MOTION Chapter 10

MEASURING MOTION Motion is an object’s change in position relative to a reference point. A reference point is the stationary object that you use to see if another object is moving. It is also called a reference frame or frame of reference When an object changes position in comparison to a reference point the object is said to be in motion.

You can describe the direction of an objects motion with a reference direction. Some examples of reference directions are north, south, east, west, up or down.

DISTANCE Distance measures the path taken Example: If you started at your home and wandered around your neighborhood for a while changing directions a few times, a string that followed your path would be as long as the distance you traveled.

DISPLACEMENT Displacement is the change of an object’s position. If you stretched a string in a straight line from your home directly to your final destination, the length of that string would be your displacement. Displacement is the change in the position of the object.

DISTANCE VS. DISPLACEMENT There are two differences between distance and displacement: straightness and direction. Distance can be a straight line, but it doesn’t have to be. Displacement must be a straight line So the displacement is shorter than the actual distance traveled Displacement must be in a particular direction, while distance does not indicate a direction

SPEED AND VELOCITY Speed describes how fast an object moves Speed measurements involve distance and time To find speed you must measure the distance the object traveled and the time it took to travel to that distance. The SI unit for speed is meters per second (m/s) It is sometimes expressed in units like kilometers per hour (km/h) or miles per hour (mi/h)

TYPES OF SPEED When an object covers equal distances in equal amounts of time, it is moving at a constant speed. Most objects do not move with a constant speed. Example: Putting a car in cruise control Instantaneous speed is the speed at any given point in time Example: driving a car and looking down at the speedometer. Average speed is the total distance traveled divided by the total time When finding speed this is the one we normally are looking for Example: Taking a road trip.

FINDING AVERAGE SPEED Average speed (v) = total distance (d) / total time (t) V=d/t

You can investigate the relationship between distance and time in many ways You can plot a graph with distant and time You can use equations and calculations You can also combine these two approaches

Motion of an object can be plotted on a distance – time graph. The slope of a line on a distance – time graph gives the speed of an object in motion. Y   DISTANCE X TIME X axis -> TIME Y axis -> DISTANCE

Time is increasing, but the distance does not change Time is increasing, but the distance does not change. The object is not moving If object is not moving a horizontal line is shown.

Time is increasing to the right and distance is increasing constantly with time. Object is moving with constant speed. Constant speed is shown by straight lines on a graph.

Steeper line indicated a larger distance moved in a given time. Steeper slope = faster moving object

This shows an increasing speed, since the line gets steeper.

Velocity is the speed of an object in a particular direction You use the same formula as you do for speed but when you get an answer you must put a direction on it.

Combine velocities to determine resultant velocities. For example if you are riding in a bus traveling east at 15 m/s, you and all the passengers are traveling at a velocity of 15 m/s east. But if you stand up and walk down the bus aisle in the opposite direction while the bus is moving you are no longer moving at 15 m/s east.

Resultant velocity: When two velocities are moving in the same direction, add them together to find the resultant velocity, which is in the direction of the two velocities. When two velocities are moving in opposite directions, add the positive velocity to the negative velocity to find the resultant velocity, which is in the direction of the larger velocity.

Bellringer 9/25 What is the difference between speed and velocity? How do we find each one?

10.2 Acceleration

Acceleration Acceleration is the rate of change in velocity. Acceleration occurs when an object changes speed, its direction, or both. Acceleration has a direction just like velocity

Uniform circular motion is a constant acceleration. An object traveling in a circular motion is always changing its direction, so even though its speed does not change its direction does. The acceleration that occurs in uniform circular motion is known as centripetal acceleration

Calculating Acceleration Acceleration = change in velocity/ time a=vf-vi/t *NOTE: change in velocity = final velocity - initial velocity v = vf-vi

Units for acceleration: meters per second per second m/s2

Example A flowerpot falls off a second-story window sill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot.

Acceleration must have a direction, so it can be positive or negative. Most of the time, when an object is speeding up the acceleration will be positive. Most of the time, when an object is slowing down the acceleration will be negative.

Acceleration can be determined from a velocity- time graph or a speed-time graph. The slope on a velocity-time graph is the acceleration

A straight horizontal line means the speed is constant A straight horizontal line means the speed is constant. It is not changing over time

•The steeper the graph, the greater the acceleration. •A horizontal line means the object is moving at a constant speed. •A downward sloping line means the object is slowing down.