# A Glider of length moves through

## Homework Statement

A glider of length moves through a stationary photogate on an air track. A photogate shown in the figure below is a device that measures the time interval Δtd during which the glider blocks a beam of infrared light passing across the photogate. The ratio vd = /Δtd is the average velocity of the glider over this part of its motion. Suppose the glider moves with constant acceleration.

(a) Argue for or against the idea that vd is equal to the instantaneous velocity of the glider when it is halfway through the photogate in space.

(b) Argue for or against the idea that vd is equal to the instantaneous velocity of the glider when it is halfway through the photogate in time.

None

## The Attempt at a Solution

On (a), I'm not exactly sure how to think of this problem. What does it mean by space?

Regarding (B), I believe the instantaneous velocity is equal to the average velocity at the halfway point. This is because the velocity decreases linearly.

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Simon Bridge
Homework Helper
On (a), I'm not exactly sure how to think of this problem. What does it mean by space?
as opposed to "time" ... you know: thing thing you have lengths and volumes in. eg. "The air track and glider take up some space."
Regarding (B), I believe the instantaneous velocity is equal to the average velocity at the halfway point. This is because the velocity decreases linearly.
I think your description is missing a symbol for the length of the glider.

I think you need to first define what "instantanious velocity" means for the glider.

as opposed to "time" ... you know: thing thing you have lengths and volumes in. eg. "The air track and glider take up some space."I think your description is missing a symbol for the length of the glider.

I think you need to first define what "instantanious velocity" means for the glider.
I'm sorry, i'm still confused. Could you perhaps describe the difference between space and time? (Just a short sentence or two. I don't want to waste your time)

The instantaneous velocity (the velocity of an object at a certain time) at the half point between the two laser points is equal to the average velocity of the entire flight. This is because the glider, which has a length of l, has a velocity that decreases linearly.

How's that?

Simon Bridge
Homework Helper
I'm sorry, i'm still confused. Could you perhaps describe the difference between space and time? (Just a short sentence or two. I don't want to waste your time)
What? You are talking about one of the deep and enduring mysteries of the Universe ... in a few sentences???

Time is because things happen ... time is the stuff that is made by clocks: you measure it with a standard clock like a stopwatch.

Space is because things have to have someplace to happen in. The amount of space is called the volume. You measure it with a ruler.

The other important aspect of the universe is matter - matter is the stuff that has inertia - the amount of inertia is called the mass. You measure mass with a balance.

All the question is saying is that the glider changes position.

The instantaneous velocity (the velocity of an object at a certain time) at the half point between the two laser points is equal to the average velocity of the entire flight. This is because the glider, which has a length of l, has a velocity that decreases linearly.

How's that?
No - I mean, how would you relate the concept of "instantaneous velocity" to an object normally?