# Newton's first law of motion [closed]

A dog sleeping in a room and after some time it suddenly woke up and started running so its state of rest changed but no external force is applied on it.

Every one will answer this like **the leg of the dog applied force on earth and earth applied force against it ** But How did it's leg of the dog moved.

• It is not a bad question in my opinion Commented Jul 20, 2020 at 6:22
• It is simple and basic - but why should such questions be down voted? Not everyone is at same level of expertise on any subject. Commented Jul 20, 2020 at 6:30

## 2 Answers

Internal parts of the system can move without any application of an external force. Consider dog as a system. Then legs can move - internal force of muscles.

The center of mass of the system (i.e. dog) can't move without an external force. When legs move they hit the ground and ground provides that external force.

If ground was smooth - dog (more precisely dog's center of mass - say it is inside dog's tummy) won't be able to move. No matter how much it moves its legs - it will just slip and the center of mass of the dog i.e. its tummy will be stationary.

To give you another example - if the road was perfectly smooth your car won't be able to move while its wheels will keep spinning.

That's an internal force that your muscles apply . In this case dog muscles apply an internal muscular force so that dog leg moves.

• This answer is wrong. Internal forces can never accelerate a system. You need external forces to accelerate the system.
– user258881
Commented Jul 20, 2020 at 6:25
• @FakeMod Then how does the leg moves ? Commented Jul 20, 2020 at 6:31
• aman_cc explains that, please see their answer.
– user258881
Commented Jul 20, 2020 at 7:08
• The op was clear on the part that how does dog's com moves/accelerate , i.e. due to the friction force . I only addressed the movement of the dog's leg. Commented Jul 20, 2020 at 7:20
• Well, legs do move due to internal muscular forces, bjt that isn't going to take us anywhere? The meat of aman_cc's answer is the rest of his answer which mentions that external forces are necessary to "move". I wouldn't have downvoted (and would even take back the downvote) if you talked about external forces in your answer. Withoutthem, your answer is misleading, if not incorrect.
– user258881
Commented Jul 20, 2020 at 7:22