A 4 kg block has a coefficient of friction of 0.3 between itself and the surface it slides across. What is the friction force between the block and the surface?
a. 1.2 N
b. 4.3 N
c. 11.7 N
d. 13.3 N

C - 11.76 N (Newtons)

Explanation:

2 steps are needed,

First find the Force being applied by the box on the surface.

F = M(a) [Force equals: mass X Acceleration (of gravity)]

You get 39.2 N of Force.

Plug in the remaining values into the equation for Frictional Force

F(f) = μ(F) [Coefficient of Friction X Force applied by the Box]

So

F(f) = 0.3(39.2) = 11.76 N (of Frictional Force)

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me