2
Replies
41
Views
2K
Acceleration & Normal Force of a Box: 4 m/s2 & 418 N
    • Introductory Physics Homework Help
Replies
9
Views
931
Newton's law problem: Helicopter lifting a box with a rope
    • Introductory Physics Homework Help
Replies
5
Views
2K

Hot Threads

Recent Insights

Back
Top