# Is it correct to say that; an object moving at a constant velocity is moving in a straight line?

I think it is correct because: v = displacement/time, if the direction changes, the displacement changes so the velocity is not constant anymore. Please correct me if I'm wrong

• The second paragraph here explains nicely: en.wikipedia.org/wiki/Velocity Jul 20, 2020 at 12:48
• That will depend on the reference from where you are choosing the straight line. If you take a car moving on a straight road with constant speed , then yes it has constant velocity. Observe the same car from Mars , its now performing a circular motion around earth Jul 20, 2020 at 13:22
• Thanks for the useful comment @NoahJ.Standerson, it's interesting! :D Jul 20, 2020 at 14:00
• No you an drive a circle path with constant velocity
– Eli
Jul 20, 2020 at 15:00
• @Eli, velocity is a vector, circular motion is accelerated motion, hence the velocity is not constant, the speed is constant in that case. Jul 20, 2020 at 17:04

Yes you are correct$$^1$$, to define an object's velocity we have to specify in which direction it is travelling and how fast - this defines a vector quantity. If we say the vector is unchanging, its length (the object's speed) and its orientation (which direction the object is travelling) must be constant in time which corresponds to an object travelling in a straight line at a constant speed.

$$^1$$ Ignoring complications for example the presence of a gravitational field.

Yes. Velocity has both magnitude as well as direction, so a constant velocity would imply constant direction as well. Constant speed, however doesn't necessarily mean straight-line motion (e.g. uniform motion along a circular path).

The vector has direction and magnitude. If one of the two quantities changes, the vector changes.

And velocity is a vector. Therefore, the direction change leads to the velocity change which is the acceleration.

• Can we say that a straight line is circumference of circle of infinite radius Jul 20, 2020 at 13:08
• @sheltonBenjamin Why do you need the definition? Jul 20, 2020 at 13:10
• Sorry its off topic but i still was wondering Jul 20, 2020 at 13:19
• @sheltonBenjamin You may open a new question including the reason why you consider the situation. Jul 20, 2020 at 13:22
• No i am not including this in the situation and question it is just a different question Jul 20, 2020 at 13:29