Question

Please help me with this! My tutor's connection or mine got disconnected.

Answer 1

a)

The free body diagram is shown below:

b)

Newton's second law leads to the following equations of motion:

`\begin{gathered} F-f_{}=ma \\ N-W=0 \end{gathered}`

From the second equation we have that:

`\begin{gathered} N=W \\ N=(15)(9.8) \\ N=147 \end{gathered}`

Hence the normal force is equal to 147 N.

We know that the maximum static friction is given by:

`f_s=\mu_sN`

then in this case we have:

`\begin{gathered} f_s=\mu_sN \\ f_s=(0.4)(147) \\ f_s=58.8 \end{gathered}`

As long as the applied force is less than the maximum static friction the object will remain stationary and the static friction will be equal to the force applied (remember that the static friction increases with the force applied until it reaches its maximum value, while this occur the force applied and the static friction will be equal). Since in this case the force applied is 20 N this means that the object will remain stationary and the magnitude of the friction will be:

`f_s=20\text{ N}`

c)

Since the object remains stationary (for what we discuss in the previous point) we conclude that the acceleration is zero.