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Chapter 4- Forces and Motion FOCUS QUESTION(S) What is force? List some examples of force FOCUS QUESTION(S) What are Newton’s three laws of motion? Objective(s) 1. Explain how force affects the motion of an object. 2. Distinguish between contact forces and field forces. 3. Interpret and construct free-body diagrams. Think about the following questions: What is this object? Where is it? Why does it look like that? Erupting Volcano!! IO is a moon of Jupiter Competing forces between Jupiter and the other Galilean moons cause the center of Io compress and melt. Consequently Io is the most volcanically active body in the solar system. Other examples of forces What is a force? IPC definition: A push or a pull exerted on some object Better definition: Force represents the interaction of an object with its environment The Unit for Force is a Newton kgm 1N 1 2 s Two major types of forces Contact Forces: Result from physical contact between two objects Examples: Pushing a cart, Pulling suitcase Field Forces: Forces that do not involve physical contact Examples: Gravity, Electric/Magnetic Force Force is a vector! (yay more vectors ) The effect of a force depends on magnitude and direction Force Diagrams (p. 126) Force Diagram: A diagram that shows all the forces acting in a situation Free Body Diagrams p.127 Free Body Diagrams (FBDs) isolate an object and show only the forces acting on it FBDs are essential! They are not optional! You need to draw them to get most problems correct! How to draw a free body diagram Situation: A tow truck is pulling a car (p. 127) We want to draw a FBD for the car only. Steps for drawing your FBD Step 1: Draw a shape representing the car (keep it simple) Step 2: Starting at the center of the object, Draw and label all the external forces acting on the object Force of Tow Truck on Car= 5800 N Add force of gravity Force of Tow Truck on Car= 5800 N Gravitational force (Weight of car)= 14700 N Add force of the road on the car (Called the Normal Force) Normal Force = 13690 N Force of Tow Truck on Car= 5800 N Gravitational force (Weight of car)= 14700 N Finally add the force of friction acting on the car Normal Force = 13690 N Force of Tow Truck on Car= 5800 N Gravitational force (Weight of car)= 14700 N Force of Friction= 775 N A Free Body Diagram of a Football Being Kicked Fkick Fg A person is pushed forward with a force of 185 N. The weight of the person is 500 N, the floor exerts a force of 500 N up. The friction force is 30 N. FN= 500 N Ff= 30 N Fapp= 185 N Fg= 500 N Forces you will need Symbol of Description Force Fg Gravitational Force is the Weight of the Object (equal to mass x g= mg) FN Normal Force= Force acting perpendicular to surface of contact Ff Frictional Force- Opposes applied force; acts in direction opposite of motion Fapp Applied Force Sample Problem p. 128 #3 Draw a free body diagram of a football being kicked. Assume that the only forces acting on the ball are the force of gravity and the force exerted by the kicker. Practice Problem A large, square box of exercise equipment sits on a storeroom floor. A rope is tied around the box. Assume that if the box moves along the floor, there is a backward force that resists its motion. Suppose that the box remains at rest. In the space provided, draw a free body diagram for the box. Label each force involved in the diagram. A large, square box of exercise equipment sits on a storeroom floor. A rope is tied around the box. Assume that if the box moves along the floor, there is a backward force that resists its motion. Suppose a warehouse worker moves the box by pulling the rope to the right horizontal to the ground. In the space provided, draw a free-body diagram for the box. Label each force involved in the diagram A large, square box of exercise equipment sits on a storeroom floor. A rope is tied around the box. Assume that if the box moves along the floor, there is a backward force that resists its motion. Suppose the warehouse worker moves the box by pulling the rope to the right at a 50° angle to the ground. In the space provided, draw a free body diagram for the box. Label each force involved in the FOCUS QUESTION(S) Explain how force affects the motion of an object. 2. Distinguish between contact forces and field forces. A Chair is pushed forward with a force of 185N. The gravitational force of the earth on the chair is 155N downward, and the floor exerts a force of 155N upward on the chair. Draw a free-body diagram showing the forces acting on the chair. FBD Worksheet FOCUS QUESTION(S) Two soccer players kick a ball at the same instant. One player kicks with a force of 65 N to the north, while the other player kicks with a force of 88 N to the east. In what direction does the ball travel? The muscle responsible for closing the mouth is the strongest muscle in the human body. It can exert a force greater than that exerted by a man lifting a mass of 400 kg. Richard Hoffman of Florida recorded the force of biting at 4.33 x 103 N. If each force shown in the diagram below has a magnitude equal to the force of Hoffman’s bite, determine the net force. FOCUS QUESTION(S) The gravitational force of earth on a cake on a plate is 8.9 N. The plate exerts a force of 11.0 N. Draw a freebody diagrams of the cake. Newton’s first Law SECTION 2 Objective(s) 1. Explain the relationship between the motion of an object and the net external force acting on it. 2. Determine the net external force on an object. 3. Calculate the force required to bring an object into equilibrium. Newton’s 1st Law of Motion The Law of Inertia An object at rest remains at rest, and an object in motion continues in motion with constant velocity (constant speed in straight line) unless the object experiences a net external force The tendency of an object not to accelerate is called inertia Acceleration The net external force (Fnet) is the vector sum of all the forces acting on an object If an object accelerates (changes speed or direction) then a net external force must be acting upon it Equilibrium If an object is at rest (v=0) or moving at constant velocity, then according to Newton’s First Law, Fnet =0 When Fnet =0, the object is said to be in equilibrium How do we use this information? Sample Problem p. 133 #2 A crate is pulled to the right with a force of 82.0 N, to the left with a force of 115 N, upward with a force of 565 N and downward with a force of 236 N. A. Find the net external force in the x direction B. Find the net external force in the y direction C. Find the magnitude and direction of the net external force on the crate. Step 1: Draw a FBD Fup = 565 N Fleft = 115 N Fright = 82 N Fdown = 236 N Find the vector sum of forces A. 82 N + (-115 N )= -33 N B. 565 N + (-236 N) = 329 N C. Find the resultant of the two vectors from part a and b. R = 331 N at 84.3 North of West 329 N 33 N Newton’s 1st Law Review Newton’s 1st Law: When Fnet=0, an object is in equilibrium and will stay at rest or stay in motion In other words, if the net external force acting on an object is zero, then the acceleration of that object is zero Practice Problems Practice 4A page 133 Classwork Section 4.2 review page 135 Homework. FOCUS QUESTION(S) What is Newton 2nd and 3rd Law of Motion? The net force exerted by a woodpecker’s head when its beak strikes a tree can be as large as 4.90 N, assuming that the bird’s head has a mass of 50.0 g. Assume that two different muscles pull the woodpecker’s head forward and downward, exerting a net force of 4.90 N. If the forces exerted by the muscles are at right angles to each other and the muscle that pulls the woodpecker’s head downward exerts a force of 1.70 N, what is the magnitude of the force exerted by the other muscle? Draw a free-body diagram showing the forces acting on the woodpecker’s head. Chapter 4 SECTION 3 Objective(s) 1. Describe the acceleration of an object in terms of its mass and the net external force acting on it. 2. Predict the direction and magnitude of the acceleration caused by a known net external force. 3. Identify action-reaction pairs. 4. Explain why action-reaction pairs do not result in equilibrium. Newton’s 2nd Law (p.137) The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass Fnet a m Example p. 138 # 4 A 2.0 kg otter starts from rest at the top of a muddy incline 85 cm long and slides down to the bottom in 0.50 s. What net external force acts on the otter along the incline? Solving the problem To calculate Fnet, we need m and a M=2.0 kg What is a? Vi= 0 m/s, t=0.50 s, displacement=85 cm=.85 m Welcome back kinematic equations! 1 2 x vi t at 2 x vi t 0.85m 00.50 m a 6.8 2 1 2 1 s 0.502 t 2 2 Fnet m ma 2kg 6.8 2 14 N s Newtons’ 3rd Law Forces always exist in pairs For every action there is an equal and opposite reaction Action- Reaction Pairs Some action-reaction pairs: Although the forces are the same, the accelerations will not be unless the objects have the same mass. Practice Practice page 138 HW section 4.3 review page 140 FOCUS QUESTION(S) FOCUS QUESTION(S) A cabinet initially at rest on a horizontal surface requires a 115 N horizontal force to set it in motion. If the coefficient of static friction between the cabinet and the floor is 0.38,what is the normal force exerted on the cabinet? What is the mass of the cabinet? FOCUS QUESTION(S) Two students reach for a jar of mustard at the same time. One student pulls to the left with a force of 13.2 N, while the other student pulls to the right with a force of 12.9 N. If the jar has a net acceleration of 0.44 m/s2 to the left, what is the mass of the jar? FOCUS QUESTION(S) 1. Are mass and weight the same? Why or why not? 2. Explain the difference between static and kinetic friction. CHAPTER 4 SECTION 4 Objective(s) 1. Explain the difference between mass and weight. 2. Find the direction and magnitude of the normal force. 3. Describe air resistance as a form of friction. 4. Use coefficients of friction to calculate frictional force. Everyday Forces Weight= Fg = mg Normal Force= FN= Is always perpendicular to the surface. Friction Force= Ff Opposes applied force There are two types of friction: static and kinetic Static Friction Force of Static Friction (Fs) is a resistive force that keeps objects stationary As long as an object is at rest: Fs = -Fapp Kinetic Friction Kinetic Friction (Fk) is the frictional force on an object in motion Fnet= Fapp- Fk Coefficients of Friction The coefficient of friction (μ) is the ratio of the frictional force to the normal force Coefficient of kinetic Friction Fk Kinetic Friction Force Coefficien t of Kinetic Friction k FN Normal Force Coefficient of Static Friction Fs Static Friction Force Coefficien t of Static Friction s FN Normal Force Sample Problem p. 145 #2 A 25 kg chair initially at rest on a horizontal floor requires a 365 N horizontal force to set it in motion. Once the char is in motion, a 327 N horizontal force keeps it moving at a constant velocity. A. Find coefficient of static friction B. Find coefficient of kinetic friction Coefficient of Static Friction In order to get the chair moving, it was necessary to apply 365 N of force to overcome static friction. Therefore Fs = 365 N. The normal force is equal to the weight of the chair (9.81 x 25= 245 N) F s 365 N s 1.5 FN 245 N Coefficient of Kinetic Friction The problem states that the chair is moving with constant velocity, which means Fnet=0. Therefore, Fapp must equal -Fk. Fk= 327 N Fapplied= 327 N Solve for Coefficient of Kinetic Friction Fk 327 N k 1.3 FN 245 N FOCUS QUESTION(S) A 20.0 kg trunk is pushed across the floor of a moving van by a horizontal force. If the coefficient of kinetic friction between the trunk and the floor is 0.255,what is the magnitude of the frictional force opposing the applied force? FOCUS QUESTION(S) A cabinet initially at rest on a horizontal surface requires a 115 N horizontal force to set it in motion. If the coefficient of static friction between the cabinet and the floor is 0.38,what is the normal force exerted on the cabinet? What is the mass of the cabinet? FOCUS QUESTION(S) A box of books weighing 319 N is shoved across the floor by a force of 485 N exerted downward at an angle of 35° below the horizontal. Draw the free body diagram for this situation. Forces at an angle A woman is pulling a box to the right at an angle of 30 above the horizontal. The box is moving at a constant velocity. Draw a free body diagram for the situation. FBD FN= Normal Force Fapp= Applied Force F app,y F app,x Ff= Friction Force Fg=Weight What is Fnet? Since the suitcase is moving with constant velocity, Fnet=0. That means the forces in the x direction have to cancel out and the forces in y direction have to cancel out Fk = Fapp,x FN + Fapp,y = Fg NOTICE THAT NORMAL FORCE DOES NOT EQUAL WEIGHT IN THIS SITUATION Let’s do an example. P. 154 #42 A 925 N crate is being pushed across a level floor by a force F of 325 N at 25° above the horizontal. The coefficient of kinetic friction is 0.25. Find the magnitude of the acceleration of the crate. What do we need to know? So we need mass and Fnet. Fnet a m We have weight (925 N). So what is mass? weight 925 N mass 94.3 kg m g 9.81 2 s How to find Fnet? Find vector sum of forces acting on crate. FBD FN= Normal Force Fapp= 325 N F app,y Ff= Friction Force F app,x Fg=Weight=925 N Finding Fnet,y Is box accelerating in y direction? No. Therefore Fnet in y direction is 0 So FN + Fapp,y = Fg So FN = Fg- Fapp,y= 925 N- 325sin(25) FN= 787.65 N Finding Fnet,x Is box accelerating in x direction? Yes. Therefore Fnet,x is not 0 Fnet,x= Fapp,x – Ff Fapp,x = Fappcos(25)=294.6 N Use coefficient of friction to find Ff Ff=μFN=(0.25)(787N)=197 N Finish the Problem Fnet,x = 294 N – 197 N= 97 N So now we know that the Fnet on the box is 97 N since Fnet,y is 0 Fnet 97 N m a 1.03 2 m 94.3kg s Another example. P. 154 #54 part a A box of books weighing 319 N is shoved across the floor by a force of 485 N exerted downward at an angle of 35° below the horizontal. If μk between the floor and the box is 0.57, how long does it take to move the box 4.00 m starting from rest? DRAW FBD FN Ff Fapp,x Fapp,y Fg=319 N Fapp= 485 N Find Fnet Is box accelerating in y direction? No. Therefore Fnet in y direction is 0 So FN = Fapp,y + Fg So FN = 485sin(35) + 319 N= 598 N Fnet,x Is box accelerating in x direction? Yes. Therefore Fnet,x is not 0 Fnet,x= Fapp,x – Ff Fapp,x = 485cos(35)=397.29 N Use coefficient of friction to find Ff Ff=μFN=(0.57)(598)=341 N Fnet, x = 397.29- 341= 57.29 N So now we know that the Fnet on the box is 57.29 N since Fnet,y is 0 Weight of box is 319 N. Find mass by dividing by 9.81 m= 32.52 kg Fnet 57.29 N m a 1.76 2 m 32.52kg s Finish the problem We want to know how long it takes for the box to move 4.00 m. Find vf so that you can solve for t Solve for t x 4m vf vi 0 m s a 1.76 m s2 m v 2ax 3.8 s 2 i m m v f vi 3.8 s 0 s t 2.13s m a 1.76 2 s Forces on An Incline A block slides down a ramp that is inclined at 30° to the horizontal. Write an expression for the normal force and the net force acting on the box. Draw a Free Body Diagram FF FN Fg,y θ Fg Fg,x θ Closer look at gravity triangle. Solve for Fg,y and Fg,x Fg , y a Fgcos mg cos( ) ,y h Fg F o sin( Fgsin( mg ) ,x ) g ,x h Fg Fg,y θ Fg Fg,x Coordinate system for inclined planes Fnet in the y direction When a mass is sliding down an inclined plane, it is not moving in the y direction. Therefore Fnet,y =0 and all the forces in the y direction cancel out. Forces In the y-direction So what are the forces acting in the y direction? Look at your FBD We have normal force and Fg,y Since they have to cancel out… FN= mgcos(θ) Forces in the x direction What is the force that makes the object slide down the inclined plane? Gravity…but only in the x direction Remember that Vectors can be moved parallel to themselves!! FF FN Fg,y θ Fg Fg,x θ Forces in the x direction So what are the forces acting in the x direction? Friction Force (Ff) and Gravitational Force (Fg,x) If the box is in equlibrium Fg,x = Ff If the box is accelerating Fnet= Fg,x - Ff What if there is an additional applied force? Example: a box is being pushed up an inclined plane… Fapp FN Fg,x Fg,y Ff θ In that case… FN= mgcosθ Fnet = Fapp- Fg,x – Ff If the object is in equilibrium then Fapp= Fg,x + Ff An Example p. 153 #40 A 5.4 kg bag of groceries is in equilibrium on an incline of 15° angle. Find the magnitude of the normal force on the bag. Draw a FBD Ff FN Fg,x Fg,y Fg θ Solve the Problem The block is in equilibrium so… Fnet=0 Fg,y= FN=mgcosθ=(5.4kg)(9.81)cos(15) FN=51 N Additionally, what is the force of friction acting on the block? Find Force of Friction Fnet= 0 Fg,x= Ff= mgsinθ=5.4(9.81)sin(15) Ff= 13.7N Example p. 147 #3 A 75 kg box slides down a 25.0° ramp with an acceleration of 3.60 m/s2. Find the μk between the box and the ramp What acceleration would a 175 kg box have on this ramp? FBD Ff FN Fg,x Fg,y Fg θ What is Fnet? They give mass and acceleration So Fnet= ma= 75kg x 3.60 m/s2 Fnet= 270 N FN= mgcosθ Fnet= Fg,x – Ff=mgsinθ - Ff Solve for Ff Fnet= Fg,x – Ff=mgsinθ – Ff Ff= mgsinθ – Fnet Ff = 75kg(9.8)sin(25) – 270 N Ff = 40.62 N Finish the Problem We are trying to solve for μk Ff 40.26 N 40.26 N k .061 FN mg cos 667 N CLASS PRACTICE Practice 4D p.147 HW 4.4 Section Review p. 149 #1-5 GET OUT YOUR HW