2 Change in motion? UNBANLANCED FORCE What causes an object to start moving, stop moving, or change direction?UNBANLANCED FORCE
3 What is a force? The strength of a force is measured in the Newton (N) A push or pullA force is described by strength and directionThe strength of a force is measured in the Newton (N)Arrows are used to represent force:
4 Combining forcesMost of the time, there are more than 1 force acting on an object at a timeNet force- the combination of all forces acting on an15 N15 N15 N15 N
5 Unbalanced vs. Balanced Forces Unbalanced ForcesResult in a net forceCan cause a change in the object’s motionBalanced ForcesEqual forces acting in opposite directionDo not cause a change in the object’s motionThey cancel each other out
6 Draw Force Diagrams in Notebook A book is at rest on a tabletop. Diagram the forces acting on the bookA gymnast is holding on the rings suspended from the ceiling waiting to being.A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance.A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance
7 Examples Continued5. A student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. 6. A skydiver is descending with a constant velocity. Consider air resistance 7. A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration 8. A car is coasting to the right and slowing down
8 Force = mass x acceleration FormulaF=m•aForce = mass x accelerationUnits: Newtons (N)N=(kg)•(m/s2)
9 FrictionThe force that one surface exerts on another when the two surfaces rub against each otherForce that oppose motion- always in opposite direction of motion
10 SURFACE AREA DOES NOT AFFECT THE AMOUNT OF FRICTION Friction produces heat! – how have you seen this?The amount of friction depends on:How hard the surfaces push together (force)Types of surfaces involvedSURFACE AREA DOES NOT AFFECT THE AMOUNT OF FRICTION
11 Static Friction Acts on an object that are not moving Strongest- most difficult to overcomeExamples:Desk on the floorParked car on the pavement
12 Sliding FrictionOccurs when one solid surface slides over another solid surfaceNot as strong as static frictionExamples:Bicycle brakesPushing a box along the floor
13 Rolling Friction Occurs when an object rolls over a surface Not as strong as sliding frictionExamples:Shopping cartCar traveling on the road
14 Fluid Friction Occurs as an object moves through a fluid Examples: Surf boarding in the oceanCar as it moves through the air
15 Gravity Is the force that pulls objects toward each other The law of Universal Gravitation- the force of gravity acts between all objects in the universe!That means: any two masses exert an attractive force on each other
16 Factors affecting gravity Mass- a measure of the amount of matter in an objectThe more mass an object has, the greater its gravitational force on other objects isDistanceThe farther apart objects are, the lesser the gravitational force between themThe sun has a gravitational force on you..why don’t you feel it?The pencil in front of you has a gravitational force on you…why don’t you feel it?
17 Gravitational Acceleration The gravitational attraction of Earth causes all falling objects to have an acceleration of 9.8 m/s2Which direction is Earth’s gravitational force?
18 Weight What is it? W=m x g g=a The gravitational force exerted on an objectW=m x g g=aCalculate the weight of a truck with a mass of 600 kg.G: m = 600 kg F: W=m x gg = 9.8 m/s/sFind: w = ? W.S.:w = 600kg x 9.8 m/s/sAnswer: 5,880 N
19 Mass vs. Weight Mass is _________ Weight is a FORCE the more mass an object has, the stronger the gravitational force between the object and Earth. So, the more mass an object has, the more it weighsWeight can be different if the gravitational force is different
20 WeightlessnessWhen an object is influences only by the force of gravity, it is said to be in free fall.
21 Air resistanceAir resistance acts in the opposite direct of an object’s motionThe force it opposes is GRAVITYThe amount of air resistance depends on:SpeedSizeShapeTerminal velocity- when the force of air resistance = weight of object no more acceleration
22 What happens if you drop 2 objects? What would happen if there was no air resistance?REMEMBER:ALL OBJECTS IN FREE FALL ACCELERATE AT THE SAME RATE, REGARDLESS OF MASS!!
23 Projectile MotionA projectile is an object that has been projected through the airA projectile has horizontal and vertical motionNeglecting air resistance, all falling objects accelerate at the same rateNo matter how fast and object is moving horizontally, the force of gravity acts the same as if it was falling straight down