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CHAPTER 4 FORCES IN ONE DIMENSION Honors Physics Glencoe, 1999 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) diagramshows vectors of all forces involved in motion Net Force (Fnet) Sum of all forces Newton’s 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.” ā = SF/m (S is the “sum of” F) 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 F1=35N E F3=53N W Choose appropriate equation F2=42N E Fnet=? Fnet=F1+F2+F3 Plug & Chug Fnet=35N+42N+(-53N) Fnet=24N E Newton’s 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/s2 The unit of force is called a Newton (N) Force needed to accelerate a mass of 1 kg by 1 m/s2 Types of Forces 4.2 Using Newton’s Laws Apparent weight- force on the object as exerted by a gravitational field On earth, g = Fg/m On the earth’s moon, g = 1.63m/s2 1.63m/s2 = F/m Your New (SI) Weight Weight is measured in Newtons, not pounds To determine your mass Your To determine your weight in Newtons Your weight (in pounds) ÷ 2.2 mass (in kg) x 9.80m/s2 Mr. Latham=185lbs (185lbs)/(2.2lbs/kg)=84.1kg (84.1kg)(9.80m/s2)=824.1N 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 Drag Force (Fd) 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 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/s2. If Kamaria’s mass is 27.2kg, with what force does her mother need to pull her? Known & Unknown m=27.2kg a= 0.80m/s2 Choose appropriate equation F=? F=ma Plug & Chug F=(27.2kg)(0.80m/s2) F=22N 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/s2, with what force is Reiko pulling? Known & Unknown FTaru=16.0N FReiko=? Fnet=? Choose appropriate equation(s) m=0.75kg a=-1.25m/s2 Fnet=ma FTaru+FReiko=ma Fnet=FTaru+FReiko Plug & Chug 16.0N+FReiko=(0.75kg)(-1.25m/s2) FReiko=(0.75kg)(-1.25m/s2)-16.0N FReiko=17N 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/s2)? Known & Unknown Fg=585N gE=9.80m/s2 m=? Choose appropriate equation(s) & rearrange gM=1.60m/s2 FgM=? Fg=mg m=Fg/g Plug & Chug A) m=Fg/g B) Fg=mg m=(585N)/(9.80m/s2)=59.7kg Fg=(59.7kg)(1.60m/s2)=95.5N 4.3 Interaction Forces Newton’s 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.” FA on B = -FB on A 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. Tension (FT) A force exerted on a string Equal to the net weight of the objects that it supports Normal Force (FN) The perpendicular contact force exerted by a surface on another object If the surface is tilted, the FN is tilted at the same angle 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?