• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

Conceptual Physics Questions

JohnCU

JohnCU

Banned
Studying for a test right now and going over some conceptual type questions:

1. If the acceleration of a particle is not zero, its velocity must also not be zero.

I'd say this is true because if something is accelerating, it's either slowing down or speeding up...

2. If the velocity of a particle is not zero, its acceleration must also be not zero.

This is false, because you can have constant velocity and that means acceleration = 0.

3. If a car is slowing down, can its acceleration be positive?

I'd say no for this one, because if it's slowing down that means the acceleration is negative... well, on the other hand, I'd guess it depends on which direction you specify as backwards and forwards. If it's in reverse but slowing down, that mean it's a positive acceleration, right?
 
1) False, you can be accelerating while stopped. Otherwise no stopped object would ever be able to begin moving.

2) False, like you said.

3) If the car is going in reverse and braking, the acceleration is positive.
 
You're wrong on #1. Example: If I throw a ball exactly straight up in the air, the acceleration is 9.81 m/s/s downward THE ENTIRE TIME. There is an instant in time when v=0 (at the top of the flight)
 
Originally posted by: notfred
1) False, you can be accelerating while stopped. Otherwise no stopped object would ever be able to begin moving.

2) False, like you said.

3) If the car is going in reverse and braking, the acceleration is positive.
Click to expand...

 
Originally posted by: JohnCU
Originally posted by: no0b
Originally posted by: lchyi
I likes conceptual physics... give me more!
Click to expand...

Does there have to be a change in velocity for an acceleration to occur?
Click to expand...

Nope, there can just be a change in direction, right?
Click to expand...
That's still a change in velocity.

Velocity != Speed

In answer to the question acceleration is dv/dt, so by definition acceleration is change in velocity.
 
You can get acceleration by taking the derivative of velocity. So how can acceleration be a non-zero number when velocity is 0? It can't. You can be accelerating when velocity is constant (i.e. rounding a curve at a constant 15 mph) but you have to have velocity to accelerate.
 
Originally posted by: no0b
Originally posted by: lchyi
I likes conceptual physics... give me more!
Click to expand...

Does there have to be a change in velocity for an acceleration to occur?
Click to expand...

Considering that the definition of Acceleration is "change in velocity", I'm going with "yes".
 
Originally posted by: DrPizza
You're wrong on #1. Example: If I throw a ball exactly straight up in the air, the acceleration is 9.81 m/s/s downward THE ENTIRE TIME. There is an instant in time when v=0 (at the top of the flight)
Click to expand...

no, the instant that v=0 acceleration = 0.
 
Originally posted by: PlatinumGold
Originally posted by: DrPizza
You're wrong on #1. Example: If I throw a ball exactly straight up in the air, the acceleration is 9.81 m/s/s downward THE ENTIRE TIME. There is an instant in time when v=0 (at the top of the flight)
Click to expand...

no, the instant that v=0 acceleration = 0.
Click to expand...
That's just blatantly wrong, gravity does not stop acting on the ball when its velocity is zero.
As notfred said earlier no stopped object could ever start moving if acceleration is zero when velocity is zero.
 
Originally posted by: JohnCU
If you are rounding a curve, your velocity is not constant, because you are changing direction.
Click to expand...

No, that change in direction affects acceleration, not velocity, so that's why you can have constant velocity with varying acceleration.
 
Originally posted by: MindStorm
Originally posted by: JohnCU
If you are rounding a curve, your velocity is not constant, because you are changing direction.
Click to expand...

No, that change in direction affects acceleration, not velocity, so that's why you can have constant velocity with varying acceleration.
Click to expand...

Velocity is a vector (a force in a direction). If you change the direction, you change the velocity...
 
Originally posted by: MindStorm
You can get acceleration by taking the derivative of velocity. So how can acceleration be a non-zero number when velocity is 0? It can't. You can be accelerating when velocity is constant (i.e. rounding a curve at a constant 15 mph) but you have to have velocity to accelerate.
Click to expand...

I concur, anyone with an answer?
 
Originally posted by: LordSnailz
Originally posted by: MindStorm
You can get acceleration by taking the derivative of velocity. So how can acceleration be a non-zero number when velocity is 0? It can't. You can be accelerating when velocity is constant (i.e. rounding a curve at a constant 15 mph) but you have to have velocity to accelerate.
Click to expand...

I concur, anyone with an answer?
Click to expand...


Acceleration is a change in velocity.

Think of it this way: accel= (beginning velocity - end velocity)/ time...

If an object has a negative beginning velocity, but you accelerate it in the opposite direction until the end velocity is positive, you at some time (for just an instant) pass the point of zero velocity when the object changes direction.

The object hasn't finished changing velocity yet, thus it still has acceleration.

Originally posted by: Haircut :
...gravity does not stop acting on the ball when its velocity is zero.
As notfred said earlier no stopped object could ever start moving if acceleration is zero when velocity is zero.
Click to expand...



 
Originally posted by: MrPickins
Originally posted by: LordSnailz
Originally posted by: MindStorm
You can get acceleration by taking the derivative of velocity. So how can acceleration be a non-zero number when velocity is 0? It can't. You can be accelerating when velocity is constant (i.e. rounding a curve at a constant 15 mph) but you have to have velocity to accelerate.
Click to expand...

I concur, anyone with an answer?
Click to expand...


Acceleration is a change in velocity.

Think of it this way: accel= (beginning velocity - end velocity)/ time...

If an object has a negative beginning velocity, but you accelerate it in the opposite direction until the end velocity is positive, you at some time (for just an instant) pass the point of zero velocity when the object changes direction.

The object hasn't finished changing velocity yet, thus it still has acceleration.

Originally posted by: Haircut :
...gravity does not stop acting on the ball when its velocity is zero.
As notfred said earlier no stopped object could ever start moving if acceleration is zero when velocity is zero.
Click to expand...
Click to expand...

but accel = (beginning velocity - end velocity)/ time, it's the derivative of delta_v(t)/t, so if v(t)=0, how can accel no be equal to zero?

...gravity does not stop acting on the ball when its velocity is zero.

true, but when we talk about accel, we're talking about whether or not the ball accelerates if velocity=0 ... there is not acceleration if the velocity is 0.
 
Originally posted by: LordSnailz

true, but when we talk about accel, we're talking about whether or not the ball accelerates if velocity=0 ... there is not acceleration if the velocity is 0.
Click to expand...

there is no acceleration if the velocity is held at 0. but the v doesn't have to stay at 0. eg an egg just released from your hand
 
Let me venture into shaky territory (for me anyway). Note, I've never taken a physics class.

Someone said that acceleration must be 0 when velocity is 0.

So if the function for velocity is v=2x²-8, v=0 when x=2. Since acceleration is the derivative of velocity, and v'=4x and x=2....

When v=0, v'=8.

Is that right?
 
You must log in or register to reply here.

TRENDING THREADS

Back
Top