#1
When a ladder leans against a wall there is a normal reaction and possibly friction acting up the wall. So horizonal and vertical forces.

When you have a hinged rod the reaction at the wall isn't a normal. I understand that it can be split into a horizontal and a vertical force but I don't get why the reaction isn't normal as it is in a ladder. And if the ladder didn't have friction at the wall the reaction would still be normal.

I can't see the difference between the two scenarios and I would like an explanation if possible,
0
1 year ago
#2
(Original post by maggiehodgson)
When a ladder leans against a wall there is a normal reaction and possibly friction acting up the wall. So horizonal and vertical forces.

When you have a hinged rod the reaction at the wall isn't a normal. I understand that it can be split into a horizontal and a vertical force but I don't get why the reaction isn't normal as it is in a ladder. And if the ladder didn't have friction at the wall the reaction would still be normal.

I can't see the difference between the two scenarios and I would like an explanation if possible,
If the vertical friction is large enough so the ladder can't move, then there isn't really a difference. Both would have a diagonal "reaction" force exerted on the end of the ladder by the wall. In both cases you can analyse the horizontal and vertical components to get the actual force.
Last edited by mqb2766; 1 year ago
0
1 year ago
#3
(Original post by maggiehodgson)
When a ladder leans against a wall there is a normal reaction and possibly friction acting up the wall. So horizonal and vertical forces.

When you have a hinged rod the reaction at the wall isn't a normal. I understand that it can be split into a horizontal and a vertical force but I don't get why the reaction isn't normal as it is in a ladder. And if the ladder didn't have friction at the wall the reaction would still be normal.

I can't see the difference between the two scenarios and I would like an explanation if possible,
I have a feeling that I know what you mean but if I do then you're not describing it correctly. If you're still confused after reading the reply above mine can you please post an example question(s)?
Last edited by Notnek; 1 year ago
0
#4
(Original post by Notnek)
I have a feeling that I know what you mean but if I do then you're not describing it correctly. If you're still confused after reading the reply above mine can you please post an example question(s)?
This is what I mean.
0
1 year ago
#5
(Original post by maggiehodgson)
This is what I mean.
Considering static equilibrium, a rough wall would be different from a hinge when:
* The horizontal force switched directions (ladder/rod tries to move away from the wall)
* The vertical friction limit was exceeded (ladder/rod would slide)
So really a rough wall imposes static equilibrium constraints in both the vertical and horizontal directions, which a hinge would not. Otherwise they'd both have the same effect (same force on the ladder/rod).

Generally the horizontal/vertical rough wall resolution is done to see if the vertical limiting friction limit is exceeded and to link the vertical friction force with the horizontal reaction (multiply by mu).
Last edited by mqb2766; 1 year ago
0
#6
(Original post by mqb2766)
Considering static equilibrium, a rough wall would be different from a hinge when:
* The horizontal force switched directions (ladder/rod tries to move away from the wall)
* The vertical friction limit was exceeded (ladder/rod would slide)
So really a rough wall imposes static equilibrium constraints in both the vertical and horizontal directions, which a hinge would not. Otherwise they'd both have the same effect (same force on the ladder/rod).

Generally the horizontal/vertical rough wall resolution is done to see if the vertical limiting friction limit is exceeded and to link the vertical friction force with the horizontal reaction (multiply by mu).
Hi

I've just been looking at a selection of questions and perhaps I understand my confusion, but perhaps not. I now think that there is a difference between a freely hinged pole and pole resting on a rough wall supported by a wire. If it's resting on a rough wall it has the same types of forces as a ladder against a wall but if it's freely hinged there's no friction but there is a force keeping it in place that can be split into a vertical and a horizontal component. Do you think that's more or less it? If so I'd be very relieved as it would now make sense to me and the difference would be clear. If it's nonsense, sadly, I'm back to not getting it.

Thanks
0
1 year ago
#7
(Original post by maggiehodgson)
Hi

I've just been looking at a selection of questions and perhaps I understand my confusion, but perhaps not. I now think that there is a difference between a freely hinged pole and pole resting on a rough wall supported by a wire. If it's resting on a rough wall it has the same types of forces as a ladder against a wall but if it's freely hinged there's no friction but there is a force keeping it in place that can be split into a vertical and a horizontal component. Do you think that's more or less it?
I'm not clear what you think the difference is between the two scenarios. In both cases you have vertical and horizontal components; you may *call* them different things, but that's not really going to affect the mathematics.

As mqb and Notnek are explaining, there actual "mathematical" differences; in the "rough wall" case, the horizontal force exerted by the wall can only act *away* from the wall (so if you had a net force (excluding the wall) pulling the ladder away from the wall, it would actually leave the wall). Somewhat similarly, the vertical force exerted by the wall is limited by the coefficient of friction, while it is unlimited in the case of a hinge (and therefore unreliant on the size of the horizontal reaction force). So, for example, you could have a ladder of length 1m freely hanging from a hinge on a wall 2m above the ground, but this would be impossible with a rough wall.
0
1 year ago
#8
(Original post by maggiehodgson)
Hi

I've just been looking at a selection of questions and perhaps I understand my confusion, but perhaps not. I now think that there is a difference between a freely hinged pole and pole resting on a rough wall supported by a wire. If it's resting on a rough wall it has the same types of forces as a ladder against a wall but if it's freely hinged there's no friction but there is a force keeping it in place that can be split into a vertical and a horizontal component. Do you think that's more or less it? If so I'd be very relieved as it would now make sense to me and the difference would be clear. If it's nonsense, sadly, I'm back to not getting it.

Thanks
Sort of. The hinge will stop any movement so it exerts whatever force on the rod is necessary to maintain that. The horizontal component could be into the wall or away from the wall. The vertical component could be in either direction and is not constrained.

I think I know the type of rod suspended by a wire question, and yes, it's similar to a ladder against a wall. The rod will push into the wall horizontally so Newton 3 says the wall pushes back horizontally. The rough wall exerts an upwards vertical force on the rod, which if limiting friction is not exceeded, will stop all motion. If the rough wall was replaced with a hinge (same wire suspension), you'd have an identical force being exerted on the rod in static equilibrium. The resolution into horizontal - vertical components is important for friction, but a bit of a red herring comparing the two scenarios.

Tbh, posting an actual question or two would help.
Last edited by mqb2766; 1 year ago
0
1 year ago
#9
(Original post by maggiehodgson)
When you have a hinged rod the reaction at the wall isn't a normal. I understand that it can be split into a horizontal and a vertical force but I don't get why the reaction isn't normal as it is in a ladder.
Go and have a close look at a hinge. One part of the hinge is wrapped round the rod that goes down the middle of the hinge. So the force exerted by the hinge on the door or whatever can act in any direction.
1
#10
(Original post by mqb2766)
Sort of. The hinge will stop any movement so it exerts whatever force on the rod is necessary to maintain that. The horizontal component could be into the wall or away from the wall. The vertical component could be in either direction and is not constrained.

I think I know the type of rod suspended by a wire question, and yes, it's similar to a ladder against a wall. The rod will push into the wall horizontally so Newton 3 says the wall pushes back horizontally. The rough wall exerts an upwards vertical force on the rod, which if limiting friction is not exceeded, will stop all motion. If the rough wall was replaced with a hinge (same wire suspension), you'd have an identical force being exerted on the rod in static equilibrium. The resolution into horizontal - vertical components is important for friction, but a bit of a red herring comparing the two scenarios.

Tbh, posting an actual question or two would help
I can actually do the questions but it’s been by rote rather than understanding. But now you have cleared it up I think. I will go back to some questions and just check. Currently I’ve switched to statistics. Lovely. Not
1
#11
(Original post by tiny hobbit)
Go and have a close look at a hinge. One part of the hinge is wrapped round the rod that goes down the middle of the hinge. So the force exerted by the hinge on the door or whatever can act in any direction.
Thank you so much. Because in so much mechanics things are not things (a car is a particle) I tried not to think of a hinge as a real hinge. But now you have said to, although it won’t actually be a real hinge, I can visualise the problems better. You have really helped me.
0
1 year ago
#12
(Original post by maggiehodgson)
Thank you so much. Because in so much mechanics things are not things (a car is a particle) I tried not to think of a hinge as a real hinge. But now you have said to, although it won’t actually be a real hinge, I can visualise the problems better. You have really helped me.
I am a retired teacher and I do miss the hands on part of teaching mechanics, e.g. pushing chairs along the floor while explaining friction and limiting friction, propping a pole up against the wall while explaining moments and ladders, etc. You had to choose your "particles" carefully when showing how pulleys worked, so that there was movement but not so fast that the students blinked and missed it. I used to use the groove along Tomy train tracks to show the possible things that could happen when two objects collide, while moving in a straight line i.e. bounce back away from each other or both continue moving in the same direction etc.
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