Thread: Need help with a 1D newtonian motion question.

A box with a mass of 45.0kg is pulled along a horizontal surface by a rope that makes an angle of 32. Degrees with the horizontal. If the force of friction is 50.0 N and the tension in the rope is 95.0 N, what is the acceleration of the object? Thanks for any help

Well now, I don't make it a habit to give out answers to homework. That's just cheating. . However, I will try to get you to understand how to solve the problem, that way, you can do any of the above type problems. So, the obvious first question I'm going to ask, is what type of physics this is? That way, we'll know what format the answer should be in. Vectors, acceleration, forces, work... and so on.

Oh the answer provided is .679m/s^2 It's a kinematics/ dynamics question that will most likely require the 'F=ma' formula. I just don't know where I'm going wrong, I used the formula and I got the wrong answer but I have no idea how else too approach the question.

Originally Posted by xXplosionZz

Oh the answer provided is .679m/s^2 It's a kinematics/ dynamics question that will most likely require the 'F=ma' formula. I just don't know where I'm going wrong, I used the formula and I got the wrong answer but I have no idea how else too approach the question.

Indeed you will need to use Newton’s second law of motion for this problem. Draw a diagram showing all the forces acting on the box, and consider their horizontal components. (Hint: The weight of the box and the normal reaction of the surface have zero horizontal component.) Set the resultant of the horizontal components of these forces equal to where is the mass of the box and is its acceleration. Now solve for .

Although it is correct to do it that way, you first need the total horizontal force that is being applied to the system. There are two ways to determine that force. 1: As Nehushtan as said, you can draw it, and 2: You can work it out using normal math. Although I don't know the exact formulae for this (using sin, cos and tan), the answer should be in there somewhere.

Originally Posted by xXplosionZz

A box with a mass of 45.0kg is pulled along a horizontal surface by a rope that makes an angle of 32. Degrees with the horizontal. If the force of friction is 50.0 N and the tension in the rope is 95.0 N, what is the acceleration of the object? Thanks for any help

The active force pulling the mass is : where and
The resisting force is the friction force
All you have to do is to form the equation of motion

maybe you should write here how you done it, so we know where/how you went wrong.

Well, curious mind, I think I know what happened. Because the answer was given, xXplosionZz saw that his answer was incorrect. Then, the only thing to do is to reverse engineer the formula from the answer. Clever. This will help in understanding why its done like that. But I think the angle was a bit daunting. And instead of using cos, he was probably trying to use sin. Am I correct in saying this xXplosoinZz? Correct me where I am wrong.