Physics

Hi guys, i have a question about newtons laws. The question is , if for example, i exert a force on a door then a equal and opposite force from the door will be exerted upon me. If this is the case then how does the door move if the forces are both equal and opposite? Thanks

Reply 1

Htx_x346

17

Original post by mxk_fr_7

Hi guys, i have a question about newtons laws. The question is , if for example, i exert a force on a door then a equal and opposite force from the door will be exerted upon me. If this is the case then how does the door move if the forces are both equal and opposite? Thanks

the door moves because there's a force being applied to it? F=ma

Reply 2

Sinnoh

22

The forces are being exerted on two different things. The door is not applying an equal and opposite force to itself, it's applying the force to the person pushing it.

Reply 3

Kallisto

Entertainment Forum Helper

22

Original post by mxk_fr_7

Hi guys, i have a question about newtons laws. The question is , if for example, i exert a force on a door then a equal and opposite force from the door will be exerted upon me. If this is the case then how does the door move if the forces are both equal and opposite? Thanks

That is actio and reactio principle, I guess. Objects - so a door too - have an inertia, that is to say a counterforce is caused equal to the excerted force to stay in the resting position: -F = F

(edited 2 years ago)

Reply 4

mxk_fr_7

OP

8

Original post by Sinnoh

The forces are being exerted on two different things. The door is not applying an equal and opposite force to itself, it's applying the force to the person pushing it.

Yes, i understand that however wouldnt that mean the forces cancel out ? So how does the door move as a result

Reply 5

Callicious

22

Original post by mxk_fr_7

Yes, i understand that however wouldnt that mean the forces cancel out ? So how does the door move as a result

The forces are on two different things... not the same body... why would they cancel out?

Reply 6

Sinnoh

22

Original post by mxk_fr_7

Yes, i understand that however wouldnt that mean the forces cancel out ? So how does the door move as a result

You also need to remember that 0 resultant force means that velocity is constant - not necessarily 0.

Original post by mxk_fr_7

Hi guys, i have a question about newtons laws. The question is , if for example, i exert a force on a door then a equal and opposite force from the door will be exerted upon me. If this is the case then how does the door move if the forces are both equal and opposite? Thanks

This is quite a common question. I can understand where you come from and believe that your confusion originates from not discerning Newton 2nd law and Newton 3rd law.

Let talk about your question. Call the force that you exert on the door be

\vec{F}_{\text{You} \to \text{door}}

while the force that the door exerts on you be

\vec{F}_{\text{ door } \to \text{ You }}

From Newton’s 3rd law, we note that

\vec{F}_{\text{You} \to \text{door}} = -\vec{F}_{\text{ door } \to \text{ You }}

what is important is to note that the force is exerting different “object” NOT same “object” as mentioned by many others.

To decide whether the object “moves” or NOT, we need to use Newton 2nd law. Sum all the forces that exert on that object vectorially:
Apply Newton 2nd law to the door:

\vec{F}_{\text{You} \to \text{door}} + \text{other forces exerting on the door} = m_{\text{door}} \vec{a}

Note that we are summing the forces that exert on the door NOT the force that the door exerts on another object.

Apply Newton 2nd law to the you:

\vec{F}_{\text{ door } \to \text{ You }} + \text{other forces exerting on You} = m_{\text{You}} \vec{a}

Similarly, we are summing the forces that exert on You NOT the force that You exerts on another object.

Do you see any cancellation of the two forces?

Hope this makes sense to you now.

Reply 8

Jackson_19

You can go through this link and understand how this happens:https://dewwool.com/examples-of-inertia-of-rest/

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