Motion

Motion An object’s change in position relative to a reference point. One thing it moving and something else is staying still. Stationary object: is the reference point or object staying still The stationary object allows us to describe the direction of movement. “the ball moved down the hill” The ball is in motion The hill is stationary

In order to accurately measure motion 1) you need a stationary object 2) you need a moving object 3) you need a direction of a moving obejct Up, down, north, south, right, left, east, west, etc. 4) you need to know the distance travelled by the moving object or the total displacement. Distance: the total length travelled no matter the direction or path. Could be a straight line or just random is any particular direction

Displacement: the total length travels from start point to end point This is always a straight line. The shortest distance betweens two points is a straight line. Requires direction So….you may take a tour of the entire country and travel all over the place with 4500 miles in total distance, but if you start in Pittsburgh and you end in Philadelphia, then your total displacement is only 305 miles east.

Speed Speed: how fast something is moving distance divided by time (mi/hr) In science we use meters/second (m/s) as a standard If I travel 25 miles in 30 min my speed 50 mi/hr or 0.83 mi/min Constant Speed: when an object moves the same amount of distance at repeating time intervals. Like travelling 50 mi/hr constantly for 2 hours, would mean that you travelled 100 miles. Most object do not do this Your car tries to do this when it uses cruise control

Graphing speed Time is always on the horizontal axis (as the independent variable  meaning time will pass no matter what we do) Distance is on the vertical axis (as the dependent variable  meaning the distance can change depending on what you do) The steeper the slope, the faster the object is moving Slopes of zero (a flat line) occur when something is not moving.

Measuring speed is better with a smaller time interval, it is more accurate. When a small time interval is used, the speed calculated is called instantaneous speed…..the speed in the that instant.

Velocity Velocity: the speed AND direction of an object. Ex: Matt was walking 2 m/s to the north towards school Ex: the swimmer traveled 110 meters towards the shore in 72 sec speed = 1.5 m/s Velocity = 1.5 m/s towards shore An object’s velocity is constant only if its speed and direction are constant. Constant velocity is always motion along a straight line.

Velocity is represented using an arrow Velocity is represented using an arrow. The point of the arrow shows the direction. The length of the arrow shows how much. Combining velocities of objects will determine resultant velocity. Resultant velocity: you can combine velocities to get a new changed velocity When combining velocities in the same direction, add them together. When combining velocities in opposite directions, subtract the smaller velocity from the larger velocity. The direction is the same as the larger.

Figure shows a person walking along the aisle of a bus while the bus is in motion. The resultant velocity of the person in the first illustration is 16 m/s east. The resultant velocity of the person in the second illustration is 14 m/s east. Why are these two velocities different?

Acceleration Acceleration is the rate at which velocity changes over time. Remember: velocity involves both speed and direction. An object accelerates if either speed or direction change. The faster the velocity changes the greater the acceleration. Any time you change velocity, you change acceleration

Types of Acceleration Acceleration is commonly associated with an increase in speed, but an object accelerates if it slows down as well. (refer to the definition of acceleration) An increase in velocity is called positive acceleration. (This is the same as speeding up in the positive direction or slowing down in the negative direction.) A decrease in velocity is called negative acceleration. (This is the same as slowing down in the positive direction or speeding up in the negative direction.) Also know as deceleration

You can constantly accelerate while never speeding up or slowing down… You can constantly accelerate while never speeding up or slowing down….huh? Well, when you change direction, you accelerate, even if your speed is the same. If you travel in a circle at the same speed, you are always accelerating because you are always changing direction. This is called Centripetal acceleration

Calculating Acceleration Acceleration is found by using the following formula: A = final velocity – initial velocity time A = Dv time Some common units for acceleration are meters per second per second (m/s/s or m/s2) or kilometers per hour per second (km/h/s).

Graphing Acceleration On a distance-time graph, acceleration is represented using a curved line. If Velocity is not changing over time, then the acceleration is constant.

Velocity-Time Graph

Acceleration Graph

Velocity-Time Graph

Negative Acceleration (deceleration)

Positive Acceleration

No Acceleration Velocity

Force Action exerted in order to change something’s motion When you push or pull on something Force has magnitude and direction Net force: combination of all the forces acting on the object

If the net force is not zero, the object will move in which ever direction is stronger When the net force is zero, then the object is considered balanced. A balanced force does not cause the object to move. (Like pushing on a box from 2 opposite directions….it is not going to move) Balanced force ex: a hat on your head, a light hanging from the ceiling

Net Force Options The net force can be balance by cancelling each other out or they can be in the direction of the stronger force, or they can be added together if they are in the same direction 2 forces working in opposite directions Can be equal and in opposite directions, so they are balanced Can be unequal and in opposite directions, so the net force is the difference between the forces and in the direction of the stronger force Can be equal/unequal in the same direction, so the net force is the sum (when you add) of the forces and in the direction that they both act

Friction One of the forces acting on an object can be friction Friction occurs because the surface of an object is rough An object doesn’t move unless the force acting on the object is greater than the friction. Static Friction: friction that happens between two unmoving objects Kinetic Friction: friction that occurs when an object is moving Rolling friction Sliding friction Fluid friction: air resistance

Good Friction: Bad Friction: Trying to clean something Keeps your car at rest on a hill Sanding a piece of wood Pencils works by friction Bad Friction: When engine parts rub together (this is why they need motor oil to lubricate the parts)