GCSE Physics - Forces box

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xx.arianagrande
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#1
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Can someone please explain this question to me?

Q1.3 George is pushing a box at a constant speed along a level surface. He applies a force of 400N to the box. As George pushes the box, the amount of chemical energy stored in his body decreases. Into which object (or objects), and into which type of energy store, is this energy transferred?

Ans: [1] Into the internal (thermal) energy store of [1] the box and ground
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thrivingfrog
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Due to friction maybe?
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xx.arianagrande
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(Original post by thrivingfrog)
Due to friction maybe?
but why isn't the answer into the box's kinetic energy store
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Ira Acedia
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Internal Energy includes Kinetic Energy as well as Potential Energy.

As the surface is level and the box is being moved at a constant speed, then there is no change in the Kinetic Energy of the box, assuming the mass is constant, which is usually going to be the case.
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Stonebridge
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(Original post by Ira Acedia)
Internal Energy includes Kinetic Energy as well as Potential Energy.

As the surface is level and the box is being moved at a constant speed, then there is no change in the Kinetic Energy of the box, assuming the mass is constant, which is usually going to be the case.
While he is pushing the box at constant speed (as the question asks) the kinetic energy of the box is not increasing.
So the energy George is using up during this time is not being converted to kinetic.

The kinetic energy the box does have while moving was given to the box right at the start. This happened before the situation* which the question is asking you about.

* 'George is pushing a box at a constant speed along a level surface.'
Last edited by Stonebridge; 2 months ago
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xx.arianagrande
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(Original post by Ira Acedia)
Internal Energy includes Kinetic Energy as well as Potential Energy.

As the surface is level and the box is being moved at a constant speed, then there is no change in the Kinetic Energy of the box, assuming the mass is constant, which is usually going to be the case.
ohh ok, so no energy is transferred into the kinetic energy store of the box since it is already moving at a constant speed? instead energy is transferred to the thermal energy store of the box and ground (due to friction)?

also if internal energy means kinetic and potential energy, why does answer mention the internal energy being 'thermal'?
Last edited by xx.arianagrande; 2 months ago
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Ira Acedia
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(Original post by xx.arianagrande)
ohh ok, so no energy is transferred into the kinetic energy store of the box since it is already moving at a constant speed? instead energy is transferred to the thermal energy store of the box and ground (due to friction)?

also if internal energy means kinetic and potential energy, why does answer mention the internal energy being 'thermal'?
Internal energy is also called thermal energy, so the mark scheme puts thermal there in brackets as an alternative name. Internal energy is equivalent to the sum of the kinetic and all potential energies.

You may be aware that temperature is defined as the average kinetic energy of the molecules in a sample, if this helps wrap your head around why "thermal" and internal can be synonymous. Or, things that move faster are hotter, in simpler terms.

And yes, due to friction, it is transferred to the thermal energy store of the box and the ground. This actually flows as heat here, meaning that the temperature here increases (which, if you think about logically, does make sense by observation. Friction makes things hotter). And by our earlier definition, that means kinetic energy has increased.

So the kinetic energy of the particles of the box in one part has actually increased, but heat's definition includes the transfer of energy, so energy is being transferred to the ground as well, and vice versa.

So overall, if we were to take the box as a whole, as both mass and speed are constant, its kinetic energy is constant. However if we were to take the particles at the part of the box in contact with the ground, the temperature (and therefore average kinetic energy) here is changing.

Due to friction and it being a level surface, the work done is not being stored as potential energy. It's wrong (I think? or at least, misleading) to say that the kinetic energy of the box is changing due to the above reasons, but the potential energy is not changing either. So technically saying that the internal energy of the box has changed would also be wrong (as it is the sum of the other two).

Rather, the energy is stored as thermal/internal energy in the box as well as the floor, which is why both are specified in the answer.
Last edited by Ira Acedia; 2 months ago
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