Motion Linear motion and Relative Velocities Physics

What is motion? Frame of reference. Who is upside down? In order to see if something has moved, we need to see if it has changed it’s positon. Position: refers to where an object is relative to a frame of reference.

Frame of reference A point or set of points, assumed to be stationary, used as the “background” to describe motion. For example: You are sitting on a train and the train on the opposite platform starts to pull out. For an instant you think you are moving. Quickly, you look at the platform and see it is the other train moving. The platform is your frame of reference.

Where is the frame of reference? video

This brings us to the meaning of scalar vs. vector. Change in position can be a measure of distance. Distance: A scalar quantity which refers to “how much ground an object has covered” during it’s motion. Displacement: A vector quantity which refers to how far out of place an object has moved from it’s original position. It is measured using a straight line from one point to another as well as a direction of movement. This brings us to the meaning of scalar vs. vector. A scalar quantity is something that has magnitude, which is an amount or size or something with a numerical value.

A vector quantity is something that has magnitude and a direction A vector quantity is something that has magnitude and a direction. It is often denoted with an arrow above it to imply it has direction.

Another way to look at motion is it’s speed. Speed is a scalar quantity which refers to “how fast an object is moving”, or the ratio of distance over time. (as you know, time is the duration of an action or an event.) v = d t We can think of speed as a type of rate. A rate is a measure of how a quantity changes with respect to time.

Velocity We can make speed a vector quantity by using displacement instead of distance and putting arrows above the v and d. This make speed become velocity. v = d t It is calculated the same way as speed and the term velocity will often be used in place of speed.

Average vs. Instantaneous speed Average speed is a measure of the speed calculated over the distance in a span of time. Often it is the total distance over total time unless a range is specified. Instantaneous speed is a measure of the speed at any moment or point within the journey. It could be a single set of x and y data points on a graph.

What if both points are moving? You and the frame of reference? This is called a relative velocity Here is an equation to help determine your relative velocity with respect to a frame of reference. vrelative velocity =vmoving object– Vframe of reference This would calculate the relative velocity of your moving object to the frame of reference. You must also assign a direction to your motion. Anything in that direction would be positive velocity, anything opposite to that direction would be negative velocity. East= +, West= -

Lets try some examples: A person throws a ball 10 m/s E from a car going 20 m/s E. What is the relative speed of the ball from your perspective as a stationary observer. What is the balls relative speed relative to the car?

A person throws a ball 10 m/s E from a car going 20 m/s E A person throws a ball 10 m/s E from a car going 20 m/s E. What is the relative speed of the ball from person B, who is running toward the car at 5 m/s W? What is the relative speed of the ball?

A person throws a ball 10 m/s E from a car going 20 m/s E A person throws a ball 10 m/s E from a car going 20 m/s E. What is the relative speed of the ball from person C, who is running toward the car at 5 m/s W on a cart going backwards at 10 m/s E?

A person is on a train going 6 m/s E and a person X is walking 2 m/s E A person is on a train going 6 m/s E and a person X is walking 2 m/s E. Person X observes another person Y on a train on parallel tracks. That train is going 4 m/s E but is walking 1 m/s W. What is the relative velocity of person X to person Y?