# Friction Concepts

1. May 21, 2014

### gummybeargirl

1. The problem statement, all variables and given/known data
A mass M is initially at rest on a horizontal surface, μs=0.30 and μk=0.20 . A horizontal string then pulls M with a tension T. N is the normal component of the contact force. All quantities below are magnitudes.
True or False:
1) If M does not accelerate, then T ≤ μsN
2) M will accelerate if T exceeds μkN
3) M will accelerate if T exceed μsN
4) The value of N equals Mg
5) T equals μkN if M remains at rest.
6) The total force on M is μkN if M remains at rest.

2. The attempt at a solution
From what i have learned i thought the answer should be:
1) True (to gain acceleration, first F must exceed μsN)
2) True (it takes µkN, force to just move M, and anything over that will accelerate M)
3) True
4) True (mg = N, the normal force)
5) False (T is less than or equal to µsN for M not to move)
6) False (The total force on M is equal to or less than µsN for no movement)
The in incorrect but i am not sure as to why they are wrong.

2. May 21, 2014

### dauto

looks good

3. May 21, 2014

### gummybeargirl

1) True
2) False (It must exceed ukN not just uks)
3) True
4) True
5) False
6) False

4. May 21, 2014

### Nathanael

The actual reason (which you may have meant to say) is it must exceed u$_{s}$N, not u$_{k}$N