How Do Newton's Laws Apply to Weight Lifting?

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SUMMARY

The discussion focuses on applying Newton's laws to analyze the forces involved when an athlete lifts a barbell with constant acceleration. The key equation derived is F = w(a/g + 1), where F represents the total force exerted by the athlete's feet on the ground, w is the weight of the barbell, a is the acceleration, and g is the acceleration due to gravity. Participants clarify that the apparent weight of the barbell changes during acceleration, similar to the sensation of increased weight felt in an accelerating lift. The importance of considering both the normal force and the gravitational force is emphasized in calculating the total force on the ground.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Basic knowledge of force, mass, and acceleration (F=ma)
  • Familiarity with concepts of weight and normal force
  • Ability to interpret free body diagrams
NEXT STEPS
  • Study the implications of Newton's third law in dynamic systems
  • Explore the concept of apparent weight in accelerating frames of reference
  • Learn about free body diagram techniques for complex systems
  • Investigate the effects of varying acceleration on force calculations in physics
USEFUL FOR

Students in physics, athletes interested in biomechanics, and anyone studying the application of Newton's laws in real-world scenarios, particularly in weight lifting and dynamic movements.

Peach
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Homework Statement


An athlete whose mass m is performing weight lifting excersies. Starting from rest positions, he lifts, with constant acceleration, a barbell that weighs w. He lifts the barbell a distance of x in time of t. Use Newton's laws to find the total force his feet exert on the ground as he lifts the barbell.

Homework Equations


F=ma


The Attempt at a Solution


I drew the free body diagram already, with the force his feet exert on the ground downward, the normal force upward, and the gravity force downward. Is this correct? Is there any other force I'm missing?
 
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interesting problem... things to note, when the barbell is being lifted its weight is different from when it is stationary (or not under acceleration). The normal upward force on the barbell is the force provided by the lifting. So, total foce on ground would I guess come from the body weight and the modified barbell weight
 
The barbell weight changes? I don't get this part...
 
Peach said:
The barbell weight changes? I don't get this part...

Nor do I. My understanding of the problem is that if the barbell accelerates upward, then this means that he is pushing upward on it with a force, F that is > its weight. Therefore, since the barbell has mass M = w/g, we have:

Ma = F - w

==> wa/g + w = F

==> w(a/g + 1) = F

Now, if he pushes up on the barbell with this force, then presumably the barbell pushes down on him with the same force (Newton's third law, this is why the problem says to use Newton's lawS (plural)). Which means that the floor, in addition to supporting his weight mg as it normally does, must also support force F. That's the best I can come up with for this problem. Somebody let me know if I'm totally on crack...
 
cepheid

what you have done have actually demonstrated that the (apparent) weight of the barbell has changed since F = w (a/g+1) which is different from its original weight w. It is like inside a lift when the lift accelerate up you feel that your feet is pushing down the floor of the lift harder. The term "weight" can sometimes be confusing.
 
Oh, okay. Fair enough.
 
So... the normal force is N = mg + w(2x/(t^2)+1) ? Something like that?
 

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