# When do i use these kinematic formulas (constant-acceleration)

• XwakeriderX
In summary, the conversation discusses when to use positive or negative signs in equations involving displacement, velocity, and acceleration. The formula with a plus sign is used for upward motion, while the formula with a minus sign is used for downward motion. However, sign conventions are not a strict requirement and can be chosen as per preference, as long as they are consistently applied throughout the problem.
XwakeriderX

## Homework Statement

When do i use:

(delta)Y=Viy+(1/2)ayt2

Compared to...

(delta)Y=Viy-(1/2)ayt2

What tells me if I am going to be using
(+) or (-)

XwakeriderX said:

## Homework Statement

When do i use:

(delta)Y=Viyt+(1/2)ayt2

Compared to...

(delta)Y=Viyt-(1/2)ayt2
Hi there.
Note my corrections to your formulae in red. Delta y must have length units. Watch your units!
What tells me if I am going to be using
(+) or (-)
Choose a direction as positive, say, positive is upwards. Then negative is downwards. So use plus signs for upward displacement, velocity, and acceleration, and use minus signs for downward displacement, velocity, and acceleration.

awesome thank you!

Sign Conventions are just a reference. They are not a must condition. Take whatever you like but be sure to use it every time and every where. You should never define sign conventions in two different way in the same equation and to save yourself from confusion you should never change the sign conversion in the whole problem.

These kinematic formulas are used to calculate the displacement (change in position) of an object under constant acceleration. The first formula, (delta)Y=Viy+(1/2)ayt2, is used when the initial velocity (Viy) is in the same direction as the acceleration (ay). This can happen when the object is moving in a straight line with a constant speed and direction, or when the object is initially at rest and then experiences a constant acceleration in the positive direction.

On the other hand, the second formula, (delta)Y=Viy-(1/2)ayt2, is used when the initial velocity (Viy) is in the opposite direction as the acceleration (ay). This can happen when the object is initially moving in a certain direction and then experiences a constant deceleration, or when the object is initially at rest and then experiences a constant acceleration in the negative direction.

The sign (+ or -) in these formulas represents the direction of the displacement. A positive value means that the object is moving in the positive direction (in the same direction as the initial velocity), while a negative value means that the object is moving in the negative direction (in the opposite direction as the initial velocity).

To determine which formula to use, you need to consider the initial conditions (initial velocity and acceleration) of the object and the direction in which it is moving. If the initial velocity and acceleration are in the same direction, you will use the first formula with a positive sign. If the initial velocity and acceleration are in opposite directions, you will use the second formula with a negative sign.

In summary, the sign in these kinematic formulas indicates the direction of the displacement and it is determined by the initial conditions and direction of motion of the object.

## 1. When do I use the kinematic formula for displacement?

The kinematic formula for displacement is used when you want to calculate the change in position of an object over a certain period of time. This formula is especially useful for objects that are moving with a constant acceleration.

## 2. How do I know when to use the kinematic formula for velocity?

The kinematic formula for velocity is used when you want to calculate the speed of an object at a specific moment in time. This formula is commonly used in situations where an object is accelerating at a constant rate.

## 3. Can I use the kinematic formulas for non-constant acceleration?

No, the kinematic formulas are specifically designed for objects that are accelerating at a constant rate. If the acceleration is changing, then the formulas will not give accurate results.

## 4. When should I use the kinematic formula for time?

The kinematic formula for time is used when you want to calculate the duration of an object's motion. This formula is particularly useful when you have information about the object's displacement, initial velocity, and acceleration.

## 5. Are the kinematic formulas only applicable to linear motion?

No, the kinematic formulas can also be applied to rotational motion. However, in this case, the displacement, velocity, and acceleration are measured in terms of angular displacement, angular velocity, and angular acceleration.

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