THREE (3) LAWS OF MOTION• LAW OF INERTIA• LAW OFACCELERATION• LAW OF ACTION-REACTION
LAW OF INERTIAAn object at rest willremain at rest unlessacted on by anunbalanced force. Anobject in motioncontinues in motionwith the same speedand in the samedirection unless actedupon by an unbalancedforce.
What does this mean?This means that there is a naturaltendency of objects to keep on doingwhat theyre doing. All objects resistchanges in their state of motion. In theabsence of an unbalanced force, anobject in motion will maintain this stateof motion.
What is the motion in this picture?What is the unbalanced force in thispicture?What happened to the skater in thispicture?
This law is the same reason why youshould always wear your seatbelt.Seat belts are used to provide safety for passengerswhose motion is governed by Newtons laws. The seatbelt provides the unbalanced force which brings youfrom a state of motion to a state of rest. Perhaps youcould speculate what would occur when no seat belt isused.
1. Blood rushes from your head to your feet while quicklystopping when riding on a descending elevator.2. The head of a hammer can be tightened onto the wooden handleby banging the bottom of the handle against a hard surface.3. A brick is painlessly broken over the hand of a physics teacherby slamming it with a hammer. (CAUTION: do not attemptthis at home!)4. To dislodge ketchup from the bottom of a ketchup bottle, it isoften turned upside down and thrusted downward at highspeeds and then abruptly halted.5. Headrests are placed in cars to prevent whiplash injuries duringrear-end collisions.Applications of Newtons first law
Check Your Understanding1. The group of physics teachersare taking some time off for alittle putt-putt golf. The 15thhole at the Hole-In-One Putt-Putt Golf Course has a largemetal rim which putters mustuse to guide their ball towardsthe hole. Mr. S guides a golfball around the metal rim Whenthe ball leaves the rim, whichpath (1, 2, or 3) will the golfball follow?
The answer is 2. Once leavingthe rim, the ball will follow an"inertial path" (i.e., a straightline). At the instant shown in thediagram, the ball is moving tothe right; once leaving the rim,there is no more unbalancedforces to change its state ofmotion. Paths 1 and 3 both showthe ball continually changing itsdirection once leaving the rim.
a. 0 N b. 0.5 N c. 2.0 N d. 8.0 Ne. depends on the speed.ANSWER: If an object is in motion, then it willstay in motion with those very same motioncharacteristics. It doesnt take any force to maintainthat same state of motion. In fact, the presence of aforce would "ruin" such a state of motion and causean acceleration.2. A 4.0-kg object is moving across a friction-freesurface with a constant velocity of 2 m/s. Which oneof the following horizontal forces is necessary tomaintain this state of motion?
INERTIA• " Objects tend to "keep on doing what theyredoing."• In fact, it is the natural tendency of objects toresist changes in their state of motion.This tendency to resist changes in theirstate of motion is described as inertia.• Inertia: the resistance an object has to achange in its state of motion.
• Newtons conception of inertia stood in directopposition to more popular conceptions aboutmotion.• The dominant thought prior to Newtons daywas that it was the natural tendency of objectsto come to a rest position. Moving objects, soit was believed, would eventually stopmoving; a force was necessary to keep anobject moving. But if left to itself, a movingobject would eventually come to rest and anobject at rest would stay at rest;
•Thus, the idea which dominatedpeoples thinking for nearly 2000 yearsprior to Newton was that it was thenatural tendency of all objects toassume a rest position.
Mass as a Measure of the Amount of InertiaAll objects resist changes in their state of motion.All objects have this tendency - they have inertia.But do some objects have more of a tendency toresist changes than others? Absolutely yes! Thetendency of an object to resist changes in its stateof motion varies with mass.Mass is that quantity whichis solely dependent upon theinertia of an object. Themore inertia which an objecthas, the more mass it has.
ANSWER:According to Newtons first law, the rock willcontinue in motion in the same direction at constant speed.Check Your Understanding1. Imagine a place in the cosmosfar from all gravitational andfrictional influences. Supposethat you visit that place (justsuppose) and throw a rock. Therock willa. gradually stop.b. continue in motion in the samedirection at constant speed.
2. A 2-kg object is moving horizontally with aspeed of 4 m/s. How much net force is requiredto keep the object moving at this speed and inthis direction?Answer: 0 NAn object in motion will maintain its state ofmotion. The presence of an unbalanced forcechanges the velocity of the object.
3. Mac and Tosh are arguing in the cafeteria.Mac says that if he flings the Jell-O with agreater speed it will have a greater inertia.Tosh argues that inertia does not dependupon speed, but rather upon mass. Who doyou agree with? Explain why.Tosh is correct. Inertia is that quantity whichdepends solely upon mass. The more mass, the moreinertia. Momentum is another quantity in Physicswhich depends on both mass and speed. Momentumwill be discussed in a later unit.
4. Supposing you were in space in aweightless environment, would it require aforce to set an object in motion?Absolutely yes!Even in space objects have mass. And if theyhave mass, they have inertia. That is, an objectin space resists changes in its state of motion. Aforce must be applied to set a stationary objectin motion. Newtons laws rule - everywhere!
5. Ben Tooclose is being chased through the woods bya bull moose which he was attempting to photograph.The enormous mass of the bull moose is extremelyintimidating. Yet, if Ben makes a zigzag pattern throughthe woods, he will be able to use the large mass of themoose to his own advantage. Explain this in terms ofinertia and Newtons first law of motion.The large mass of the bull moose means that the bullmoose has a large inertia. Thus, Ben can more easilychange his own state of motion (make quick changes indirection) while the moose has extreme difficultychanging its state of motion. Physics for better living!
Drawing Free-Body Diagrams1. A book is at rest on atable top. A free-bodydiagram for thissituation looks like this:2. A girl is suspendedmotionless from the ceilingby two ropes. A free-bodydiagram for this situationlooks like this:
3. An egg is free-falling froma nest in a tree. Neglect airresistance. A free-bodydiagram for this situationlooks like this:4. A flying squirrel isgliding (no wing flaps) froma tree to the ground atconstant velocity. Considerair resistance. A free-bodydiagram for this situationlooks like this:
5. A rightward force isapplied to a book in order tomove it across a desk with arightward acceleration.Consider frictional forces.Neglect air resistance. A free-body diagram for thissituation looks like this:
6. A rightward force isapplied to a book in orderto move it across a desk atconstant velocity. Considerfrictional forces. Neglectair resistance. A free-bodydiagram for this situationlooks like this:
7. A force is appliedto the right to drag asled across loosely-packed snow with arightwardacceleration. A free-body diagram forthis situation lookslike this:
8. A car is coasting to theright and slowing down.A free-body diagram forthis situation looks likethis:
LAW OF ACCELERATIONAcceleration is produced when aforce acts on a mass. Thegreater the mass (of the objectbeing accelerated) the greaterthe amount of force needed (toaccelerate the object).
What does this mean?Everyone unconsciously knows theSecond Law. Everyone knowsthat heavier objects requiremore force to move the samedistance as lighter objects.
However, the Second Law gives us anexact relationship between force, mass,and acceleration. It can be expressedas a mathematical equation:orFORCE = MASS x ACCELERATION
This is an example of howNewtons Second Law works:Mikes car, which weighs1,000 kg, is out of gas.Mike is trying to push thecar to a gas station, andhe makes the car go 0.05m/s/s. Using NewtonsSecond Law, you cancompute how much forceMike is applying to thecar. Answer = 50 newtons
Practice #1An applied force of 50 N isused to accelerate an object tothe right across a frictionalsurface. The objectencounters 10 N of friction.Use the diagram to determinethe normal force, the netforce, the mass, and theacceleration of the object.(Neglect air resistance.)
Fnorm = 80 Nm = 8.16 kgFnet = 40 N, righta = 4.9 m/s/s, right
Practice #2An applied force of 20 N is usedto accelerate an object to theright across a frictional surface.The object encounters 10 N offriction. Use the diagram todetermine the normal force, thenet force, the coefficient offriction ("mu") between theobject and the surface, the mass,and the acceleration of theobject. (Neglect air resistance.)
Fnorm = 100 Nm = 10.2 kgFnet = 10 N, right"mu" = 0.1a =0.980 m/s/s, right(a = Fnet / m = (10 N) / (10.2 kg) = 0.980 m/s/s.)
Practice #3A 5-kg object is sliding to theright and encountering a frictionforce which slows it down. Thecoefficient of friction ("mu")between the object and thesurface is 0.1. Determine theforce of gravity, the normalforce, the force of friction, thenet force, and the acceleration.(Neglect air resistance.)
Fgrav = 49 NFnorm = 49 NFfrict = 4.9 NFnet = 5 N, left; a = 0.98 m/s/s, left( Fgrav = 50 N; Fnorm = 50 N; Ffrict = 5N;Fnet = 5 N, left; a = 1 m/s/s, left )a = Fnet / m = (4.9 N) / (5 kg) = 0.98 m/s/s.
Check Your Understanding1. Determine the accelerations which result when a12-N net force is applied to a 3-kg object andthen to a 6-kg object.2. A net force of 15 N is exerted on anencyclopedia to cause it to accelerate at a rate of5 m/s2. Determine the mass of the encyclopedia.3. Suppose that a sled is accelerating at a rate of 2m/s2. If the net force is tripled and the mass is doubled,then what is the new acceleration of the sled?
4. Edwardo applies a 4.25-N rightward force to a0.765-kg book to accelerate it across a table top. Thecoefficient of friction between the book and thetabletop is 0.410. Determine the acceleration of thebook.5. In a physics lab, Kate and Rob use a hangingmass and pulley system to exert a 2.45 N rightwardforce on a 0.500-kg cart to accelerate it across alow-friction track. If the total resistance force to themotion of the cart is 0.72 N, then what is the cartsacceleration?
Finding Individual ForcesFree-body diagrams for four situations
In situations in which there is air resistance, moremassive objects fall faster than less massiveobjects. But why?
LAW OF ACTION-REACTIONFor every action thereis an equal andopposite re-action.
What does this mean?This means that for every force there is areaction force that is equal in size, butopposite in direction. That is to say thatwhenever an object pushes another object itgets pushed back in the opposite directionequally hard.
Lets study how a rocket works tounderstandNewtons Third Law. The rockets action is to push down on the ground with theforce of its powerful engines, and the reaction is that theground pushes the rocket upwards with an equal force.