# Newton's First Law

1. Nov 24, 2014

### avito009

Everything should move in a straight line if there is no friction. But when an aircraft goes into space it needs to move burning its fuel. Why does it not move constantly in straight line?

So is the reason behind the fact that a rocket needs to be propelled, that the gravity of the sun and other planets does not allow it to move in straight line without fuel?

2. Nov 24, 2014

### CWatters

A spacecraft would travel in a straight line if there were no forces acting on it (eg no gravity from a star/planet).

3. Nov 24, 2014

### zoki85

Where did you hear that?

4. Nov 24, 2014

### Staff: Mentor

That's not right. Everything should (and does) move in a straight line if there is no external force acting on it. Friction is just one of the many forces that might act to push an object off its inertial trajectory.

And in fact, just as Newton's first law says that an object not experiencing any external forces moves in a straight line at a constant velocity, Newton's second law says that if there is an external force at work, the object will not move in a straight line at a constant velocity - mathematically we have $F=ma$.

5. Nov 24, 2014

### hafiz ns

first law states that every objects in the universe continues in its state of motion unless an external force act on it. it dosen't mean friction.

6. Nov 24, 2014

### Staff: Mentor

Note that an object can still move in a straight line with a force acting on it, the force would just be in the same or opposite direction that the object is already traveling in and would just accelerate or decelerate the object.

7. Nov 24, 2014

### Staff: Mentor

Are you thinking of when a satellite is launched, it starts out going straight up, then curves into a horizontal path? That is done by rotating the rocket engines slightly so as to change the direction of thrust.