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Course: High school physics (DEPRECATED)>Unit 3

Lesson 6: Angled forces

Free body diagram with angled forces: worked example

Sal draws a free body diagram for a box held stationary against a wall with a force at an angle theta.

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• I don’t get it. Friction is against the direction of motion. So why is the frictional force(Ff) pointing upwards in the direction of motion instead of downwards?
• So in this scenario, the vertical component of the angled force is less than gravity. Therefore, if there was no friction, then the block would accelerate downwards. Since friction always counteracts the motion, the friction force would point upwards.

In order for the friction force to point downwards, the block would have to be trying to accelerate upwards. That would happen if the horizontal component of the angled force were greater than gravity.

Hope this helps!
• whats the purpose of doing math without numbers?
(1 vote)
• Math doesn't have to be all about numbers. Mathematicians like to generalize things so it can be applicable to all cases, when there are numbers.
(1 vote)
• Think about it. If a force is there FsinQ then the friction should be exact opposite, but Sal draws it in the same direction. Why?
• Non of those forces have to equal our right or is it just this particular set of question?
• (I know this is a year late, but I’ll answer the question anyway just in case someone else has the same one.)

The forces don’t always have to equal (even) out or add up to a net force of zero when we do these kinds of problems, but they do cancel out here.

In this problem, we’re told that the box is “held stationary”, meaning it’s at rest. Staying stationary/at rest means that the box has 0 velocity and 0 acceleration. Newton’s 2nd law says that the sum of the forces equals mass times acceleration ΣF=ma

We know the acceleration is 0m/s^2, so we can substitute it to get ΣF=m(0), which multiplies to ΣF=0.

ΣF=0 means that the sum of the forces equals 0, or that all of the forces have to add up to zero (cancel out). Even though the forces won’t always cancel out, that’s why they do here.
• so what do i do if force isn't given in newtons? one of my homework questions has the magnitude of an angled force given in mg (milligrams) and no matter where i look, i can't find any help on how to deal with that.
edit: turns out the force was 1/2 of the mass * gravity, which had been written as 0.500mg. :( lost ten marks because the textbook couldn't write out full words...
• If you have mg (milligrams), then you know that is the mass. To find the downwards force, you need to find out weight, which is w=mg, weight = mass x gravitational field strength. So if the object is on earth, you would do mass x 9.81 or mass x 10. Remember weight is a force, and is measured in newtons.
• What about the equal and opposite force exerted by block which is pulling the earth?
(1 vote)
• You are right in saying that the block is pulling up the earth, but it is really small. THis is seen through f=ma. THe forces have to be the same according to newton's 3rd law, but the mass would be far far far far far larger - the earth is very heavy, thus the acceleration is really really really really small as mass is indirectly proportional to acceleration when the force is the same. Because it is too small, it is mostly disregarded.
• At , Sal says that he doesn't draw the arrow atop F sub g because he's talking about the magnitude of the vector. Do you always need to draw the arrow? It seems like a bit of extra work for the same purpose. Thanks!
(1 vote)
• F sub g refers to the magnitude of the vector, while F sub g with an arrow refers to the whole vector (both magnitude and direction). Here the downward arrow already tells us the direction so it is fine to write only F sub g, which tells us the magnitude of the vector Fsub g. We need to draw the arrow if we want to refer to the “whole” vector and not just its magnitude.
• Is the normal force the electrons (and protons) repelling each other?
• Yes. Just (or mainly) the electrons, the outer electron shell.

The charge of electrons and protons together usually cancel each other out, so the entire atom (seen as one thing) is neutral (neither positive nor negative). The proximity (the distance) between electrons belonging to two different atoms is the main factor. The protons located in the nucleii, the center of each atom, are “too far” away to make a difference (when talking about normal force).
(1 vote)
• Why is the normal force going sideways?