# Can anyone clear my dilemma?

1. Mar 27, 2013

### mashwood

how can we explain newton's third law when a body is moving upward against earth's gravity with constant velocity?in another way if the applied force on the body is equal to it's weight according to the third law, why and how does it move?

2. Mar 27, 2013

### Staff: Mentor

If the (net) applied force on the body is always equal to its weight, and it's initially at rest, then it will stay at rest, and not move.

In order for it to start moving upwards, you have to apply a bit more upward force initially. After it starts moving, you can ease off to make the upwards force equal to its weight, and it continues moving at constant velocity. To bring it to a stop, you ease off some more, so the upwards force is less than its weight, until it comes to a stop. Then you make the upwards force equal to its weight again, and it stays at rest at its final height.

3. Mar 27, 2013

### mashwood

what if the same body is moving up with very high constant velocity.....is the applied force still equal to it's weight...but how?

4. Mar 27, 2013

### Bandersnatch

It's just Newton's second law of motion. No net force acts on the body, so it keeps on moving with constant velocity, no matter how high.

5. Mar 27, 2013

### mashwood

so, it seems to me that third law can't explain this phenomena?is it so? and how come there is no net force and still the body is moving with high velocity?

6. Mar 27, 2013

### Bandersnatch

The laws of motion are observations, not explanations. It's about what happens, not why it happens.

The third law, as applied here, states that the body pushes on whatever keeps it from falling in the gravitational field with the same in magnitude, but opposite force. That's all it says.

That it is moving with constant velocity in the absence of net forces has got little to do with the third law, and everything to do with the second.

It's got such a high velocity because you "gave" it to the body before the motion started.
Once it's got some velocity(including 0), it won't slow down nor speed up, unless a non-zero net force acts upon the body.

7. Mar 27, 2013

### sophiecentaur

I could be worth mentioning that, for a body under gravity, there is also a 'third law', reaction force on the Earth itself (to comply with the conservation of momentum).

8. Mar 27, 2013

### Staff: Mentor

You are skipping a step: the step where the force is much higher than the weight and it accelerates to that high velocity. Then the force can be reduced to be equal to the weight and the velocity will remain constant.

9. Mar 27, 2013

### Khashishi

It's hard to figure out where to begin. For something to be moving at constant velocity, the forces on the object have to cancel out. If you fire a gun upward, the bullet isn't going to travel with constant velocity. It's going to go up, slow down, and come back down (dangerously).

10. Mar 28, 2013

### CWatters

Because there is nothing to stop it moving. To stop moving it would have to slow down. Slowing down implies an -ve acceleration. To accelerate you need a force and since the net force is zero it cannot slow down.

The question is not "why does it move" but "why would it stop moving".

11. Mar 28, 2013

### CWatters

Consider a man in a helicopter taking off...

On the ground the vertical velocity is zero and he feels normal. Newton tells us that the floor of the helicopter exerts a force on the man = mg

In order to lift off the vertical velocity has to increase from zero to some non-zero value. That means there is a vertical acceleration and he feels heavier BUT only while the aircraft is accelerating vertically.

Once the helicopter is climbing at a constant velocity he will feel normal again. Newton tells us that the floor of the helicopter exerts a force on the man = mg.