Physics Help Forum Homework regarding relation between friction and velocity/acceleration. Just need my

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Dec 2nd 2017, 02:19 PM #1 Junior Member   Join Date: Dec 2017 Posts: 3 Homework regarding relation between friction and velocity/acceleration. Just need my Hi there. Our teacher gave us a table related to friction and velocity/acceleration. It basically gives a single scenario, and we're required to figure out which force is stronger (pulling force vs. friction) and which direction the object will go (left, right, up the slope, down the slope). I think I got it right, but I could really use some help correcting any mistakes or just generally checking if what I wrote is correct. It's a small table so it shouldn't be much of a time waster. Link to the table!
 Dec 2nd 2017, 08:22 PM #2 Senior Member   Join Date: Apr 2017 Posts: 182 First line of question is "determine the friction value and direction" They want you to give the direction of the friction ... this will be opposite to the direction the object is pulled ... the rope is pulling one way , the friction 'pulling' the other ..So always the friction is in the opposite direction to the movement ...You need to change ALL the ticks in the second group of boxes to the opposite direction .. In the first group of boxes if there is any movement then Fp is grater than Ff .... no movement then Ff is greater and friction will act up the slope Last edited by oz93666; Dec 2nd 2017 at 11:43 PM.
 Dec 3rd 2017, 02:33 AM #3 Junior Member   Join Date: Dec 2017 Posts: 3 Thank you for the answer! So there isn't a situation where Fp = Ff? Link to the revised table!
 Dec 3rd 2017, 03:55 AM #4 Senior Member   Join Date: Apr 2017 Posts: 182 sorry ...my error ... on a horizontal plane Fp= Ff if there is NO acceleration so coulomb 1 and 3 , change to = I've been rushing things ... let me go slowly and make sure I'm right ... First rule is friction ALWAYS acts opposite to the direction of movement , it wants to stop things .... if velocity is constant then there is no net force acting on the object and Fp = Ff If there is acceleration on a horizontal plane then pulling force is greater than friction acceleration will be in direction of pull If moving down a slope with constant v , then gravity is helping move it , the pull is not so strong Ff is greater than Fp If movin.............up ............................then g is fighting against movement so Fp is greater than Ff If accelerating up a slope the pull has to be very strong Fp is greater than Ff If accelerating down the slope also Fp is greater than Ff Henlo likes this. Last edited by oz93666; Dec 3rd 2017 at 04:41 AM.
 Dec 3rd 2017, 05:32 AM #5 Junior Member   Join Date: Dec 2017 Posts: 3 Thank you very much for the answers! I think I understood it now. Friction always works opposite of the movement. Here is the revised table, hopefully I got it right this time!
 Dec 3rd 2017, 05:50 PM #6 Senior Member   Join Date: Apr 2017 Posts: 182 That all seems good , except for the last question ... We are told that the object is accelerating down a slope , so the downward force parallel to the inclined surface is greater than the opposing friction , but this force is a compound of the pull from the rope and the objects weight component which acts parallel to the surface , Weight x sin(angle) the bigger the angle of inclination ,the more the weight of the object will act parallel to the surface helping slide it down. All we know for sure is (Weight x sin Angle of slope) + Fp (pull from rope) is greater than Ff So there are two possible correct answers Fp could be greater , Or it could be less than Ff In both cases Ff will act UP the slope .

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