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Found in: Page 141

### Fundamentals Of Physics

Book edition 10th Edition
Author(s) David Halliday
Pages 1328 pages
ISBN 9781118230718

# A worker pushes horizontally on a ${\mathbf{35}}{\mathbf{kg}}$crate with a force of magnitude ${\mathbf{110}}{\mathbf{}}{\mathbf{N}}$. The coefficient of static friction between the crate and the floor is ${\mathbf{0}}{\mathbf{.}}{\mathbf{37}}$. (a) What is the value of ${{\mathbit{f}}}_{\mathbf{a}\mathbf{,}\mathbf{}\mathbf{m}\mathbf{a}\mathbf{x}}$under the circumstances? (b) Does the crate move? (c) What is the frictional force on the crate from the floor? (d) Suppose, next, that a second worker pulls directly upward on the crate to help out. What is the least vertical pull that will allow the first worker’s ${\mathbf{110}}{\mathbf{}}{\mathbf{N}}$ push to move the crate? (e) If, instead, the second worker pulls horizontally to help out, what is the least pull that will get the crate moving?

(a) The value of ${f}_{a,max}$under the circumstances is 127 N.

(b) The crate does not move.

(c) The frictional force on the crate from the floor is 110 N.

(D) The least vertical pull that will allow the first worker’s $110\mathrm{N}$ push to move the crate is 47 N.

(E) The least magnitude of F’ to move the box should be 17N.

See the step by step solution

## Step 1: Given

Mass, $\mathrm{m}=35\mathrm{kg}$

Coefficient of static friction, ${\mathrm{\mu }}_{\mathrm{s}}=.037$

External force on the crate, $\mathrm{F}=110\mathrm{N}$

## Step 2: Determining the concept

The problem is based on Newton’s second law of motion which states that the rate of change of momentum of a body is equal in both magnitude and direction of the force acting on it. Use the Newton's 2nd law of motion along vertical and horizontal direction. According to Newton's 2nd law of motion, a force applied to an object at rest causes it to accelerate in the direction of the force.

Formula:

where, F is the net force, m is mass and a is an acceleration.

## Step 3: Determining the free body diagram

Free body Diagram of Crate:

## Step 4: (a) Determining the value of

By using Newton’s 2nd law along vertical direction (along y),

Since crate is not moving upward, ${a}_{y}‐0$

$\mathrm{N}-{\mathrm{F}}_{\mathrm{g}}=0$

$\mathrm{N}={\mathrm{F}}_{\mathrm{g}}$

Relation between maximumstatic frictional force and normal force is,

${\mathrm{f}}_{\mathrm{s},\mathrm{max}}={\mathrm{\mu }}_{\mathrm{s}}\mathrm{N}\phantom{\rule{0ex}{0ex}}={\mathrm{\mu }}_{\mathrm{s}}{\mathrm{F}}_{\mathrm{g}}\phantom{\rule{0ex}{0ex}}=\left(0.37\right)\left(35\right)\left(9.81\right)\phantom{\rule{0ex}{0ex}}=127\mathrm{N}$

Hence, the value of ${\mathrm{f}}_{\mathrm{s},\mathrm{max}}$ under the circumstances is 127 N.

## Step 5: (b) Determining if crate will move or not

As. $\mathrm{F}=110\mathrm{N}<{\mathrm{f}}_{\mathrm{s},\mathrm{max}}\phantom{\rule{0ex}{0ex}}$

Hence, the crate does not move.

## Step 6: (c) Determining the frictional force on the crate from the floor

By using Newton’s 2nd law along the horizontal direction,

Since crate is not moving, ${\mathrm{a}}_{\mathrm{x}}=0$

$\mathrm{F}-{\mathrm{f}}_{\mathrm{s}}=0\phantom{\rule{0ex}{0ex}}{\mathrm{f}}_{\mathrm{s}}=\mathrm{F}\phantom{\rule{0ex}{0ex}}=110\mathrm{N}$

Hence, the frictional force on the crate from the floor is $110\mathrm{N}$.

## Step 7: (d) Determining the least vertical pull that will allow the first worker’s  push to move the crate

Let, the upward force apply by the worker is F’, then by using Newton’s 2nd law,

$\sum _{}{\mathrm{F}}_{y}={\mathrm{ma}}_{y}$

since crate is not moving upward, ${\mathrm{a}}_{\mathrm{y}}=0$

$\mathrm{N}+{\mathrm{F}}^{1}-{\mathrm{F}}_{\mathrm{g}}=0\phantom{\rule{0ex}{0ex}}\mathrm{N}={\mathrm{F}}_{\mathrm{g}}-{\mathrm{F}}^{1}$

In order to move the crate F must satisfy the condition,

$\mathrm{F}>{\mathrm{\mu }}_{\mathrm{s}}\left(\mathrm{mg}-{\mathrm{F}}^{1}\right)\phantom{\rule{0ex}{0ex}}110>\left(0.37\right)\left[\left(35\right)\left(9.81\right)-{\mathrm{F}}^{1}\right]\phantom{\rule{0ex}{0ex}}{\mathrm{F}}^{1}>46.05\mathrm{N}$

Since,F’ is slightly greater than $46.05\mathrm{N}$.

Hence, the least vertical pull that will allow the first worker’s $110\mathrm{N}$ push to move the crate is 47 N.

## Step 8: (e) Determining the least pull that will get the crate moving

If the horizontal force applied by the worker is F’ and to move the box the total forward force should overcome the backward static frictional force $127\mathrm{N}$

Thus, ${\mathrm{F}}_{1}{\mathrm{F}}^{1}>127\mathrm{N}$

$110\mathrm{N}+{\mathrm{F}}^{1}>127\mathrm{N}$

Therefore, the least magnitude of F’ to move the box should be $17\mathrm{N}$ .