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Chapter 12 forces and motion power pointPresentation Transcript
CHAPTER 12FORCES ANDMOTION12.1 FORCES
12.1 FORCESThere are 4 distinct forces in our universe:Gravitational, electromagnetic, strong nuclearand weak nuclear forces.Ex: everyday force – windForce – is a push or pull that acts on an object.A force can cause a resting object to move, or itcan accelerate a moving object by changingthe object’s speed or direction.
*Gusts of wind push you eitherway.Weight is a type of force (springscale) measure force
*Units of ForceForce is measured in Newtons (N)One Newton is the force that causes a l-kilogram mass to accelerate at a rate of1 meter per each second (1 m/s2)1N = 1kg x m/s2Named after Sir Isaac Newton, Englishscientist who explained how force, massand acceleration are related.
*Representing ForceUse arrows to represent the direction andstrength, or magnitude, of a force.Figure 2 page 357Force arrows represent the weight of items onthe scaleThe longer the arrow the more weight acting onthe scale.Combining ForcesNet force is the overall force acting on anobject after all the forces are combined.
*Balanced Forces (ex: tug of war)Sometimes the net force acting on an object iszero.These are called balanced forces.Equal and Opposite Forces =0When the forces on an object are balanced thenet force is zero and there is no change in theobject’s motion.EX: tug of war ~ Where no one wins
*Unbalanced forcesIs a force that results when the net force actingon an object is not zero.When an unbalanced force acts on anobject, the object accelerates.Forces acting in opposite directions can alsocombine to produce an unbalanced force.The net force equals the size of the larger forceminus the size of the smaller force. Accelerate in the direction of the unbalancedforce ( larger force)
FRICTIONFriction (4 types)- it is a force thatopposes the motion of objects.It acts on any surface where 2 objects ormore are in contact.Without friction all surfaces would bemore slippery than ice.All objects experience friction.We need friction to walk. Finger printshelps up pick things up.
*4 main types1. Static Friction : Is the friction force that acts on the objects that are not moving. It always acts in the opposite direction of the force being applied. Ex: foot and ground allows you to walk. Ground pushing up.2. Sliding Friction: It is the force that opposesthe direction of motion of an object as it slidesover a surface. This force is less than staticfriction. Less force is needed to keepsomething sliding. Ex: Pushing somethingalong the floor.
*3. Rolling Friction: The friction force that actson a rolling object100 to 1000 times less than the force of staticfriction. Why we use dollies to move heavyobjects. Ball bearings are used in machines toreduce friction. Ex: Bikes, cars, inlineskates, etc.4. Fluid Friction: Opposes the motion of anobject through a fluid. Can be air or water.Air = airplane Water = submarineFluid Friction increases as the speed of theobject moving through the fluid increases.
*Gravity – a force that acts between any twomasses- Attractive force that pulls objects together- Can act over long distances- Acts downward toward the center of the Earth
*Falling Objects- 2 forces act upon this object- Gravity and Air resistance- Reach terminal velocity when gravity and airresistance equal each other (forces balanceeach other out)Projectile Motion – a curved path of an object infree fall.Force – initial force forward, force of gravitypulling it down.
12.2 NEWTON’S FIRST AND SECOND LAWS OF MOTIONAristotle thought force was needed to keepan object moving at constant speed. Notcorrect and held up the progress in thestudy of motion for about 2 thousand years.Galileo found out how gravity producedconstant acceleration. Concluded thatmoving objects that are not subjected tofriction or any other force would moveindefinitely.Newton defined mass and force. Wrote hislaws of motion
*Newton’s first law of motion- (the state ofmotion)• The state of motion of an object does not change as long as the net force acting on the object is zero.• Basically an object in motion stays in motion with the same speed and direction.• An object at rest stays at rest unless an unbalanced force acts on it.• Sometimes called the law of inertia.• Inertia is the tendency of an object to resist change in its motion.
12.2Newton’s Second Law of MotionThe acceleration of an object is equal to the netforce acting on it divided by the objects mass.Acceleration of an object depends upon itsmass.Mass is a measure of the inertia of an objectand depends on the amount of matter the objectcontains.Acceleration is directly proportional to the netforce acting on it.
12.2Formula:Acceleration = Net Force a=F Mass mUnits- N/kg and m/s2 are equivalent becauseN= kg·m/sAcceleration of an object is always in the samedirection as the net forceThe 2nd law also applies when a net force actsin the direction opposite to the objects motion.Force produces a deceleration.
WEIGHT AND MASSAn object’s weight is the product of theobject’s mass and acceleration due togravity.We have a weight formula which alsoconsidered Newton’s second law.Weight = Mass × Acceleration due to gravityW = mg (just a different form of a=F/m)g= 9.8m/s2
SECTION 12.3 NEWTON’S 3RD LAW OF MOTION ANDMOMENTUMForces never exist alone. Forces allexist in pairs.Newton’s 3rd Law:Whenever one object exerts a force on asecond object, the second object exertsan equal and opposite force on the firstobject.
*Action and reaction forcesNot all action-reaction force pairs result inmotion.Ex: Swimmer in water- yes Push on wall-noWhy don’t action-reaction forces cancel outand produce a net force of zero?It is because reaction and action forces donot act on the same object.
*Ex: Figure 16 P. 373The action force of the swimmers armsact upon the water.The reaction force acts upon theswimmer ( the water pushing on theswimmer)Only equal and opposite forces act onthe same object and give a net force ofzero.
*Momentum- is the product of an object’s massand its velocity.An object with a large momentum is hard tostop.An object has a large momentum if the productof its mass and velocity is large.Formula:Momentum = Mass × velocity
*Law of conservation of momentumIf no net force acts on a system, then thetotal momentum of the system does notchange.See diagrams page 376
SECTION 12.4 UNIVERSAL FORCESElectromagnetic forces• Associated with charged particles• Electric force and magnetic force are the only forces that can attract and repel.Electric forces- between charged objects orparticles such as electrons and protons.• Same charge – repel• Opposite charge - attract
*Magnetic forces – act on certain metals, onthe poles of magnets, and on moving charges.• 2 poles- N and S, attract each other• 2 like poles (N,N or S,S) repel each otherNuclear forces2 forces, the strong and weak nuclear forcesact within the nucleus to hold it together.
*Strong nuclear force- overcomes the electricforce that acts among the protons.• goes the diameter of a proton (distance it acts on)• Powerful force of attraction• Acts only on the neutrons and protons in the nucleus, holding them together.
* Weak nuclear force- involved in certain types of radioactive processes • Weaker than strong nuclear force • Also located in the nucleus of an atom • Acts over a shorter range than the strong nuclear force.
*Gravitational force• Is the weakest universal force• Is an attractive force that acts between any two masses• Keeps the moon in its orbit• Pulls the moon towards the Earth.• Newton’s law of universal gravitation states that every object in the universe attracts every other object.
*• Gravitational force between 2 objects is proportional to their masses.• It decreases as the distance between the masses increase.• Greater the mass of the objects, the greater the gravitational force.• Most effective universal force over long distances.
*Why do we 2 high tides and 2 low tides.Gravitational pull from the moon produces 2bulges in the Earth’s oceans.One on the side of the Earth closest to themoon.One on the side of the Earth farthest fromthe moon.This is also why satellites stay in the sky.