# F=dP/dT Force equals the rate of change of momentum question?

Force equals the rate of change of momentum with respect to time. F = dP/dT

Momentum equals mass times velocity. P = mv

So, if the mass or velocity doesn't change, dP/dT = 0. F = 0

Here's my question. If I throw an object in space, that object would move at a constant speed since there is no friction in space. So, the velocity doesn't change and the mass of the object doesn't change. So, in this case dP/dT = 0.

F = 0. How could such object not exert any force at all? The object is still moving. So, it must be exerting some force to the environment.

### 6 Answers

- Anonymous9 years agoBest Answer
If an object is moving in a vacuum (with no gravity, electric force), then there is nothing to exert force upon.

This is the whole point of Newton's laws. In an ideal case an object will move (and continue to move) with NO force at all. It cannot, however, change its direction/speed of movement unless a force is applied

- 4 years ago
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RE:

F=dP/dT Force equals the rate of change of momentum question?

Force equals the rate of change of momentum with respect to time. F = dP/dT

Momentum equals mass times velocity. P = mv

So, if the mass or velocity doesn't change, dP/dT = 0. F = 0

Here's my question. If I throw an object in space, that object would move at a constant speed since there...

Source(s): dp dt force equals rate change momentum question: https://shortly.im/BJ0hW - SallyLv 44 years ago
I hope the person responds. In the meantime, your equation is correct. This applies when mass is constant and velocity does not approach the speed of light. A different equation is used in special relativity. Observed from an inertial reference frame, the net force on a particle is proportional to the time rate of change of its linear momentum: F = d[mv] / dt. Momentum is the product of mass and velocity. This law is often stated as F = ma (the net force on an object is equal to the mass of the object multiplied by its acceleration). This can also be stated as net force on an object is equal to its rate change of momentum Notice that the second law of motion only holds when the observation is made from an inertial reference frame.

- 9 years ago
According to Newton's first law a body continues to be in the state of rest or uniform motion unless an external force is applied on it. Here the object is experiencing no force in its state of uniform motion.so it will continue moving. However if you need to alter the speed or bring the body to rest, a force needs to be applied.

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- gdrLv 59 years ago
you are describing an ideal case where there are no forces on the object. That is why it would n=move at a constant speed.

By the third newton law, if no force is exerted on the object, then the object would exert no force on the environment. indeed in this ideal case, there is no gravity, no electrical forces, no friction etc. So how could your object exert any force on other objects?

- sha wei stedLv 59 years ago
it creates a gravitational force while it is in existance and more so when its moving... that is to say that the faster it moves, the stronger its gravitational pull. So technically it IIISSSS exerting a force. But that doesn't apply to your equation. Look up Lagrangian's.