How Much Energy Is Needed to Drag a Block Over a Distance?

AI Thread Summary
To calculate the energy required to drag a block over a distance L, the friction force acting against the movement is given by Fr = μmg, where μ is the coefficient of friction and m is the mass of the block. The force in the direction of movement is F = ma, leading to a resultant force of F - Fr = m(a - μg). The energy required can be expressed as the work done against friction, which is Fr multiplied by the distance L. The assumption that the energy required is equivalent to the change in kinetic energy is incorrect if the block is moved at constant speed, as the work done is solely to overcome friction. Therefore, the focus should be on calculating the work done against friction to determine the energy needed.
indie452
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Homework Statement



whats the energy required to drag a block a distance L

Homework Equations



coefficiant of friction between block and floor = \mu
density of block = p

force*distance = change in kinetic energy

The Attempt at a Solution



friction acting against movement Fr = \mumg m = mass of block

force in direction of movement F = ma

resultant force = F - Fr = m(a-\mug)

resultant force*L = energy = m(a-\mug)L

i don't think the last step is right though as I am not sure sure that the assumption made that energy required is the same as the change in KE is correct...

help or guidance appreciated
 
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Although not stated as such, I think you are looking for the energy (work) required by a person or machine to just overcome friction and move the block at constant speed (no acceleration).
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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