Why Are These Dynamics Problems So Challenging?

[akademiks]

Some Dynamics Questions

I've been working at these stupid questions for hours and i just can't figure them out. Someone please help.

1) A student is pushing horizontally on a table (m = 16kg) to move it across a horinzontal floor. The coefficient of kinetic friction between the table and the floor is 0.61
a) Determine the magnitude of the applied force needed to keep the table moving at a constant velocity.
b) If the applied force were 109N and the table were to start from rest, how long would the table take to travel 75cm?



2) A rope exerts a force of magnitude 21N, at an angle 31 degrees above the horizontal, on a box at rest on a horizontal floor. The coefficients of friction between the box and the floor are uS (Coefficient of Static Frict) = 0.55 and uK (coefficient of Kinetic friction) = 0.50. The box remains at rest. Determine the smallest possible mass of the box.


I'd need this help in less than an hour though, so Thanks

 

[James R]

Hints:

Question (1)

(a)
What are the forces on the table?
What can you say about the net force if the table is moving at constant velocity?

(b)
Can you find the acceleration? Is it constant?
Given the acceleration, how can you calculate distance travelled?

2.
What can be said about the friction force vs. the applied force if the box doesn't slide?
How are you going to deal with the angle?
Have you drawn a force diagram?

Think about these things, and get back if you still have problems.