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CHAPTER 4 FORCES IN ONE DIMENSION Honors Physics Glencoe, 1999 Slide 2 4.1 Force & Motion Force- any agent that can cause an object to change its state of motion Contact forces- must touch to interact (walking, throwing, crashing) Field forces- no touching necessary (gravity, magnetism) Force (free-body) diagram- shows vectors of all forces involved in motion Slide 3 Net Force (F net ) Sum of all forces Slide 4 Newtons Second Law The acceleration of an object is equal to the sum of the forces acting on the object, divided by the mass of he object. = F/m ( is the sum of F) Slide 5 Practice Problem 4.1.8 Three confused sleigh dogs are trying to pull a sled across the Alaskan snow. Alutia pulls east with a force of 35N, Seward also pulls east but with a force of 42N, and big Kodiac pulls west with a force of 53N. What is the net force on the sled? Known & Unknown F 1 =35N EF 2 =42N E F 3 =53N WF net =? Choose appropriate equation F net =F 1 +F 2 +F 3 Plug & Chug F net =35N+42N+(-53N) F net =24N E Slide 6 Newtons First Law- Inertia Every object continues in its state of rest or uniform velocity as long as no net force acts on it. Inertia is measured in terms of a ratio, the amount of force need to cause one unit of acceleration Ratio is called mass and is measured in kilogram (kg) A 150 kg object requires 150 units of force to cause an acceleration of 1 m/s 2 The unit of force is called a Newton (N) Force needed to accelerate a mass of 1 kg by 1 m/s 2 Slide 7 Types of Forces Slide 8 4.2 Using Newtons Laws Apparent weight- force on the object as exerted by a gravitational field On earth, g = F g /m On the earths moon, g = 1.63m/s 2 1.63m/s 2 = F/m Slide 9 Your New (SI) Weight Weight is measured in Newtons, not pounds To determine your mass Your weight (in pounds) 2.2 To determine your weight in Newtons Your mass (in kg) x 9.80m/s 2 Mr. Latham=185lbs (185lbs)/(2.2lbs/kg)=84.1kg (84.1kg)(9.80m/s 2 )=824.1N Slide 10 Weightlessness Apparent Weight of 0N Your acceleration has the same magnitude and direction as the gravity field If you are not in a measurable gravity field http://www.youtube.com/w atch?v=2V9h42yspbo http://www.youtube.com/w atch?v=2V9h42yspbo Slide 11 Drag Force (F d ) The force exerted by a fluid on the object moving through the fluid Dependant on size and shape of object, properties of fluid, velocity of object Terminal velocity- reached when drag force equals the force of gravity Slide 12 Practice Problems 4.2.16 Kamaria is learning how to ice-skate. She wants her mother to pull her along so that she has an acceleration of 0.80m/s 2. If Kamarias mass is 27.2kg, with what force does her mother need to pull her? Known & Unknown m=27.2kgF=? a= 0.80m/s 2 Choose appropriate equation F=ma Plug & Chug F=(27.2kg)(0.80m/s 2 ) F=22N Slide 13 Practice Problem 4.2.17 Taru & Reiko simultaneously grab a 0.75kg piece of rope and begin tugging on it in opposite directions. If Taru pulls with a force of 16.0N and the rope accelerates away from her at 1.25m/s 2, with what force is Reiko pulling? Known & Unknown m=0.75kgF Taru =16.0N a=-1.25m/s 2 F Reiko =? Fnet=? Choose appropriate equation(s) F net =ma F net =F Taru +F Reiko F Taru +F Reiko =ma Plug & Chug 16.0N+F Reiko =(0.75kg)(-1.25m/s 2 ) F Reiko =(0.75kg)(-1.25m/s 2 )-16.0N F Reiko =17N Slide 14 Practice Problem 4.2.19 On Earth, a scale shows that you weigh 585N. A) What is your mass? B) What would the scale read on the Moon (g=1.60m/s 2 )? Known & Unknown F g =585N g E =9.80m/s 2 g M =1.60m/s 2 m=?F gM =? Choose appropriate equation(s) & rearrange F g =mgm=F g /g Plug & Chug A) m=F g /gm=(585N)/(9.80m/s 2 )=59.7kg B) F g =mgF g =(59.7kg)(1.60m/s 2 )=95.5N Slide 15 4.3 Interaction Forces Newtons Third Law Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. F A on B = -F B on A Slide 16 Practice Problem 4.3.31 A suitcase sits on a stationary airport luggage cart. Draw a free-body diagram for each object and specifically indicate any interaction pairs between the two. Slide 17 Tension (F T ) A force exerted on a string Equal to the net weight of the objects that it supports Slide 18 Normal Force (F N ) The perpendicular contact force exerted by a surface on another object If the surface is tilted, the F N is tilted at the same angle Slide 19 Practice Problem 4.3.32 You are helping to repair a roof by loading equipment into a bucket that workers hoist to the rooftop. If the rope is guaranteed not to break as long as the tension does not exceed 450N and you fill the bucket until it has a mass of 42kg, what is the greatest acceleration that the workers can give the bucket as they pull it to the roof?