What is the Work Done by Several Forces?

[Adrian Pete]

Homework Statement

Problem # 1

Homework Equations

Work done by several forces

w=f * d cos theta
w=mg
n=w

The Attempt at a Solution

I can't figure out how to solve a) and b)
NOT SURE:
so my attempt a) f = mg = 30kg*9.8m/s^2 = 294N =magnitute of the force must the worker apply.(NOT SURE)
b) WofF= f d cos theta = 294N * 4.5m * cos0 = 1323N
?

 

[kuruman]

Hi Adrian Pete and welcome to PF.

Your "relevant equations" show no equations. What do you mean by "work done by several forces"? How many forces do you have acting on the crate? How many of them are doing work?

 

[Adrian Pete]

5 forces acting on it Sir

 

[kuruman]

Can you name these 5 forces and find how much work each one does?

 

[Adrian Pete]

a) work must the worker apply = :(
b) work done by the force = :(
c) work done by friction = Wf = 73.5N*4.5m
d) work done by gravity = Wg = 294N*4.5m
e) work done by normal force Wn = 294N*4.5m
only these Sir :( can't figure out to solve a) and b)

 

[kuruman]

First, it seems you have misunderstood the questions (a) and (b). It also seems that question (a) has a typo. I believe the first two questions should read
(a) What magnitude of force (not work) must the worker apply?
(b) How much work is done by this force?
There are only 4 forces exerted by (1) the worker, (2) gravity, (3) normal force and (4) friction.
It also seems you think that Work = Force × Distance. This is not necessarily true and is not true for some of the forces here. Please look up the definition for work done by a constant force and apply it correctly.
Finally, you are told that the crate is moving at constant velocity. What must be true for this to be the case?

 

[Adrian Pete]

force = m * a?
so a is 0 because it moves at constant velocity?

so force = 30kg * 0 m/s^2

a) f = 0N

so work is w = f d cos theta so,

w = 0 * 4.5m cos 0

so

b) w = 0J = work done by the worker?

 

[kuruman]

orce = m * a?
so a is 0 because it moves at constant velocity?

This is correct. It is not correct that the force exerted by the worker is zero. The worker must exert a force otherwise the crate will not move. Given that the crate is moving and that its acceleration is zero, what must be tru about the forces acting on the crate?

 

[Adrian Pete]

since there is no acceleration therefore,

F - fk = ma = 0

F = fk ?

so F is basically fk?

so work done by F is the same as work done by fk?

 

[kuruman]

so work done by F is the same as work done by fk?

In magnitude. The work done by f_k is negative while the work done by F is positive.

 

[Adrian Pete]

oh okay, because cos 180 is -1. okay Sir. Thank you for the help ^ _ ^

 

[kuruman]

oh okay, because cos 180 is -1. okay Sir. Thank you for the help ^ _ ^

That's correct. You are welcome.