# Homework Help: Force exerted on Bench Press

1. Jan 1, 2012

This is my own question, so sorry if the question might not be as clear.

1. The problem statement, all variables and given/known data
During the Bench Press exercise, a person unracks a 200kg barbell and places it 46.5 cm above his/her chest. It took the person 1.06 seconds to lower the barbell to his/her chest at a constant speed. Then the person takes 0.92 seconds to get the bar 17 cm off his/her chest and 0.8 seconds to lockout the barbell at 46.5 cm. Calculate the amount of force it took for the entire rep.

2. Relevant equations
Fnormal = Mass x (Acceleration + Gravity)
X(final) = X(start) + V(start)t + 1/2 at^2
a=accleration t=time

3. The attempt at a solution
I think I solved the ascending part of the bench press but I don't know where to begin with the descending part.

The ascending phase is basically:

X(final) = X(start) + V(start)t + 1/2at^2
.17m = 0 + 0(0.92) + 1/2a(0.92)^2
a = 0.4017m/s^2

Fnormal = m(a+g)
Fnormal = 200(0.4017+9.81)
Fnormal = 2054.34 N

X(final) = X(start) + V(start)t + 1/2at^2
.295 = 0 + 0(0.8) + 1/2a(0.8)^2
a = 0.922m/s^2

Fnormal = m(a+g)
Fnormal = 2146.4 N

2146.4 + 2054.34 = 4200.74 N
I think that is the amount of force exerted during the ascending part of the bench press

Now I have no idea how to do the descending part but this is what I think. Im pretty sure the equations used for the descending phase are the same, since force and gravity will always be 200 kg and 9.81, the difficult part would be calculating the accleration. Since in descending, the bar is being "pulled" down to your chest, so that means gravity is working for the lifter. This is all I could get. Any help will be appreciated!

2. Jan 2, 2012

### cupid.callin

You can use:
Change in kinetic energy = sum of all the work done on the body

3. Jan 2, 2012

Wat do you mean? Im not trying to find work or kinetic energy, Im having difficulties with finding the force exerted by the body during the descending phase of the bench press.

4. Jan 2, 2012

### neyzenyelda

wow!, 200kg bench press, a superman!
I havent been able to exceed 50kg! :)

5. Jan 2, 2012

### cupid.callin

For descending part, speed is constant so change in KE is ______

Thus total work done is ____.

Now list all the forces acting on the bench press and find the force
HINT: Work = Force * distance & for gravity F=mg

6. Jan 3, 2012

sorry but im not following you. I haven't been taught any physics yet and even though I understand the formula, I don't really know the concept or the relation between anything to be honest.

7. Jan 3, 2012

### SammyS

Staff Emeritus
I don't usually comment about the wording of problems, but (depending upon the intent of the author) this problem is either:
Somewhat poorly worded,
OR
very poorly worded.​

I assume that the intent is to find the average force, in any case. -- averaged over time, of course.

I further assume that you are to find the average force exerted by this (very strong) person, rather than finding the average net force.

What is the net Work exerted by the person for one whole rep ?

Actually, I think it would have been more instructive to find the average force exerted by the person during each of the 3 stages of a "rep".

8. Jan 3, 2012

I wrote this question my self based on my own inquiry. I don't have much or if at all any experience with doing physics problem. This question is due to my curiousity as I am planning to do a science fair project on examining strength curve of training with chains and conventional weightlifting.

What I am confused bout is basically how to calculate the amount of force exerted by the athlete during the descending phase of the bench press. http://fitnessanddefense.com/wp-content/uploads/2011/03/increase-bench_press.jpg
If you are unclear about the exercise, from the picture, you can see that the barbell goes down then up then down then up. I understand it takes 200x9.8=1960N of force just to keep it controlled and balanced but how much force would it take to have a controlled/balanced descend.
Anywayz, if no body can solve it then please check if my calculation for the ascending phase is correct. Because for some reason, when I calculated the amount of force taken to fully extend the barbell, the result isn't the same as what I got in the first post.

9. Jan 18, 2012

### Aert

If you had the distance of the bar from it's max height to your chest:
Potential Energy at max height = mgh = (200kg)(9.8)(h)
Potential Energy at min height = 0

Difference in potential energy from the min to the max height is the work done.
Work= PE = Fh (h is the distance).
PE/h = average force

10. Jan 19, 2012

### genericusrnme

50kg?
surely you jest

In reply to the original question
1. I know no girl that could bench 200kg so let's just rule out that possibility to start off with :tongue2:
2. 'amount of force' doesn't really mean anything, magnitude would be a better way of wording it but the magnitude changes during the whole rep
it sounds to me like the question is asking for the work done during the rep since work is the integral of the force along the path which would be less than you'd expect (if you expect it to be something big, that is) what happens when you take closed loop in a uniform field?