Change in direction affecting velocity

[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?).

 

[phinds]

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?).

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

 

[Zack K]

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

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 let's say turn 20° to the left. Would that count as deceleration?

 

[phinds]

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 an go backwards, your velocity changes in that time. But I wanted to know if that happens when you are driving a car and let's say turn 20° to the left. Would that count a deceleration?

You first asked if it affected velocity. Does it?

 

[Nugatory]

Does changing your direction while in motion affect your velocity?

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.

What forces cause its velocity to decrease?

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.

 

[jtbell]

But I wanted to know if that happens when you are driving a car and let's say turn 20° to the left. Would that count as deceleration?

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

 

[Zack K]

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.

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?

 

[Zack K]

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

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

 

[jtbell]

But I wanted to know if that happens when you are driving a car and let's say turn 20° to the left. Would that count as deceleration?

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

just use that instead of acceleration in the negative direction.

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

 

[Zack K]

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

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

 

[PeroK]

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

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.

 

[jtbell]

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 language, 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).

 

[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.

 

[Doc Al]

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?

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:

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.