Forces Force - A push or a pull Long Range forces (four fundamental forces) “Jedi – Forces” Gravity Magnetism Weak force Strong force Contact forces Forces that result from direct contact between objects Friction Normal force Any force that’s not a Long-Range force

Past notions on forces Objects needed a force in order to move And object could not possibly move without a force pushing it Aristotle (~300 BC) first formulated these laws and were not proven wrong until much later by Galileo

Galileo force is not necessary to keep objects in motion Galileo argued, that only when friction is present (which it almost always is) is a force necessary to keep an object in motion Force - A push or a pull Friction - a force that acts between materials that touch as they move past each other oActs opposite the direction of motion He tested this with balls rolling up and own inclines hypothesized ….if no friction, a ball rolling on a level surface would stay in motion forever w/ no force pushing it…. It just had a natural tendency to stay in motion This natural tendency of objects to stay in motion and resist changes in its motion is called INERTIA

Newton’s 1 st Law - The Law of Inertia Isaac Newton … was Galileo’s successor in the world of Physics. formulated Three Laws of Motion replaced 2000 year old Aristotelian ideas NEWTON’s 1 st Law - The Law of Inertia “An object in motion will stay at a constant speed in a straight line, an object at rest will stay at rest unless acted upon by outside forces” Inertia -- property of matter that resists changes in its motion.

Examples of inertia Why seatbelts are necessary Astronauts in space… get back to spaceship? The more mass something has the more inertia it has Large boats not being able to stop their motion A force is the only thing that can put an object in motion OR stop a object from moving. Objects cannot change their own motion w/out an outside force Nickel on a piece of paper Rollercoaster 1994 Nissan Maxima Car with cinder blocks Car going around a circle (stuff on dashboard?) Satellite

Example--- Mr. Schober goes to the shooting range …

Which gun to choose??

Heavy gun, w/ small caliber bullets= means small kick back because gun has more inertia– Equals easier for smaller person to shoot Video

The moving Earth Throw a coin upward in a moving car Bird catching a worm Helicopter hovering above Earth…. Does the Earth move under it?? No, helicopters inertia keeps it moving with the Earth

Net Force (∑F) (F net ) Net Force  Total force acting on an object ∑F = F 1 + F 2 + F 3… force is a vector quantity Sign indicates direction Forces in opposite directions will cancel out A 5 N force to the right would cancel out a 5 N force to the left ∑F = 6 N Only the NET force acting on an object affects its motion -7 N 13 N Free Body Diagrams. Simple drawings showing all force vectors acting on an object

Skydiver (m=75 kg) falling towards Earth, if the force of air resistance is 100 N what is the Net Force on the skydiver? Force of Gravity (aka Weight) pulling skydiver towards Earth. F g =mg Force from Air Resistance directed upwards 75 kg x -9.8 m/s 2 = -735 N 100 N ∑F = -735N + 100N = -635 N

 We learned w/ the 1 st Law that objects do not like changes in their motion…  if a net force is applied to an object the motion of an object will change…  ‘changing motion’ basically means the same thing as Acceleration  So then we can say…. ‘force causes acceleration’

 All forces do not accelerate all objects equally  Objects with more mass are more difficult to accelerate, and thus require more force

 ‘The acceleration of an object is directly proportional to the net force on it, and inversely proportional to its mass’  This means…. The larger the force, the larger the acceleration  AND …. The larger the mass, the smaller the acceleration  This can be summed up w/ the equation  ∑F=ma  a  acceleration (m/s 2 ); ∑F  Net Force (N); m  mass (kg)

 ∑F=F 1 + F 2 + F 3.. AND  ∑F=ma  Soo we can say  ∑F = ma = F 1 + F 2 + F 3..  This also works in 2-D  ∑F x = ma x =F 1x + F 2x + F 3x..  ∑F y = ma y = F 1y + F 2y + F 3y..

Mass & Weight Mass (m) How much matter is in an object A measure of an object’s inertia Mass never changes depending on location… you have the same mass in space as you do here on Earth Weight (W) How much gravity is pulling on an object Aka… The FORCE of gravity acting on an object Is a function of how strong gravity is can change from place to place…. You have a much different weight in space than you do here on Earth B/c mass is a measure of inertia it is just as hard to move a large boulder in space as it is here on Earth

How to determine an object’s weight? Mass and weight are always directly proportional We find weight by using the equation Weight = mass x acc. of gravity Or.. W = mg g = -9.8 m/s 2 So if an object has a mass of 1 kg, then its weight will be 9.8 N. The “Newton” is the standard unit for force, and since Weight is a force, the Newton is used here. 1 N = 1 kg*m/s 2

 Newton’s 2 nd says that the more force an object has on it the faster it should accelerate….. So then if heavier objects have a larger weight (force of gravity), why do they not accelerate faster?  Ans. True, heavier objects have more force, but they also have more mass, which means that it requires more force to accelerate as fast as lighter objects….  Essentially…. The larger force and larger mass cancel each other out  All falling objects accelerate downward at 9.8 m/s 2

 You push a 10 kg cart with a force of 8 N. There is a frictional force of 3 N that acts between the cart and ground. What is the acceleration of the cart? ∑F = ma = F A + F F 10kg (a) = 8 N + -3 N 10kg (a) = 5 N a=.5 m/s 2

NEWTON’S THIRD LAW Equal & Opposite Forces

3 rd Law  Forces are not a thing of themselves, but are part of a mutual interaction  Forces only exist in pairs  Third Law says….  “For every force there is exists another force that is equal in magnitude but opposite in direction”  You cannot push on something without it pushing on you by the same amount in the opposite direction

Action & Reaction  Action forces produce the interaction, and the reaction force must be in existence at the same time, have the same size, and be opposite in direction  You apply a 10 N eastward force to wall, wall applies a 10 N westward force to you

How do objects move? What pushes them?  Car??  Rocket??  Person??

What is the reaction to the Force of Gravity?  Earth’s gravity pulls objects towards it  So if all forces have are part of a pair, what is the reaction force to this??  Object’s gravity pulling Earth up!  An object pulls upward on the Earth as much as the Earth pulls it downward  Object’s mass is much less than Earths, so object falls down rather than Earth rising up to object ACTION- Earth’s Gravity pulls person down REACTION- Person’s gravity pulls Earth up

Examples

Draw all forces on the car/Earth system as it is driving down the road Normal force of the Earth pushing up on the car Contact force of the car pushing down on the Earth Earth’s Gravity pulling the car down The car’s gravity pulling the Earth up The car’s tire pushing on the road The road pushing the car forward The road’s friction on the car’s tires The cars friction on the road **Action/Reaction pairs have matching colors in drawing

Astronauts & Reaction Forces

Stuck in Space??  An astronaut in space…not moving with just a spacesuit on and he’s not tethered to the ship. If he is only a few meters away from his ship and there are no other people to help him how could you get back to his spaceship? Oh No!!

Forces on different masses  Cannon and cannonball  Do both the cannon and the cannonball experience the same force the cannon is fired?  Yes, the force on each are the Action/Reaction pairs  Does the cannon or the cannon ball have a larger acceleration?  The ball, b/c it has a small mass. It is easier to accel. a small mass  a = ∑ F/m

5.2 - Applying Newton’s Laws  Different types of Forces  Normal Force (F N )  Weight (W)  Apparent Weight (W a )  Force from a Spring  Tension  Friction (covered in section 5.3)

Normal Force (F N )  “Normal” means perpendicular  Force of the Earth (ground) pushing up on an object when they are in contact with each other  IF object is….  1) on a horizontal surface and  2) no other vertical forces are applied and  3) no vertical acceleration. THEN Normal Force is Equal to the weight (force of gravity) of the object.  Normal Force is NOT the reaction to the force of gravity acting on an object  Responsible for how heavy we “feel”  AKA “Apparent Weight” --- W a

Elevators & Normal Force Elevator Accelerating DownElevator at rest OR Moving at constant velocity Elevator accelerating Up W=900 N F N =890 N F N =900 N W=900 N F N =910 N F NET =+10 N F NET =0 NF NET =-10 N

Equilibrium Equilibrium occurs when the Net Force acting on an object is equal to zero (∑F = 0 N) If an object is in equilibrium 1 of 2 things must be true The object is moving at a constant speed in a straight line, OR The object is at rest ∑F= 0 13 N

Equilibrium in 2-D If forces are acting in more than one dimension then the equilibrium equation holds true separately for the x and the y Σ F x = 0 = F 1x + F 2x … Σ F y = 0 = F 1y + F 2y …

A stretched or compressed spring exerts a force Hooke’s Law F = kx K – spring constant x- the stretch or compression distance

5.3 - More about Friction Always opposes motion If direction of motion is positive, Force of Friction is negative Two types of friction Force of Static Friction (F s ) Friction force that opposes the START of motion Equation F s,max = μ s F N If no pushing force, no F s. To start an object moving the applied force must be larger than the maximum F s possible. Force of Kinetic Friction (F k ) Friction force that opposes the continuation of motion, exists only on objects that are moving Equation F k = μ k F N F k = force of kinetic friction (N) μ k = coefficient of kinetic friction (no units) F N = Normal Force (N) Static Friction is always larger than kinetic friction Harder to start motion than it is to sustain it

More on Friction Equations F k = force of kinetic friction (N) μ k = coefficient of kinetic friction (no units) Constant that depends on the nature of the two surfaces in contact No units Ratio of Friction force to Normal force (F k / F N ) μ s - coefficient of static friction is a different value for two surfaces (will be larger) but same concept F N = Normal Force (N) Force pushing two surfaces together “normal” means perpendicular If object is on horizontal surface AND no other vertical forces are being applied Normal Force is equal to object’s Weight (mg) More on Normal Force Later

 A 40 kg cart is getting pulled by a 140 N Force, if there exists a coefficient of kinetic friction between the cart and road of.10 what will be the acceleration of the cart? 0