# B Change in direction affecting velocity

1. Jul 13, 2016

### Zack K

I just have a simple question to ask people of physics since I keep on forgetting to ask my physics teacher.
Does changing your direction while in motion affect your velocity? If so why? What forces cause it's velocity to decrease. Assuming that friction is present.(Also would it affect an objects velocity in a friction-less environment?).

Last edited: Jul 13, 2016
2. Jul 13, 2016

### phinds

Do you understand the defintions of "speed" and "velocity"? What are they and how do they apply to this question?

3. Jul 13, 2016

### Zack K

Speed is a scalar quantity which means that there is no direction involved and is used for simple situations. When you get into physics, you start using velocity which is a vector quantity and it has a direction. I know that if you are going forward at a certain velocity and suddenly decelerate and go backwards, your velocity changes in that time. But I wanted to know if that happens when you are driving a car and lets say turn 20° to the left. Would that count as deceleration?

Last edited: Jul 13, 2016
4. Jul 13, 2016

### phinds

You first asked if it affected velocity. Does it?

5. Jul 13, 2016

### Staff: Mentor

Here's an easy case: swing a weight on a string in a fast circle around your head. The weight is moving at a constant speed but changing its direction all the time. Is there a force on it? Is it accelerating? If it is, then its velocity must be changing, because that's what acceleration is.
Velocity is a vector, and vectors can change without increasing or decreasing their magnitude... So you should not be assuming that the velocity is decreasing just because there's a force involved, nor that absence of increase/decrease implies that there's no force involved.

6. Jul 13, 2016

### Staff: Mentor

Why do you say "deceleration" and not "acceleration"?

7. Jul 13, 2016

### Zack K

Well I'm confused now since we just learned that forces cause acceleration. Hence the equation F=ma. So how is it possible for something to not accelerate if there is a force involved?

8. Jul 13, 2016

### Zack K

I just use that instead of acceleration in the negative direction.

9. Jul 14, 2016

### Staff: Mentor

What is the "negative direction" in your example, which implies two dimensions?

10. Jul 14, 2016

### Zack K

Well it's all relative. But on a graph, negative direction is usually left or down. Like if you throw a ball in the air. It's experiencing negative acceleration since gravity is forcing it downwards. Usually in equations we also put -9.8 m/s2

11. Jul 14, 2016

### PeroK

There is a distinction between "acceleration/deceleration" in everyday speech and the formal physics term "acceleration". The definition of "acceleration" in physics is any change in velocity, be it speeding up, slowing down and/or changing direction. And any such change must be caused by a force. So, you have:

$F = ma$

This is for motion in one dimension (along a single straight line) and here $a$ can be positive or negative. It's still possible to talk about "deceleration" when something is slowing down, but slowing down is still, formally, acceleration.

You also have:

$\vec{F} = m\vec{a}$

Which is a vector equation and applies in 1, 2 or 3 dimensions. In this case, you are best simply to talk about the acceleration $\vec{a}$.

In particular, you should start to think of "acceleration" not as a "speeding up", but as any change in velocity.

12. Jul 14, 2016

### Staff: Mentor

Right, in everyday language, "acceleration" (increasing speed) and "deceleration" (decreasing speed) are considered as opposites. In physics language, "deceleration", when it is used at all, means "decreasing speed", the same as in everyday langauge, and is simply one kind of "acceleration" (any change of velocity, whether it is by changing speed or changing direction).

In everyday language, as far as I know in English, the term for "changing direction while keeping speed constant" is simply "turning" which is separate from both "acceleration" and "deceleration" (again, in everyday language).

13. Jul 14, 2016

### CWatters

The key is to understand that velocity has components speed and direction and that a change to either component implies a change in velocity which by definition is an acceleration. So objects moving in a circle or changing direction in some other way are accelerating.

It can be confusing to talk about decelerating. For example a car accelerating at say -1m/s2 is not necessarily decelerating.

Velocity and acceleration vectors don't always point in the same direction.

14. Jul 14, 2016

### Staff: Mentor

If there's a net force, then there will be acceleration. But acceleration does not necessarily imply a changing speed.

When Nugatory made this statement, I think he meant to say that you should not assume that the speed is increasing or decreasing when there is acceleration: