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# Mechanics/Dynamics Question Tweet

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1. Mechanics/Dynamics Question
First of all l don't understand the last line and don't know how to do part a of the question.

A horizontal force of magnitude 20N acts on a block of mass 1.5kg, which is in equilibrium resting on a rough plane inclined at 30 degrees to the horizontal. The line of action of the force is in the same vertical plane as the line of greatest slope of the inclined plane.

a) find the magnitude and direction of the frictional force acting on the block.

2. Re: Mechanics/Dynamics Question
What does your working out look like?
3. Re: Mechanics/Dynamics Question
Well... i don't really know when to start.
It says that the block is in equilibrium so i can't use uR.
Hmm so don't know what to do, grrr
4. Re: Mechanics/Dynamics Question
(Original post by MathsBoy94)
It says that the block is in equilibrium so i can't use uR.
5. Re: Mechanics/Dynamics Question
Draw the block with all the forces on (weight, friction, normal, applied force) then resolve parallel and perpendicular to the plane
6. Re: Mechanics/Dynamics Question
In fact, I think you only need to resolve parallel to plane
7. Re: Mechanics/Dynamics Question
i have, but friction is unknown
8. Re: Mechanics/Dynamics Question
Ohhh Don't Worry I Got The Right Answer.
9. Re: Mechanics/Dynamics Question
By resolving the perpendicular forces
10. Re: Mechanics/Dynamics Question
(Original post by MathsBoy94)
First of all l don't understand the last line and don't know how to do part a of the question.

A horizontal force of magnitude 20N acts on a block of mass 1.5kg, which is in equilibrium resting on a rough plane inclined at 30 degrees to the horizontal. The line of action of the force is in the same vertical plane as the line of greatest slope of the inclined plane.

a) find the magnitude and direction of the frictional force acting on the block.

The below diagram might help:

11. Re: Mechanics/Dynamics Question
(Original post by MathsBoy94)
By resolving the perpendicular forces
I think you only need to resolve parallel
12. Re: Mechanics/Dynamics Question
(Original post by raheem94)
The below diagram might help:

Thanks, I did the first one.
On the second one it wanted me to find the normal reaction between the plane and the block.

The books says -2.7N
and i got 22.7N

Am not sure about the book
13. Re: Mechanics/Dynamics Question
It is 22.7 if the horizontal force acts to the right as shown but it is -2.7 if it acts to the left
14. Re: Mechanics/Dynamics Question
(Original post by leapingfrog)
It is 22.7 if the horizontal force acts to the right as shown but it is -2.7 if it acts to the left
I know but here is the diagram the book gives me next to the question
which shows the horizontal force pushing the block to the right.
Attached Thumbnails

15. Re: Mechanics/Dynamics Question
It is 22.7
16. Re: Mechanics/Dynamics Question
(Original post by MathsBoy94)
I know but here is the diagram the book gives me next to the question
which shows the horizontal force pushing the block to the right.
Its a mistake in the book. I have seen it. The answer at the back is -2.7 but the answer in the solution bank is 22.7.

See the image from the solution bank:

17. Re: Mechanics/Dynamics Question
(Original post by raheem94)
Its a mistake in the book. I have seen it. The answer at the back is -2.7 but the answer in the solution bank is 22.7.

See the image from the solution bank:

If you know this then then you know in the second question after that there is also a mistake on question 10 can you please check to verify where i got 145.7 instead of 102 as it says in the book.
18. Re: Mechanics/Dynamics Question
(Original post by MathsBoy94)
If you know this then then you know in the second question after that there is also a mistake on question 10 can you please check to verify where i got 145.7 instead of 102 as it says in the book.
The answer for it is 102N.

Both book and solution bank say the answer to be 102 and i also did it my self to ensure it, i get 102.

Post your working, so that i can correct you.
19. Re: Mechanics/Dynamics Question
(Original post by raheem94)
The answer for it is 102N.

Both book and solution bank say the answer to be 102 and i also did it my self to ensure it, i get 102.

Post your working, so that i can correct you.
I got T-22gsin35-(0.125x22gcos35) = 0
20. Re: Mechanics/Dynamics Question
(Original post by MathsBoy94)
I got T-22gsin35-(0.125x22gcos35) = 0

Why did you took the friction to be negative?

The particle is at the point of sliding down the plane, so the friction should act upwards because friction always opposes motion.

Do you get it?