Physics Help!!?

Question

A crate rests on a flatbed of a truck that is initially traveling at 15m/s on a level road. The driver applies the brakes and the truck is brought to a halt in a distance of 38m. If the deceleration of the truck is constant, what is the minimum coefficient of friction between the crate and the truck that is required to keep the crate from sliding??? Please Explain. Thank you.

Answer 1

The deceleration, a, can be found from
V^2 = Vo^2 +2*a*d.

For the crate not to slide, the force of friction, Ff, needs to provide the force, F, to decelerate the crate along with the rest of the truck. So F = Ff.
F = m*a
Ff = mu*N where N = m*g. So

mu*m*g = m*a

You don't need mass of the crate!!

Answer 2

I typed this out on Word before and it looked much nicer but... mcrate means mass of the crate

1.Most important step—trust your teacher/book and know that if the problem didn’t give you the mass it is because as you work with equations the mass will cancel. I say this is most important because if you saw this problem on the test you would first freak out, second ask your teacher why he/she didn’t give you the mass (he/she would just shake his head) and then third freak out some more. If not having a mass bothers you sooo much you lose concentration… make one up. It will be better however, if you are able just continue with the problem knowing that in the end everything will work. Okay… so now to solving the problem
2.The problem gives you a Δv (change in velocity) and a distance—find the acceleration

Think about what variables you have Vi (initial velocity) Vf (final velocity) and ΔX (change in distance) and you want acceleration.

Use Vf = Vi + 2a(ΔX)

Plug and chug— 0=152 + 2a(38)
a=-225/76≈-2.961m/s2 (KEEP THE NEGATIVE EVEN THOUGH IT ONLY DESCRIBES DIRECTION)

3.Now you want to find µ (the coefficient of friction) I assume you know the equation Ff = µ Fn (Force of friction=µ * Force of normal). I also assume you know that Fn = mg (mass * g)

So Ff = mcrate g µ
Remember this.
4.Finally, you know that Fnet = Ff =mcrate a
Now you have your two equations with Ff so set them equal to each other

mcrate a= mcrate g µ
Because you trusted your book the mcrate cancel from both sides so

a= g µ
Plug and chug: -2.961= µ
g
Because g≈-9.8 m/s2 µ≈.302 which is a plausible value for µ between a crate and a truck.

Anymore questions? email me.

Answer 3

Sorry, we need more information. What is the mass of the crate? Without this, we cannot figure out the answer.