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: 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

• 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….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 velocitytime

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

Velo

city

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: – 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)