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## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1>Unit 1

Lesson 1: Position, acceleration, and velocity

# Solving for time

Rate of change in position, or speed, is equal to distance traveled divided by time. To solve for time, divide the distance traveled by the rate. For example, if Cole drives his car 45 km per hour and travels a total of 225 km, then he traveled for 225/45 = 5 hours. Created by Sal Khan.

## Want to join the conversation?

• I don't understand how displacement can be negative, after all it is the shortest distance between the initial and final point ?
• I'd like to add to this - I know this is an old question, but new readers might find this useful.

The + and - designations are important in computer programming, especially with video game development.

In video games, the motion of characters or objects and defined with vectors, just like this. However, in computer code, we have no way of cleanly saying "to the east," or "to the left."

So instead, we will have something written like this: (assuming that "num" is a Vector object).

num = (+1,0,0);

This states that the vector called num is +1 on the X axis. If it were written as (0,-1,0), then this would denote that the object moves -1 on the Y axis, or what we might call "down".

So the + and - don't mean that the actual value is negative, it just means that it is forward / backward relative to the * starting point * on the axis.

So if the object is originally at (4, 2, 7), and the object moves, or is displaced, by a vector of (-1, 2, 0), then the final position is at (3, 4, 7).

So + and - don't represent true positive and negative values, in this case, they just try to show whether or not a value on the X, Y, or Z access has increased or decreased from it's original spot / value.

This is an important concept in video game development, and takes up a large portion of programming a typical video game.
• what is negative velocity
• Velocity in the negative spatial direction.
• At , how did you manipulate the equations? Can you define it here please?
• To manipulate an equation you use an inverse operation to simplify it.

2 X 5 = 10

To manipulate you would inverse the 2 X by dividing both sides by 2.

5 = 5

It works with everything and comes in real handy with equations.
• So can we solve this velocity question in the same way we solve a speed question? Is the only difference the addition of directions in this question?
• the simple answer is yes we can
(1 vote)
• Why do we use arrow on top of vector ?
• A vector has a magnitude and direction. If we didn't have an arrowhead on the tip of the vector, it's direction might be ambiguous. Is it pointing straight forward, or straight behind? You wouldn't know unless the vector has an arrowhead.
• Why is the direction so important? Isn't the answer the same without a direction? Also, when you answer those physics questions, it's the working that gives you the mark is it?
• Direction will be important later on. If you say "Ethan exerted a force of 5 Newtons and Newt exerted a force of 3 Newtons", we cannot calculate the net force. We don't know if they are exerting a force in the same direction or in opposites or etc.
• At 02.30 Sal multiplied the equation with t and then proceeded to do the rest. Is there a way to do it without multiplying it by t?
• Yes, instead of multiplying by time you can just plug the numbers into the equation:
time = distance / rate (speed). In this case it would be:
time = 720m / 3m per sec

When you divide 720m by 3m/s the meters cancels out and you are then left with time which would be 240 seconds. This equation though is just a manipulation of
rate = distance/time.
• How do these positives and negatives determine directions?
• It can determine directions such as + is going to the East, and - is going to the West.
The positives and negatives just tell you if you are going with the original direction or the opposite direction. So if the problem asks how much you are going to the East, but you are actually going to the West, your answer would be negative.