Question

If a 10.0 kg object is on a surface that is inclined 30o and the coefficient of static friction is 0.65, what is the force of static friction?

Answer 1

We are given the mass of the object, we know gravity is 9.8 m/s, and we know that the force of static friction is equal to the normal force times the coefficient of static friction

Answer 2

and the normal force is equal to the mass of the object times g times cos theta so\[N=mg*\cos \Theta\]

Answer 3

ok

Answer 4

so now we solve for the normal force then multiply that by the coefficient of static friction to get the force

Answer 5

\[N=10*9.8*\cos30\]

Answer 6

how did you know that u had to choose cos??

Answer 7

this has to deal with the picture of the forces on the mass Ill draw it

Answer 8

now the normal force must equal the force due to gravity or else it would fall through the inlince

Answer 9

and we can see that they are not directly opposite to each other so gravity must have an x and y component

Answer 10

that answer is 55.2 am i right ?

Answer 11

that sounds right I dont have a calculator with me

Answer 12

here is a another one ,
A 10.0 kg box is placed on an inclined plane that makes an angle of 30.0o relative to the horizontal.

a. What is the force due to friction necessary to keep the box stationary?
for a i got 98.1 is it right ??

Answer 13

well for this we want to find the force along the plane so it would be the x part in this case so it would be mgsintheta

Answer 14

so fg also equals mgsintheta?

Answer 15

sorry i mean Ff

Answer 16

Ff isnt what equals mgsintheta or mgcostheta because Ff of friction always opposes the direction of motion

Answer 17

the place we get mgsin and cos from is due to gravity like in the picture above

Answer 18

but for a we need to find Ff

Answer 19

yes but see the force of friction always opposes the direction of motin

Answer 20

and if we want the box to stay stationary we need it to equal the force in the direction of motion

Answer 21

ok then how would we set up an equation to find Ff

Answer 22

well like I said if we want the box to stay in place all we need is for friction to equal the forward motion force so the net force is zero so there would be no motion

Answer 23

then its Ff=-mg right?

Answer 24

well the problem is that mg has 2 components because it is not along the same direction as the incline like in the picture