標題:
sign of P.E.
發問:
I saw a definition about P.E. in my physics textbook. It stated that ' gravitational potential energy of an object can also be defined as the work done by an external agent in bringing the object from infinity to that point. As opposed to the gravitational force acting on the object, the external agent applies... 顯示更多 I saw a definition about P.E. in my physics textbook. It stated that ' gravitational potential energy of an object can also be defined as the work done by an external agent in bringing the object from infinity to that point. As opposed to the gravitational force acting on the object, the external agent applies a restraining force and reduces the energy of the object.Work done by the external agent is negative.' I don't understand. What is the external agent? Does the external agent belong to gravitational force? If yes, the direction of the displacement and the gravitational force are the same and both are negative, then the work done should be positive(negative x negative)? I am Really confused!!! Please give further explanation.
最佳解答:
The external agent here means some external force (e.g. the force exerted by a rocket, say, attached to the object of concern). It is not the graviational force. From the definition, the object is moved from infinity to a point under a gravitational field. Because gravitation is an attractive force, the object could thus "fall" under gravity on its own from infitiy to that point. This is similar to the case that when you hold a book in your hand and then release it, the book will fall under gravity to the ground. Now, if you exert an upward force onto the book with your hand such that the book is gently lowered to the ground. The force exerted by your hand (which is the external force and is pointing upward) and the displacement of the book, which is downward are in opposite direction. The work done by your hand is thus negative. In physical terms, your hand (the external force) has done negative work onto the book to decrease (or lower) its potential energy. When such explanation is transpired to an object at infinity, and then moved to a certain point, the work done by an external force acts in the direction towards infinity (i.e. opposite to the direction of gravity) in order to lower the potential energy of the object. The external force direction and displacement are thus acting in opposite direction, this leads to a negative work done.
其他解答:
sign of P.E.
發問:
I saw a definition about P.E. in my physics textbook. It stated that ' gravitational potential energy of an object can also be defined as the work done by an external agent in bringing the object from infinity to that point. As opposed to the gravitational force acting on the object, the external agent applies... 顯示更多 I saw a definition about P.E. in my physics textbook. It stated that ' gravitational potential energy of an object can also be defined as the work done by an external agent in bringing the object from infinity to that point. As opposed to the gravitational force acting on the object, the external agent applies a restraining force and reduces the energy of the object.Work done by the external agent is negative.' I don't understand. What is the external agent? Does the external agent belong to gravitational force? If yes, the direction of the displacement and the gravitational force are the same and both are negative, then the work done should be positive(negative x negative)? I am Really confused!!! Please give further explanation.
最佳解答:
The external agent here means some external force (e.g. the force exerted by a rocket, say, attached to the object of concern). It is not the graviational force. From the definition, the object is moved from infinity to a point under a gravitational field. Because gravitation is an attractive force, the object could thus "fall" under gravity on its own from infitiy to that point. This is similar to the case that when you hold a book in your hand and then release it, the book will fall under gravity to the ground. Now, if you exert an upward force onto the book with your hand such that the book is gently lowered to the ground. The force exerted by your hand (which is the external force and is pointing upward) and the displacement of the book, which is downward are in opposite direction. The work done by your hand is thus negative. In physical terms, your hand (the external force) has done negative work onto the book to decrease (or lower) its potential energy. When such explanation is transpired to an object at infinity, and then moved to a certain point, the work done by an external force acts in the direction towards infinity (i.e. opposite to the direction of gravity) in order to lower the potential energy of the object. The external force direction and displacement are thus acting in opposite direction, this leads to a negative work done.
其他解答:
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