You are Here: Home >< Physics

# finding the time when the rate of change in speed=0 watch

1. in vector system,

at t=2, a= -6i-2j and v= 3i-4j

in order to calculate the rate at which the object's speed is changing,
we were told to use a dot product.

so a.v=|a||v|cos (x)

since |a| cos (x) would give us the rate, we do the dot product of a and v to give -10, and divide this value by the magnitude of v which gives the value of acceleration to be -2 (which is correct according to the mark scheme).

Then I was asked 'at what time is the rate of change of the object's speed equal to zero, and what is the speed at this instant?

Can anyone please explain to me how to approach the last question?

2. (Original post by liemluji)
in vector system,

at t=2, a= -6i-2j and v= 3i-4j

in order to calculate the rate at which the object's speed is changing,
we were told to use a dot product.

so a.v=|a||v|cos (x)

since |a| cos (x) would give us the rate, we do the dot product of a and v to give -10, and divide this value by the magnitude of v which gives the value of acceleration to be -2 (which is correct according to the mark scheme). ...
A few things to note.

The rate of change of speed is NOT the value of acceleration.

The rate of change of speed is the component of the acceleration in the direction of the velocity.

is the component of the acceleration that is parallel to the direction of the velocity when you do the dot product of the acceleration and velocity vectors.

(Original post by liemluji)
... Then I was asked 'at what time is the rate of change of the object's speed equal to zero, and what is the speed at this instant?

Can anyone please explain to me how to approach the last question?

Are you given the velocity vector as a function of time? If yes, find the dot product of acceleration and velocity vectors and equate to zero to find the time.

Or else you post the whole question.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 1, 2018
The home of Results and Clearing

### 2,444

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Keele University
Sun, 19 Aug '18
2. University of Melbourne
Sun, 19 Aug '18
3. Sheffield Hallam University
Tue, 21 Aug '18
Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE