# Weight lifter - Acceleration of the barbell ?

1. Sep 27, 2011

### nukeman

Weight lifter - Acceleration of the barbell ??

1. The problem statement, all variables and given/known data

Here is the info, and below are the questions I am a little stuck on...

"An athlese who mass is 90.0 kg is performing a lift exercise. Starting from the rest position, he vertically lifts a barbell weight that weighs 490N (50kg). He lifts the barbell a distance of 0.60 m in 1.6 seconds. Assume that the barbell has a constant upward acceleration and the athlete has no accerlation as he lifts the weight.

1) How would I draw a FBD for the barbell and the athlete? Which forces are acting on each the barbell and athlete?

2) Find the force that the athlete applies to the barbell to accerlate it?

3) Find the total force that the athlete's feet exert on the ground as he lifts the barbell?

2. Relevant equations

3. The attempt at a solution

1) How would I draw a FBD for the barbell and the athlete? Which forces are acting on each the barbell and athlete?

For the FBD on the barbell - Would it just be gravity(Fg) and applied(Fa) force pulling the weight up?

Not sure how to make a FBD for the athlete?

2) Find the force that the athlete applies to the barbell to accerlate it?

Would it just be applied force?

3) Find the total force that the athlete's feet exert on the ground as he lifts the barbell?

Really stuck on this one!

2. Sep 27, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

FBD for the athlete:

What are the forces acting on the athlete?

3. Sep 27, 2011

### nukeman

Re: Weight lifter - Acceleration of the barbell ??

Hi Sammy!

Well I would think Fg(gravity) downwards.

and Fn(normal force) from the floor upwards?

Is that it? Would the weights be an applied force?

P.S. - Am i correct for the FBD forces on the barbell?

4. Sep 27, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

Does the athlete exert force on the barbell ?

Then what does Newton's 3rd Law say about the barbell exerting force on the athlete?

5. Sep 27, 2011

### nukeman

Re: Weight lifter - Acceleration of the barbell ??

Yes, the athlete exerts a force on the barbell.....For every action, there is a positive and equal reaction. ?

So on the barbell there is only Fg and Fa forces?

And on the person, there is Fg, Fnormal, and Fapplied?

6. Sep 27, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

For every action (force) there is an equal (in magnitude) and opposite (in direction) reaction (force).

I repeat, "Then what does Newton's 3rd Law say about the barbell exerting force on the athlete? "

7. Sep 27, 2011

### nukeman

Re: Weight lifter - Acceleration of the barbell ??

So your saying the barbell is exerting an equal in magnitude, and opposite reaction in direction?

8. Sep 27, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

Yes.

(And Newton says the same thing.)

9. Sep 27, 2011

### nukeman

Re: Weight lifter - Acceleration of the barbell ??

would that force be an applied force?

Any chance u can help with other questions?

"Find the force that the athlete applies to the barbell to accerlate it?"

10. Sep 27, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

What's the net force exerted on the barbell if it accelerates as described?

11. Sep 27, 2011

### nukeman

Re: Weight lifter - Acceleration of the barbell ??

By Netforce, do you mean mass*acceleration?

So, mass of 90.0 kg * acceleration of .375 m/s correct?

so 33.75 ? ???

12. Sep 27, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

Acceleration is incorrect. also incorrect units.

The barbell accelerates uniformly. It travels 0.6 m in 1.6 s for an average velocity of 0.375 m/s. But since it starts from rest (zero velocity), its final velocity is 0.75 m/s, which gives the correct average velocity.

So the barbell's velocity changes by 0.75 m/s in a time of 1.6 s. What's its acceleration during that time?

Last edited: Sep 27, 2011
13. Sep 29, 2011

### nukeman

Re: Weight lifter - Acceleration of the barbell ??

So the acceleration would be 0.468 m/s^2 ?

14. Sep 29, 2011

### SammyS

Staff Emeritus
Re: Weight lifter - Acceleration of the barbell ??

0.46875 m/s2