B Upwards movement -- Lifting weights while standing on a scale

[Eric Mackintosh]

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...

 

[ergospherical]

If upward acceleration of the weight (of mass $m$) is $a$ then additional reading on scales, compared to no weight present at all, is $m(g+ a)$.

 

[PeroK]

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...

You've induced me to indulge in a spot of experimental physics.

I stood on my scales with an extra $20 \ kg$ weight and the reading jumped up and down from about $112 \ kg$ to $118 \ kg$ as I raised the weights. Once I stood still, however, the scales settled on $112 \ kg$ and didn't change subsequently even when I raised the weights.

You need to make sure that your scales do not function the way mine do.

 

[Eric Mackintosh]

Ok, I was assuming that since force =ma that the weight should go up while the weight is in motion upwards?

 

[PeroK]

Ok, I was assuming that since force =ma that the weight should go up while the weight is in motion upwards?

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.

 

[berkeman]

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]

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.

It must accelerate to begin with?

 

[Eric Mackintosh]

Is it an analog or digital scale? If it is a digital scale, it is likely using digital filtering to filter out movement artifacts...

My scales were a simple mechanical set

 

[PeroK]

It must accelerate to begin with?

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]

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.

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]

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.

What do you think? What do Newton's laws tell you?

 

[Eric Mackintosh]

What do you think? What do Newton's laws tell you?

To every force there is an equal and opposite reaction?

 

[Eric Mackintosh]

To every force there is an equal and opposite reaction?

I am not fully sure that this applies when physiology is involved, I think the force may somehow be absorbed into the lifting muscles.

 

[PeroK]

I am not fully sure that this applies when physiology is involved, I think the force may somehow be absorbed into the lifting muscles.

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]

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.

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]

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?

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 ...

Holding a mass above your head is no different. It's just an extra part of your body as far as the scales are concerned.

Unless your centre of mass accelerates external forces on you must be balanced.

 

[Eric Mackintosh]

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?

 

[Steve4Physics]

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?

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.

During this short time-interval, the pressure in the tyres will increase above the rest-pressure. Then during the constant velocity motion, the pressure will be the same as the rest-pressure.

(If the truck is on sufficiently responsive weighing scales, there will be a brief increase in the reading. Then during the constant velocity motion, the reading will return to its rest-value.)

At the end of the upwards motion there will be deceleration - a downwards acceleration - so the load’s velocity reduces to zero. You can work out what the pressure does (and what would happen to the reading on weighing scales) during and after this deceleration..