Acceleration of a person jumping off of a bathroom scale

[Physics_Amazing]

Homework Statement

A 72 kg person jumps up off a bathroom scale. Determine the acceleration of the person when the scale reads 840 N.

Relevant Equations

Fnet=Ma
Fg=mg

fnet=ma
Fn-fg=ma
840 - (72)(-9.8) = 72a
a= 840 - (-705.6) /72
a=21.19m/s^2

The correct answer is 1.9m/s^2 unsure of how to get that

 

[hutchphd]

There are two external forces acting on the man. Name them.

 

[Physics_Amazing]

There are two external forces acting on the man. Name them.

Normal and Gravity

 

[hutchphd]

Yes I see you have that correct.
Did you ever learn arithmetic? Carefully carry the units...I don't see how you could screw this up! (and I'm a professional).

 

[kuruman]

g = +9.8 m/s² should be substituted in the equation. You already put the minus sign in the equation Fn - fg = ma in front of fg to indicate that it is down. Then you put an extraneous negative sign when you substituted the numerical value, essentially saying that the weight force is up. You can only say that the weight is "down" once, not twice.

 

[Physics_Amazing]

g = +9.8 m/s² should be substituted in the equation. You already put the minus sign in the equation Fn - fg = ma in front of fg to indicate that it is down. Then you put an extraneous negative sign when you substituted the numerical value, essentially saying that the weight force is up. You can only say that the weight is "down" once, not twice.

Thank you.

 

[Orodruin]

Thank you.

A good sanity check would be if you did exactly the same procedure for when the scale reading is the actual weight of the man. You should, of course, get zero.

 

[PeroK]

Homework Statement:: A 72 kg person jumps up off a bathroom scale. Determine the acceleration of the person when the scale reads 840 N.
Relevant Equations:: Fnet=Ma
Fg=mg

fnet=ma
Fn-fg=ma
840 - (72)(-9.8) = 72a
a= 840 - (-705.6) /72
a=21.19m/s^2

This is correct if the bathroom scale is attached to the ceiling and the person is jumping down.

 

[theanswer2physicsisu]

Homework Statement:: A 72 kg person jumps up off a bathroom scale. Determine the acceleration of the person when the scale reads 840 N.
Relevant Equations:: Fnet=Ma
Fg=mg

fnet=ma
Fn-fg=ma
840 - (72)(-9.8) = 72a
a= 840 - (-705.6) /72
a=21.19m/s^2

The correct answer is 1.9m/s^2 unsure of how to get that

The double negative which arises where you plug-in values for Fg (-9.8) for the downward gravitational acceleration is incorrect because you already account for directionality by minusing Fg from Fn (Fn - Fg).

 

[theanswer2physicsisu]

Homework Statement:: A 72 kg person jumps up off a bathroom scale. Determine the acceleration of the person when the scale reads 840 N.
Relevant Equations:: Fnet=Ma
Fg=mg

fnet=ma
Fn-fg=ma
840 - (72)(-9.8) = 72a
a= 840 - (-705.6) /72
a=21.19m/s^2

The correct answer is 1.9m/s^2 unsure of how to get that

It would be helpful to carry units throughout the math/arithmetic. Also, the parantheses in the second-to-last step should encompass the whole numerator term, (840-705.6)/72 which will get you to around just below 2 or 1.9 m/s^2.