# Basic kinematics - conceptual questions

• mizulyn
In summary, the conversation discusses various conceptual questions about kinematics, such as how traffic light timing is determined, the possibility of negative instantaneous velocity during a trip, the path of two balls dropped and thrown from different heights, and the relationship between muzzle velocity and acceleration in different rifles. Possible explanations and equations are provided for each question.
mizulyn
Hello -- I'm an upcoming junior in high school, and I'm trying to study some physics by myself in preparation for the physics class I will be taking. However, I am confused on the following conceptual questions about kinematics:

1). Traffic lights are often timed so that if you travel at a certain constant speed, you can avoid all red lights. Discuss how the timing of the lights is determined, considering that the distance between them varies from one light to the next.

^ for this question, I said something about how timing is determined by the number of cars coming to the intersection vs. the cars passing the light, but I'm not sure that that's the answer that they're looking for.

2). The average velocity for a trip has a positive value. Is it possible for the instantaneous velocity at any point during the trip to have a negative value?

3). A ball is dropped from rest from the top of a building and strikes the ground with a speed of v. From ground level, a second ball is thrown straight upward at the same instant that the first ball is dropped. The initial speed of the second ball is v0 = v, the same speed with which the first ball will eventually strike the ground. Ignoring air resistance, decide whether the balls cross paths at half the height of the building, above the halfway point, or below the halfway point.

4). The muzzle velocity of a gun is the velocity of the bullet when it leaves the barrel. The muzzle velocity of one rifle with a short barrel is greater than the muzzle velocity of another rifle that has a longer barrel. In which rifle is the acceleration of the bullet larger?"

Thank you very much for any help!

2) Yes, if you consider one direction as positive and fix the convention, then when you're traveling back, you'll be moving in an opposite direction so at any instant then you velocity will be negative.

1) Perhaps they wanted you to relate the distance between lights and the average speed of a driver to the timing of the lights. For example if the average speed on a street is 5 m/s, and the distance between adjacent lights is 100 m, they try to make sure that the light is green about 20 secs after the previous light turns green.

3) think about the equation $$V_f^2 = V_i^2 +2ad$$. Set that equation equal to D and plug in some dummy values or apply reasoning to the two cases.

4) Once again $$V_f^2 = V_i^2 +2ad$$ and solve for a this time. d will be the length of the barrel. You can also plug in dummy values for Vf and d, making sure they adhere to directions.

## What is kinematics?

Kinematics is the branch of physics that deals with the motion of objects without considering the forces that cause the motion. It focuses on the description of position, velocity, and acceleration of an object.

## What is the difference between speed and velocity?

Speed is the measure of how fast an object is moving, while velocity is the measure of the rate of change of an object's position in a particular direction.

## What is acceleration?

Acceleration is the rate of change of an object's velocity over time. It can be calculated by dividing the change in velocity by the change in time.

## What is the difference between average and instantaneous velocity?

Average velocity is the total displacement of an object divided by the total time taken, while instantaneous velocity is the velocity of an object at a specific moment in time.

## How are displacement, velocity, and acceleration related?

Displacement is the change in an object's position, velocity is the rate of change of displacement, and acceleration is the rate of change of velocity. In other words, displacement is the integral of velocity, and velocity is the integral of acceleration.

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