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Deriving Basic Physic Problem Using Calculus

  • Thread starter Hypnos_16
  • Start date
153
1
1. Homework Statement

At a distance of 45 meters from a traffic light, a car traveling 15 m/s is brought to a stop at a constant deceleration

a) What is the value of Deceleration (-5m/s)
b) How far has the car moved when its speed has been reduced to 3 m/s
c) How many seconds would the car take to come to a full stop?

2. Homework Equations

I really want to solve this Physics based, but we need to use calculus to derive the equations. so there's no way of showing what equations i have used already

3. The Attempt at a Solution

I don't have an attempt at b and c because i don't know how to start them.
Help would be appreciated, it's the last two questions i have to do on an assignment for tomorrow. Please help
 

dynamicsolo

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The acceleration function for the car is a(t) = -5 , a constant function. What are the velocity and position functions for this? (What is the relationship between a(t) and v(t)? between v(t) and x(t) ?
 
153
1
the relationship between a(t) and v(t), and v(t) and x(t) is that if you integrate a(t) you get v(t) and integrating v(t) gets x(t)
 

dynamicsolo

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1,648
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the relationship between a(t) and v(t), and v(t) and x(t) is that if you integrate a(t) you get v(t) and integrating v(t) gets x(t)
Good! So [itex]v(t) = \int a(t) dt = \int (-5) dt [/itex] gives you...?
 
153
1
Well, from what i can tell, it give you the speed at any given time.
But if that's the case i don't have the time to solve B
 

dynamicsolo

Homework Helper
1,648
4
Well, from what i can tell, it give you the speed at any given time.
But if that's the case i don't have the time to solve B
You get an "arbitrary constant" from the integration. What you know from the statement of the problem is that when the car begins slowing down, which is at t = 0, the speed is v(0) = 15 m/sec. What does this make the constant from your velocity integral?
 
153
1
Acceleration???
Sorry, this really isn't clicking.
 

Ray Vickson

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1. Homework Statement

At a distance of 45 meters from a traffic light, a car traveling 15 m/s is brought to a stop at a constant deceleration

a) What is the value of Deceleration (-5m/s)
b) How far has the car moved when its speed has been reduced to 3 m/s
c) How many seconds would the car take to come to a full stop?

2. Homework Equations

I really want to solve this Physics based, but we need to use calculus to derive the equations. so there's no way of showing what equations i have used already

3. The Attempt at a Solution

I don't have an attempt at b and c because i don't know how to start them.
Help would be appreciated, it's the last two questions i have to do on an assignment for tomorrow. Please help
You might find it easier to consider the problem in reverse: starting from rest, a car undergoing constant acceleration reaches a speed of 15 m/s at a distance of 45 m.

RGV
 

dynamicsolo

Homework Helper
1,648
4
Acceleration???
Sorry, this really isn't clicking.
Are you asking about the "-5" ? Acceleration is any change in velocity, so slowing down is a negative acceleration (what people also called a "deceleration"). The calculations of motion can be done with positive or negative accelerations; the ability to work out the velocity and position functions does not depend on the sign of a(t).
 

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