Posted by John on .
A crate of oranges weighing 209 N rests on a flatbed truck 2.0 m from the back of the truck. The coefficients of friction between the crate and the bed are μs = 0.41 and μk = 0.20. The truck drives on a straight, level highway at a constant 7.5 m/s.
(a) What is the force of friction acting on the crate?
N
(b) If the truck speeds up with an acceleration of 2.1 m/s2, what is the force of the friction on the crate?
magnitude N
(c) What is the maximum acceleration the truck can have without the crate starting to slide?
m/s^2
I posted this earlier i understand A now which is zero but part B and C are still confusing I couldn't get those please help me thnks

Physics please help its urgent i need help 
MathMate,
b.
frictional force on the truck equals
ma if the crate did not move.
If the force due to acceleration (ma) has overcome static friction, then the friction force will be μ_{k}mg (when the crate is sliding).
To determine if the crate started to slide see (c) below.
(c) The truck can accelerate up to a m/s^2 where
ma = μ_{k}mg, i.e. when static friction is exceeded.