Chapter: Motion, Acceleration, And Forces

Table of ContentsChapter: Motion, Acceleration,and ForcesSection 1: Describing MotionSection 2: AccelerationSection 3: Motion and Forces

Describing Motion1Motion Are distance andtime important indescribing runningevents at the trackand-field meets inthe Olympics?

Describing Motion1Motion Distance and time are important. In order towin a race, you must cover the distance in theshortest amount of time. How would youdescribe the motion ofthe runners in the race?

Describing Motion1Motion and Position You don’t always need to see somethingmove to know that motion has taken place. A reference point is needed to determinethe position of an object. Motion occurs when an object changes itsposition relative to a reference point. The motion of an object depends on thereference point that is chosen.

Describing Motion1Relative Motion If you are sitting in a chair reading thissentence, you are moving. You are not moving relative to your deskor your school building, but you aremoving relative to the other planets in thesolar system and the Sun.

Describing Motion1Distance An important part of describing the motionof an object is to describe how far it hasmoved, which is distance. The SI unit of length or distance is themeter (m). Longer distances are measuredin kilometers (km).

Describing Motion1Distance Shorter distances are measured in centimeters(cm).

Describing Motion1Displacement Suppose a runner jogs to the 50-m mark andthen turns around and runsback to the 20-m mark. The runner travels 50 m inthe original direction(north) plus 30 m in theopposite direction (south),so the total distance sheran is 80 m.

Describing Motion1Displacement Sometimes you may want to know not onlyyour distance but also yourdirection from a referencepoint, such as from thestarting point. Displacement is thedistance and direction ofan object’s change inposition from the startingpoint.

Describing Motion1Displacement The length of the runner’sdisplacement and thedistance traveled would bethe same if the runner’smotion was in a singledirection.

Describing Motion1Speed You could describe movement by thedistance traveled and by the displacementfrom the starting point. You also might want to describe how fastit is moving. Speed is the distance an object travels perunit of time.

Describing Motion1Calculating Speed Any change over time is called a rate. If you think of distance as the change inposition, then speed is the rate at whichdistance is traveled or the rate of change inposition.

Describing Motion1Calculating Speed The SI unit for distance is the meter and theSI unit of time is the second (s), so in SI,units of speedaremeasured inmeters persecond(m/s).

Describing Motion1Calculating Speed Sometimes it is more convenient to expressspeed in other units, such as kilometers perhour (km/h).

Describing Motion1Motion with Constant Speed Suppose you are in a car traveling on a nearlyempty freeway. You look at the speedometerand see that the car’s speed hardly changes. If you are traveling at a constant speed, youcan measure your speed over any distanceinterval.

Describing Motion1Changing Speed Usually speed is not constant. Think aboutriding abicycle for adistance of 5km, as shown.

Describing Motion1Changing Speed How would you express your speed on such atrip? Wouldyou use yourfastest speed,your slowestspeed, or somespeed betweenthe two?

Describing Motion1Average Speed Average speed describes speed of motionwhen speed is changing. Average speed is the total distance traveleddivided by the total time of travel. If the total distance traveled was 5 km andthe total time was 1/4 h, or 0.25 h. Theaverage speed was:

Describing Motion1Instantaneous Speed A speedometer shows how fast a car is goingat one point in time or at one instant. The speed shown on aspeedometer is theinstantaneous speed.Instantaneous speedis the speed at a givenpoint in time.

Describing Motion1Changing Instantaneous Speed When something is speeding up or slowingdown, its instantaneous speed is changing. If an object is moving with constant speed,the instantaneous speed doesn’t change.

Describing Motion1Graphing Motion The motion of anobject over aperiod of time canbe shown on adistance-timegraph.Click image to play movie. Time is plotted along the horizontal axis ofthe graph and the distance traveled isplotted along the vertical axis of the graph.

Describing Motion1Plotting a Distance-Time Graph On a distance-time graph, the distance isplotted on the vertical axis and the time onthe horizontal axis. Each axis must have a scale that covers therange of number to be plotted.

Describing Motion1Plotting a Distance-Time Graph Once the scales for each axis are in place,the data points can be plotted. After plotting the data points, draw a lineconnecting the points.

Describing Motion1Velocity Speed describes only how fast something ismoving. To determine direction you need to knowthe velocity. Velocity includes the speed of an objectand the direction of its motion.

Describing Motion1Velocity Because velocity depends on direction aswell as speed, the velocity of an object canchange even if the speed of the objectremains constant. The speed of this carmight be constant,but its velocity is notconstant because thedirection of motionis always changing.

Acceleration2Acceleration, Speed and Velocity Acceleration is the rate of change ofvelocity. When the velocity of an objectchanges, the object is accelerating. A change in velocity can be either a changein how fast something is moving, or a changein the direction it is moving. Acceleration occurs when an object changesits speed, its direction, or both.

Acceleration2Speeding Up and Slowing Down When you think of acceleration, youprobably think of something speeding up.However, an object that is slowing down alsois accelerating. Acceleration also has direction, just asvelocity does.

Acceleration2Speeding Up and Slowing Down If the acceleration is in the same direction asthe velocity,the speedincreases andtheacceleration ispositive.

Acceleration2Speeding Up and Slowing Down If the speed decreases, the acceleration is inthe oppositedirection fromthe velocity,and theacceleration isnegative.

Acceleration2Changing Direction A change in velocity can be either a changein how fast something is moving or a changein the direction of movement. Any time a moving object changes direction,its velocity changes and it is accelerating.

Acceleration2Changing Direction The speed of thehorses in thiscarousel isconstant, but thehorses areacceleratingbecause theirdirection ischangingconstantly.

Acceleration2Calculating Acceleration To calculate the acceleration of an object, thechange in velocity is divided by the length oftime interval over which the change occurred. To calculate the change in velocity, subtractthe initial velocity—the velocity at thebeginning of the time interval—from the finalvelocity—the velocity at the end of the timeinterval.

Acceleration2Calculating Acceleration Then the change in velocity is:

Acceleration2Calculating Acceleration Using this expression for the change invelocity, the acceleration can be calculatedfrom the following equation:

Acceleration2Calculating Acceleration If the direction of motion doesn’t changeand the object moves in a straight line, thechange in velocity is the same as the changein speed. The change in velocity then is the final speedminus the initial speed.

Acceleration2Calculating Positive Acceleration How is the acceleration for an object that isspeeding up different from that of an objectthat is slowing down? Suppose a jet airliner starts at rest at the endof a runway and reaches a speed of 80 m/s in20 s.

Acceleration2Calculating Positive Acceleration The airliner is traveling in a straight linedown the runway, so its speed and velocityare the same. Because itstarted fromrest, itsinitial speedwas zero.

Acceleration2Calculating Positive Acceleration Its acceleration can be calculated as follows:

Acceleration2Calculating Positive Acceleration The airliner isspeeding up, so thefinal speed isgreater than theinitial speed andthe acceleration ispositive.

Acceleration2Calculating Negative Acceleration Now imagine that a skateboarder is movingin a straight line at a constant speed of 3 m/sand comes to astop in 2 s. The final speedis zero and theinitial speedwas 3 m/s.

Acceleration2Calculating Negative Acceleration The skateboarder’s acceleration is calculatedas follows:

Acceleration2Calculating Negative Acceleration The skateboarder is slowing down, so thefinal speed is less than the initial speed andthe acceleration isnegative. The accelerationalways will bepositive if an objectis speeding up andnegative if the objectis slowing down.

Acceleration2Amusement Park Acceleration Engineers use the laws of physics to designamusement park rides that are thrilling, butharmless. The highestspeeds andaccelerationsusually areproduced onsteel rollercoasters.

Acceleration2Amusement Park Acceleration Steel roller coasters can offer multiple steepdrops and inversion loops, which give therider large accelerations. As the rider moves down a steep hill or aninversion loop, he or she will acceleratetoward the ground due to gravity.

Acceleration2Amusement Park Acceleration When riders go around a sharp turn, theyalso are accelerated. This acceleration makes them feel as if aforce is pushing them toward the side ofthe car.

Motion and Forces3What is force? A force is a push or pull. Sometimes it is obvious that a force has beenapplied. But other forces aren’t as noticeable.

Motion and Forces3Changing Motion A force can cause the motion of an object tochange. If you haveplayed billiards,you know thatyou can force aball at rest to rollinto a pocket bystriking it withanother ball.

Motion and Forces3Changing Motion The force of the moving ball causes the ballat rest to move in the direction of the force.

Motion and Forces3Balanced Forces Force does not always change velocity. When two or more forces act on an object atthe same time, the forces combine to form thenet force.

Motion and Forces3Balanced Forces The net force on the box is zero because thetwo forces cancel each other. Forces on an objectthat are equal in sizeand opposite indirection are calledbalanced forces.

Motion and Forces3Unbalanced Forces When two students are pushing with unequalforces in opposite directions, a net forceoccurs in the direction of the larger force.

Motion and Forces3Unbalanced Forces The net force that moves the box will be thedifference betweenthe two forcesbecause they are inopposite directions. They are consideredto be unbalancedforces.

Motion and Forces3Unbalanced Forces The students are pushing on the box in thesame direction. These forces arecombined, or addedtogether, becausethey are exerted onthe box in the samedirection.

Motion and Forces3Unbalanced Forces The net force thatacts on this box isfound by adding thetwo forces together.

Motion and Forces3Inertia and Mass Inertia (ih NUR shuh) is the tendency of anobject to resist any change in its motion. If an object is moving, it will have uniformmotion. It will keep moving at the same speed and inthe same direction unless an unbalanced forceacts on it.

Motion and Forces3Inertia and Mass The velocity of the object remains constantunless a force changes it. If an object is at rest, it tends to remain atrest. Its velocity is zero unless a force makesit move. The inertia of an object is related to its mass.The greater the mass of an object is, thegreater its inertia.

Motion and Forces3Newton’s Laws of Motion The British scientist Sir Isaac Newton(1642–1727) was able to state rules thatdescribe the effects of forces on the motionof objects. These rules are known as Newton’s laws ofmotion.