2Section 1: Measuring Motion A Reference Point is: an object that appears to stay in placeMotion: an object’s change in position overtime relative to a reference pointMotion can be north, south, east, west, up and down.Common reference points are: the Earth’s surface, trees, buildings, and sometimes other moving objects
3During the interval between the times that these pictures were taken, the hot-air balloon changed position relative to a reference point—the mountain.
4Speed: the distance traveled divided by the time interval during which the motion occurred Example: Time = 10s and Distance=50mSpeed = distance ÷ time= 50m ÷ 10s= 5 m/s
5Standard units (SI) for speed is meters per second (m/s). Other commonly used units are:Kilometers per hour (km/hr)Feet per second (ft/s)Miles per hour (mi/h)
6Not everything moves at a constant rate though so you need to be able to calculate average speed. Average Speed =total distance total time
7Speed on a Graph: shown on a graph of distance versus time
8Practice Problem:An athlete swims a distance from one end of a 50 m pool to the other end in a time of 25 sWhat is the athlete’s average speed?Step 1: Write the equation for average speed:Average Speed =total distance total time
9Step 2: Replace the total distance and total time with the values given, and solve
10Now you Try!!!Kira jogs to a store 72 m away in a time of 36 s. What is Kira’s average speed?
11If you travel 7.5 km and walk for 1.5 hr, what is your average speed?
12An airplane traveling from San Francisco to Chicago travels 1,260 km in 3.5 hr. What is the airplane’s average speed?
13Velocity: the speed of an object in a particular direction Is like an object’s rate of change of that objects positionIf speed and direction don’t change then velocity is constant, so constant velocity is motion along a straight lineThe speeds of these cars may be similar, but the velocities of the cars differ because the cars are going in different directions.
14Resultant Velocity: is a combination of 2 or more velocities
15Acceleration: the rate at which velocity changes over time An object accelerates if its speed, direction or both changePositive acceleration means an increase in velocityNegative Acceleration means a decrease in velocity or what is called decelerationfaster the velocity changes, the greater the acceleration is
16Calculating Average Acceleration Acceleration is measured in meters per second per second, or m/s2
17Example:A plane passes over point A a velocity of 240 m/s north. Forty seconds later, it passes over point B at a velocity of 260 m/s north. What is the plane’s average acceleration? Starting velocity = 240 m/s Final velocity = 260 m/s Time = 40 s Plug in your numbers: = 20m/s = 0.5m/s2 north 40 s 40s
18What is the average acceleration of a subway train that speeds up from 9.6 m/s to 12 m/s in 0.8 s on a straight section of track?
19Circular Motion:An object traveling in a circular motion is always changing its direction so its velocity is always changing, so it’s accelerating.This is called centripetal accelerationThe blades of these windmills are constantly changing direction. Thus, centripetal acceleration is occurring
21Force: a push or pull exerted on an object in order to change the motion of the object Force has size and direction
22Newton: the SI unit for force, symbol is N All forces act on objectsForces do not always cause motionExample: you are exerting a force on your chair, however the chair doesn’t move. That is because the floor is exerting a force on the chair You cannot see all forces! For example the air around you is held near Earth’s surface by the force of gravity.Something that you cannot see exerts a force that makes this cat’s fur stand up.
23The bulldozer is exerting a force on the pile of soil The bulldozer is exerting a force on the pile of soil. But the pile of soil also exerts a force by just sitting on the ground!
24Net Force: the combination of all of the forces acting on an object When forces are moving in the same direction, you add them togetherWhen forces are moving in opposite direction you subtract the smaller force from the larger one.When the forces on an object produce a net force of 0N, then those forces are balancedBalanced forces do not cause a change in motion of an objectWhen the Forces of an object are not 0N, the forces are unbalanced and a change in motion occurs.
25When forces act in the same direction, you add the forces to determine the net force. The net force will be in the same direction as the individual forces.
26When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force. The net force will be in the same direction as the larger force.
27Because all the forces on this house of cards are balanced, none of the cards move.
28The soccer ball moves because the players exert an unbalanced force on the ball each time they kick it
29A boy pulls a wagon with a force of 6 N east as another boy pushes it with a force of 4 N east. What is the net force?
30A dog pulls on his chew toy with a force of 10 N towards the east A dog pulls on his chew toy with a force of 10 N towards the east. His owner pulls on the other end f the toy with a force of 15 N to the west. What is the net force?
31Section 3: Friction: A Force that Opposes Motion
32Friction: a force that opposes motion between two surfaces that are in contact. Can cause a moving object to slow down or eventually stopOccurs because the surface of any object is roughWhen the hills and valleys of one surface stick to the hills and valleys of another surface, friction is created
33Two factors that affect the amount of friction are: 1) the force pushing the surfaces together and2) the roughness of the surfaces.If the force pushing objects together increases then the hills and valleys come in closer contact and the friction increasesThe rougher the surface, the greater the friction is
35Two Types of Friction:Kinetic Friction: friction between moving surfacesMount depends on how the surfaces moveSliding objects experience more friction than rolling objectsStatic Friction: when a force is applies to an object but does not cause the object to moveThe force of the friction is balanced with the force appliedCan be overcome with a large enough force
37Ways to Reduce Friction: Lubricants: substances applied to reduce friction between objects (motor oil, wax, grease, air on a hockey table, etc)Switching from sliding to rolling friction (using ball bearings in wheels)Make surfaces that rub together smoother (sanding wood surfaces)When you work on a bicycle, watch out for the chain! You might get dirty from the grease or oil that keeps the chain moving freely. Without this lubricant, friction between the sections of the chain would quickly wear the chain out.
38Ways to increase Friction: Make surfaces rougher (sand on ice, textured batting gloves, etc)Increase the force pushing the surfaces together (pressing harder on sandpaper)No one likes cleaning dirty pans. To get this chore done quickly, press down with the scrubber to increase friction.
40Gravity: a force of attraction between two objects that is due to their masses Gravity is a result of mass, so matter is affected by gravityObjects experience a pull toward all other objects, called gravitational forceBecause Earth has such a large mass, it also has a very large gravitational force, pulling everything toward its center
41Sir Isaac Newton – a British scientist Proposed the idea of gravityHe said that an unbalanced force on apples caused them to fall from trees, and that an unbalanced force on the moon kept it circling the Earth; and that both of these forces were really the same, a force of attraction called gravityWrote the Law of Universal Gravitation, which describes the relationships between gravitational force, mass, and distanceSir Isaac Newton realized that the same unbalanced force affected the motions of the apple and the moon
42Law of Universal Gravitation: All objects in the universe attract each other through gravitational force; the size of the force depends on the masses of the objects and the distance between the objects.
43Two Parts:Gravitational Force Increases as Mass Increases
44Gravitational Force Decreases as Distance Increases The gravitational force between objects decreases as the distance between the objects increases. The length of the arrows indicates the strength of the gravitational force between two objectsVenus and Earth have approximately the same mass. But because Venus is closer to the sun, the gravitational force between Venus and the sun is greater than the gravitational force between Earth and the sun
45Weight: is a measure of the gravitational force on an object; SI unit is Newtons (N) Mass: is the amount of matter in an object; SI unit is a kilogram (kg) but is also measured in grams (g) or milligrams (mg)