Forces with Friction questions?

Question

Can someone help me out here and show work please:?

1. Consider the block on a surface below. The coefficient of friction (µ) is 0.20. Block is 20 kg

-------------- F
| Block | --------------------->
--------------

If a 10. N force is applied to the block, what is its acceleration?

How fast will the block be moving if the force is applied for 5.0 s?

How far will the block have traveled during this 5.0 s period?

2. A 70.0 kg box is pulled across a frictionless surface by a 300. N force at an angle of 30° to the horizontal.

What is the normal force on the box?

What is the acceleration of the box in the x-direction if there is no friction?

What coefficient of friction would be required if the box were to move at constant speed?

Answer 1

F=m*a
F is the net force
so 10-m*g*µ=m*a

once you have a
v(t)=v0+a*t

and s(t)=s0+v0*t+.5*a*t^2

For this specific case, s0 and v0 are 0
compute a
(I will use g=10m/s^2)
10-20*10*.2=20*a

this is negative, so 10N is insufficient to move the box.

Was it 2.0 kg?
using 2.0
10-20*10*.2=2*a
a=3 m/s^2

v(5)=15 m/s

s(5)=.5*3*25
=37.5 m

For 2, the weight of the box is 700 N (using g=10)
Assuming the angle of the pulling force is upward, this lightened the normal force by sin(30)*300 or 150 N
The normal force is 550 N

Without friction
300=70*a
a=300*cos(30)/70
a=3.71 m/s

with friction the horizontal component 300*cos(30) must equal µ*normal force for no acceleration
or
300*cos(30)=µ*550
µ=0.472

j