The discussion centers on the physics of lifting weights while standing on a scale, specifically addressing how force exerted on the feet changes during the lifting process. When a weight is lifted with upward acceleration, the normal force on the scale increases, resulting in a higher reading. However, if the weight is lifted at constant velocity, the scale reading remains unchanged. The conversation highlights the importance of understanding Newton's laws, particularly the second law, which states that the net force determines acceleration.
PREREQUISITESPhysics students, fitness enthusiasts, and anyone interested in the mechanics of weightlifting and force measurement will benefit from this discussion.
You've induced me to indulge in a spot of experimental physics.Eric Mackintosh said:if I move a weight upwards, will the force exerted on my feet change? I tried standing on scales and lifting a weight but nothing changed as the force seemed to be all in my arm muscles and no change in the weight on the scales...
The normal force between your feet and the scales must increase while the weights are accelerating upwards.Eric Mackintosh said:Ok, I was assuming that since force =ma that the weight should go up while the weight is in motion upwards?
Is it an analog or digital scale? If it is a digital scale, it is likely using digital filtering to filter out movement artifacts...Eric Mackintosh said:Ok, I was assuming that since force =ma that the weight should go up while the weight is in motion upwards?
It must accelerate to begin with?PeroK said:The normal force between your feet and the scales must increase while the weights are accelerating upwards.
But not if the motion is constant velocity.
My scales were a simple mechanical setberkeman said:Is it an analog or digital scale? If it is a digital scale, it is likely using digital filtering to filter out movement artifacts...
Yes, but the acceleration phase may be over quite quickly. So, the increase in force may only last a short time. I can see the digital display change only fleetingly on my scales.Eric Mackintosh said:It must accelerate to begin with?
Ok thank you ,:) tell me this got me thinking... Imagine I have a heavy weight balanced on my head and I want to lift it so I push it up... But it is too heavy to lift... Will the force I am pushing it upwards be felt as increased weight on the scales. Even though it doesn',,t move? Note here I am pushing up as hard as I can but I am not strong enough to move it. I can't work out the physics here.PeroK said:Yes, but the acceleration phase may be over quite quickly. So, the increase in force may only last a short time. I can see the digital display change only fleetingly on my scales.
What do you think? What do Newton's laws tell you?Eric Mackintosh said:Ok thank you ,:) tell me this got me thinking... Imagine I have a heavy weight balanced on my head and I want to lift it so I push it up... But it is too heavy to lift... Will the force I am pushing it upwards be felt as increased weight on the scales. Even though it doesn',,t move? Note here I am pushing up as hard as I can but I am not strong enough to move it. I can't work out the physics here.
To every force there is an equal and opposite reaction?PeroK said:What do you think? What do Newton's laws tell you?
I am not fully sure that this applies when physiology is involved, I think the force may somehow be absorbed into the lifting muscles.Eric Mackintosh said:To every force there is an equal and opposite reaction?
Newton's laws apply even to the human body. If there is an upward force from the ground greater than the combined downward gravitational force of you and everything you are holding (the "system"), then the centre of mass of the system must be accelerating upwards. That's Newton's second law. You can't avoid it.Eric Mackintosh said:I am not fully sure that this applies when physiology is involved, I think the force may somehow be absorbed into the lifting muscles.
Yes but what weight does the object need to be to observe that as you push upwards against it, the force goes through your feet and the scales register an increase?PeroK said:Newton's laws apply even to the human body. If there is an upward force from the ground greater than the combined downward gravitational force of you and everything you are holding (the "system"), then the centre of mass of the system must be accelerating upwards. That's Newton's second law. You can't avoid it.
That's not the way forces work. There are internal tensions in your body all the time. You ankle bone is holding your leg bone up and your leg bone is holding your hip bone up ...Eric Mackintosh said:Yes but what weight does the object need to be to observe that as you push upwards against it, the force goes through your feet and the scales register an increase?
When the load starts moving upwards, its velocity changes from zero to some constant value. Therefore there is upwards acceleration of the load – though only during a short time-interval.Eric Mackintosh said:Ok so if we now consider a forklift truck with a load on its front, if the load is lifted at constant velocity, no added load on the tyres but if the load was accelerated then the tyres would be compressed as long as the acceleration was happening?