Solving Force on a Block Problem

[joemama69]

Homework Statement

A block is pulled along a rough surface by the force as shown in attachment

1)draw freebody diagram

2)write an expression for the normal force

3)write an expression for coefficient of kinetic friction

4)sketch graph of v vs. t & x vs. t

5)if the force is large enough, the block will lift of ground. find expression for the greatest acceleration that the block can have and still maintain contact with surface

Homework Equations

The Attempt at a Solution

1)FBD ill just explain

Left = uN
Right = FcosQ
Down = mg
Up = N

2) N = mg - FsinQ

3)F(friction) = uN = u(mg - FsinQ)

u = F/(mg-FsinQ)

4) ill explain

v vs t ... I am not sure, is the acceleration constant

x vs t ... the is a slanted line coming out of the origin to the right & up

5) FsinQ - mg = ma

a = (FsinQ-mg)/m

 

[turin]

Part 1) is incorrect; you are neglecting something. I suggest identifying each force that acts on the block, and then, for each force, identify its x and y components. Then, determine what are the x and y components of the acceleration.

Parts 2) and 3) seem to be correct, somehow, inspite of part 1).

Part 4): is the mass constant? is the force constant? is x vs. t a straight line?

You need to think about part 5) some more. You might get a better idea after you fix part 1).

 

[joemama69]

1) should the up be N - FsinQ

4) mass is constant. force is constant. so the acceleration is constant.

then the velocity is the increasing line up to the right, and displacement is the right side of a parabola

 

[turin]

1) should the up be N - FsinQ

Close, but no cigar. Which way does N point: up or down? Which way does F point: up or down?

4) mass is constant. force is constant. so the acceleration is constant.

Brilliant induction, Watson!

then the velocity is the increasing line up to the right, and displacement is the right side of a parabola

Superb!

 

[joemama69]

both go up so... N + FsinQ

 

[turin]

both go up so... N + FsinQ

Now you're talkin'.

 

[joemama69]

should it be cos

you use sin for the x, and cos for y correct

 

[turin]

You are using sin for y (up/down) and cos for x (left/right). There is no rule that dictates this (although it is typical); it is the choice that you have made. Is your diagram exactly as given, or should there be an angle defined on the diagram. That might make a difference.

What about part 5)?