Question

@ProfBrainstorm Physics Last Problem

Answer 1

I'm thinking that we're supposed to imagine a table of mass 22kg, with those various weights sitting on it.

Answer 2

Well the weights on the table will just add to the weight of the table itself, to give the total force acting on the floor

Answer 3

So do i add all the Newtons?

Answer 4

yes, but remember you need the weight of the table in newtons, the question only gives you its mass

Answer 5

Would I use f=ma?

Answer 6

i got 165N for all the objects

Answer 7

yes, with a being g, the acceleration due to gravity

Answer 8

164 looks right

Answer 9

sorry, 165 i meant

Answer 10

no, we've gone wrong

Answer 11

we're trying to calculate the total downward force on the floor, and that is due to the weight of all those objects, plus the weight of the table, okay ?

Answer 12

okay

Answer 13

now we are given the weights of the various objects directly in the question, so that is fine
but we only know the mass of the table
if an object has mass m, then its weight is mg, where g is the acceleration due to gravity, numerical value 9.8

Answer 14

so if the table has a mass 22kg, what is it's weight ?

Answer 15

if an object has mass m, how do I calculate it's weight ?

Answer 16

22 times 9.81

Answer 17

that's right

Answer 18

okay, so now we have all the weights and we can add them up to find the total

Answer 19

380.82N

Answer 20

correct

Answer 21

so now, what about this coefficient of static friction ?
if we have a weight W bearing down on a surface, the coefficient tells us how much force we need to apply sideways, in order to make the weight move on the surface

Answer 22

force needed to make object move = coefficient of static friction * weight of object

Answer 23

do we multiply the weight by the coefficient of static friction?

Answer 24

yes, that's all there is to it

Answer 25

380.82*0.235=89.4927 ?

Answer 26

why would it be unitless ? what kind of quantity have we calculated ?

Answer 27

We calculated how much force need to move the table

Answer 28

right, and force is certainly not unitless, we were working in Newtons for the forces

Answer 29

yes that's right, 89.5N is probably fine
notice that the static coefficient is higher than the kinetic coefficient of friction for a given surface
that means you have to give an object a bit more of a push to overcome the static friction, compared to just keeping it moving across the surface

Answer 30

yes, i think we have answered the question
remember, it's much better to ask a bunch of questions and make progress, than to keep quiet and stay stuck : )

Answer 31

Okay, thank you so much. You really helped me! I hope you enjoyed your medals :)