Newton's Laws - Weight Lifting

In summary, the athlete's feet exert a force on the ground that is greater than the weight of the barbell he is lifting.
  • #1
Peach
80
0

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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  • #2
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
 
  • #3
The barbell weight changes? I don't get this part...
 
  • #4
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...
 
  • #5
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.
 
  • #6
Oh, okay. Fair enough.
 
  • #7
So... the normal force is N = mg + w(2x/(t^2)+1) ? Something like that?
 

1. What are Newton's Laws of Motion?

Newton's Laws of Motion are three fundamental principles that describe the behavior of objects in motion. They were developed by Sir Isaac Newton in the 17th century and are still used today to explain how objects move and interact with each other.

2. What is the first law of motion?

The first law of motion, also known as the law of inertia, states that an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an external force.

3. How does Newton's second law apply to weight lifting?

Newton's second law states that the force applied to an object is equal to its mass multiplied by its acceleration. In weight lifting, this means that the heavier the weight being lifted, the more force is required to accelerate it and the harder the muscles must work.

4. What is the relationship between force and acceleration in weight lifting?

The relationship between force and acceleration in weight lifting is directly proportional. This means that as the force applied to an object (in this case, the weight) increases, the acceleration of the object also increases. This is why lifting heavier weights requires more force and results in greater muscle activation and growth.

5. How does Newton's third law apply to weight lifting?

Newton's third law states that for every action, there is an equal and opposite reaction. In weight lifting, this means that when you lift a weight, the weight is also exerting an equal force on your muscles. This is why muscles can become fatigued and sore after weight lifting, as they are constantly working to overcome the force of the weight being lifted.

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