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Old Mar 12th 2019, 04:36 AM   #1
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Friction and Weight


Last edited by niki22834; Mar 19th 2019 at 08:05 PM.
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Old Mar 12th 2019, 10:26 AM   #2
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How are you with part (a)?
Are you OK with the concept of splitting a force into 2 components?
If you are struggling with this stage, see your tutor.

I would suggest that there are two fairly equal options
1) based on an alignment with gravity.
2) based on an alignment with the slope of the slide.

You can swap from one to the other using trigonometry.

(b) look at the X and Y components of force,
{The two primary forces are due to Gravity and Olivia's push}
use trigonometry to find the size of the 2 force components, along and at right angles to the surface of the slide.
The force at right angles to the surface of the slide multiplied by the coeff. of friction gives the friction force aligned with the surface of the slide.
Is this friction force greater than the component of force due to Gravity+Olivia's push that acts along the surface of the slide?
If it is then Barry slides, if not then he doesn't.
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Old Mar 12th 2019, 04:38 PM   #3
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Hi I am not okay with part a as I don't know how to define the x and y components of each force and I am confused as to what 30 degrees below the horizontal means
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Old Mar 12th 2019, 06:28 PM   #4
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Talk to your tutor

Imagine taking a photo of the slide from the side
You line up the camera carefully so that the horizon in the background will line up perfectly with the top of the slide in the photo.
Imagine further that the slide is on a perfectly flat beach, so the horizon is nice and straight.
Measure the angle between the slide and the horizon on the photo
that angle will be 30 degrees.

Horizontal means parallel to the horizon
In most real situations the horizon is hilly, or obscured, or otherwise unavailable for a direct comparison.
However even if you can't see the horizon, you do know the direction of gravity (straight down),
the horizon will be at right angles (90 degrees) to gravity.

In many of these types of problems it helps if you have a good visual imagination,
or can draw a picture of the problem.
Don't put in unnecessary details, a line for the slide, a line for the ground,
remember the ground is parallel to the horizon, so set the line for the slide at 30 degrees to the ground.

Draw an arrow for the force of Olivia pushing,
Draw an arrow for gravity pulling Barry down,
Draw an arrow for the slide pushing back to hold Barry's weight
Draw an arrow for friction acting to slow Barry's slide.

If the strength of these arrows don't balance Barry will move.
If they do balance Barry will stay still.

By convention the strength of the forces is indicated by the length of the arrows,
(the longer the arrow the stronger the force).

I could go on for pages...

If this is a problem your tutor has set then these must have been topics covered by your tutor and they thought you understood.
If you don't understand, it is important that your tutor be made aware so that suitable additional teaching can be provided.
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Last edited by Woody; Mar 12th 2019 at 06:47 PM.
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Old Mar 12th 2019, 09:33 PM   #5
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I spoke to my tutor and she said to define the x and y axis as along the surface of the slide for easier calculations, I have done this and labelled the forces but how do I go about solving the problem
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Old Mar 13th 2019, 04:09 AM   #6
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X axis along the slide,
Y axis perpendicular (at right angles) to the slide (downwards).

I would personally have chosen X horizontal Y straight down,
but this is largely down to personal preference.
As I said you can move from one to the other by using trigonometry
and this is actually the key to the solution.

For example You have Olivia's Push expressed in horizontal axes
What would it be in slide axes?
(sine and cosine of 30 degrees multiplied by the strength of the force)
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Old Mar 13th 2019, 04:23 AM   #7
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would you mind doing an example using the problem?
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Old Mar 13th 2019, 05:38 AM   #8
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I am not going to do your homework for you.

The key purpose for homework is to inform the tutor of students having problems.
If your tutor is any good,
you should be able to admit your difficulties and gain additional help
(rather than criticism).

Of course your tutor may be irritated if the reason for not understanding is poor behaviour during the lesson...
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