MC question!
The coefficient of static friction between a box and a ramp is 0.5. The ramp's incline angle is 30 degrees. If the box is placed at rest on the ramp, the box will:
A. accelerate down the ramp.
B. accelerate briefly down the ramp but then slow down and stop.
C. move with constant velocity down the ramp.
D. not move.
E. Cannot be determined from the information given.
- The answer's A, but I'm not sure why.

Question

MC question!
The coefficient of static friction between a box and a ramp is 0.5. The ramp's incline angle is 30 degrees. If the box is placed at rest on the ramp, the box will:
A. accelerate down the ramp.
B. accelerate briefly down the ramp but then slow down and stop. 
C. move with constant velocity down the ramp.
D. not move.
E. Cannot be determined from the information given.
- The answer's A, but I'm not sure why.

anonymous · Answer

Part of the answer says that I should use $$mgcos \theta$$ , but I'm not sure why...?

anonymous · Answer

I bet the only vertical forces on the box is the resolved vector of its weight . For the vertical vector it's mgcos(delta) . So as the box is at rest, opposite vertical forces are equal. in which the contact force is also equal to mgcos(delta) . 

As a result, we conclude that the forward force is greater than the backward force. Therefore the box will accelerate down the ramp.

Hope this is useful.

anonymous · Answer

Nice working out @Lama97!

anonymous · Answer

Thanks @Lama97 ! Made a lot of sense now :)

anonymous · Answer

@Dan.Harrison 
@priyanka8 
Thanksss for your encouragement people :$