Hi, does anyone have any tips/efficient ways of drawing the components of forces in mechanics? Mainly when you have to find the components horizontal and perpendicular to the plane of a friction force acting on the bottom of a plank resting against a wall in equilibrium. I always end up wasting time drawing a small diagram which makes it hard to tell where the angle should go. Any help appreciated. Thanks.

Original post by Har6547

Hi, does anyone have any tips/efficient ways of drawing the components of forces in mechanics? Mainly when you have to find the components horizontal and perpendicular to the plane of a friction force acting on the bottom of a plank resting against a wall in equilibrium. I always end up wasting time drawing a small diagram which makes it hard to tell where the angle should go. Any help appreciated. Thanks.

A sketch is usually the way to go. Are you unsure about which of the two complementary angles in the right force component triangle is equal to the original one? I generally just think about what happens when the original angle is small (~0), then the angle that goes to zero in your right force triangle is that one and the parallel/perpendicular for sin/cos is straightforward.

Of course doing a small amount of angle chasing is really the way to proceed.

(edited 11 months ago)

Original post by mqb2766

A sketch is usually the way to go. Are you unsure about which of the two complementary angles in the right force component triangle is equal to the original one? I generally just think about what happens when the original angle is small (~0), then the angle that goes to zero in your right force triangle is that one and the parallel/perpendicular for sin/cos is straightforward.

Of course doing a small amount of angle chasing is really the way to proceed.

Of course doing a small amount of angle chasing is really the way to proceed.

Ok thank you that is helpful. I mostly meant the ones where you have a plank at rest against a wall and you need to take moments around the point in contact with the vertical wall, how you go about finding the component that is perpendicular to the plank to find the moment but I think sketching will help

Original post by Har6547

Ok thank you that is helpful. I mostly meant the ones where you have a plank at rest against a wall and you need to take moments around the point in contact with the vertical wall, how you go about finding the component that is perpendicular to the plank to find the moment but I think sketching will help

sketching is useful and often with the moment due to com say, its easier to extend the force line until it crosses one of the "axes", and use

moment = force * perpendicular distance

as it may be "easier" to think of the perpendicular distance rather than the resolved (perpendicular) force

(edited 11 months ago)

Original post by Har6547

Hi, does anyone have any tips/efficient ways of drawing the components of forces in mechanics? Mainly when you have to find the components horizontal and perpendicular to the plane of a friction force acting on the bottom of a plank resting against a wall in equilibrium. I always end up wasting time drawing a small diagram which makes it hard to tell where the angle should go. Any help appreciated. Thanks.

One method I find very helpful is to visualise what must happen to the force component if the angle in question was increased or decreased, or taken to its logical conclusion, what would happen to the force component if the angle was decreased to 0 or increased to 90.

If it's clear that the force component must increase as the angle is increased, then the component is Fsin@. (An example is the component of a particle's weight acting down a slope inclined at @).

If it's clear that the force component must decrease as the angle is increased, then the component is Fcos@. (An example is the normal reaction to a particle's weight on a slope inclined at @).

If in doubt about this, consider the shape of the sine and cosine curves between 0 and 90deg. The same line of thought can be applied to moments.

(edited 11 months ago)

Original post by old_engineer

One method I find very helpful is to visualise what must happen to the force component if the angle in question was increased or decreased, or taken to its logical conclusion, what would happen to the force component if the angle was decreased to 0 or increased to 90.

If it's clear that the force component must increase as the angle is increased, then the component is Fsin@. (An example is the component of a particle's weight acting down a slope inclined at @).

If it's clear that the force component must decrease as the angle is increased, then the component is Fcos@. (An example is the normal reaction to a particle's weight on a slope inclined at @).

If in doubt about this, consider the shape of the sine and cosine curves between 0 and 90deg. The same line of thought can be applied to moments.

If it's clear that the force component must increase as the angle is increased, then the component is Fsin@. (An example is the component of a particle's weight acting down a slope inclined at @).

If it's clear that the force component must decrease as the angle is increased, then the component is Fcos@. (An example is the normal reaction to a particle's weight on a slope inclined at @).

If in doubt about this, consider the shape of the sine and cosine curves between 0 and 90deg. The same line of thought can be applied to moments.

Thank you, I see what you mean that is helpful in understanding

Theirs a good video on YouTube by science shorts about vectors I usually apply it when I feel lazy and it gives me components correct 100% of the time if I don’t feel like doing trig https://youtu.be/0TGTSLn3dsc

Original post by Har6547

Ok thank you that is helpful. I mostly meant the ones where you have a plank at rest against a wall and you need to take moments around the point in contact with the vertical wall, how you go about finding the component that is perpendicular to the plank to find the moment but I think sketching will help

You need a large diagram - at least a third of a page until you get more confident

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