Which uses more force, moving a stationary object or moving a moving object pulling in the opposite direction?

Question

My best example would be a door. Does it use more energy to open it or to push it open again when it is closing? The object would be the same weight and have the same mass.

Answer 1

When it is moving in the opposite direction. This is true because you have to push it until it slows down to a stop, and then push it open. This is due to momentum and force. An other way to think of it is to first eliminate friction. When it is at rest you only have to barely touch the door, but when it is moving you have to use the same amount of force that it has coming at you (to get it to a complete stop), plus a very little bit more (for it to move it the opposite direction). An example that will answer your question the easiest is to think of a small car. If the car is at rest, you might be able to push it. But if it is coming at you at 50 MPH........ I hope this helps.

Answer 2

Assuming that friction is negligible, any net force will produce the same net acceleration in a given object, no matter what its motion is before the force is applied.

However, if the object is stationary, it immediately begins moving in the direction of the acceleration. If the object is already moving in the opposite direction of the applied force, it first slows down and only reverses direction after it comes to a brief stop. Therefore, the force would need to be applied for a longer time to produce the same final velocity, unless the force were greater in magnitude.

Also, if the door had been stationary to begin with, the work done on it will be equal to the final kinetic energy. If the door ended up with that same final kinetic energy but had originally been moving in the opposite direction, the work done on it would have to include an additional amount equal to the original kinetic energy it had.

Answer 3

The force used is simply the force which is applied. You cannot say that in principle one of the situations you describe requires more force than the other. Either will react to whatever force is applied. The reaction will be slower for less applied force.

With energy on the other hand, If the system can be stopped for example by having it lift a weight then the energy used for stopping can be stored and even (conceivably) used to perform the subsequent accceleration in the opposite direction. In general however it depends on what method is used to perform the slowing down part of the process. If mechanical work is performed to slow the object down;e.g. you push against it; then you will perform work and both your energy and the energy of the system will be irretrievable. If a braking system were used then the energy taken out of the system to slow it down will be lost in heat and new energy will be required to get it going in the opposite direction.

In summary, you can use the energy of motion in one direction to get a system moving in the other (see e.g. a pendulum) but if you use an inefficient method to do the job you will need to put in extra energy.

I hope this generates more clarity than confusion.

Bramble

Answer 4

Newton's second law of motion. Yes it would take more energy in the form of Force to appose another force. Energy can not be created or destroyed so you must appose this kinetic energy already in the door.