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Transcript
Chapter 4. Force and Motion Chapter Goal: To establish a connection between force and motion. Student Learning Objectives • To recognize what does and does not constitute a force. • To identify the specific forces acting on an object. • To draw an accurate free-body diagram of an object. • To begin the process of understanding the connection between force and motion. A force is an interaction between two objects • A force is a push or a pull on an object. • If I push a book across a table, the book pushes me back (inanimate objects can exert force!) • A force is a vector. It has both a magnitude and a direction. • The force (interaction) has the same magnitude for both me and the book. However the direction of the force on me is opposite to the direction of the force on the book. A force is an interaction between two objects •A force on an object requires an agent. The agent is another object. •This is another way of confirming that a force is an interaction between objects. It takes two to tango! • A force is either a contact force or a long-range (noncontact) force. •Gravity is the only long-range force we will study this semester. It is an interaction between two objects, but they are not necessarily in contact. •All other forces (this semester) only exist when the two objects are in contact. A Short Catalog of Forces - Gravity • Gravity is a long-range attractive force between two objects. •In this class, our emphasis is on the interaction between the Earth and objects on or near its surface (the weight force). •Your text uses W for the weight force. •Near the surface of the earth, W = mg, where m is the object mass in kilograms and g= 9.8 m/s2 is the acceleration due to gravity. Earth A Short Catalog of Forces: Tension Ropes, strings, cables and hooks exert a tensional force. These agents always pull, they never push. Although it seems strange, the sled exerts a tension force on the rope. A Short Catalog of Forces - Normal The normal force is a contact force between 2 surfaces that is perpendicular to the surfaces. Your book uses FN for the normal force A Short Catalog of Forces - friction The frictional force is a contact force between 2 surfaces that is parallel to the surfaces A Short Catalog of Forces Fp Other forces are usually shown as an F with an identifying subscript. You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the rock at this instant? Choose all that apply A. Gravity, acting downward B. The normal force, acting upward C. The force of the kick, acting in the direction of motion D. Friction, acting opposite the direction of motion E. All of the above Drawing force vectors Free-body diagrams • Identify all forces acting on the object. • Draw a coordinate system. If motion is along an incline, the coordinates system should be tilted relative to true horizontal and true vertical. • Represent the object as a dot at the origin. • Draw vectors representing each of the identified forces and label with the appropriate symbol. truck not moving due to friction Freebody diagrams Draw a free body diagram for the truck. There is no friction, but a cable attached to the wall keeps the truck from moving. Freebody diagrams Draw a free body diagram for the sled. Assume friction between the snow and the sled. 4.3 Newton’s Second Law of Motion Newton’s Second Law An object of mass m, subject to forces F1 , F2 , etc. will undergo an acceleration with a magnitude directly proportional to the net force and inversely proportional to the mass: a F m F ma The direction of the acceleration is the same as the direction of the net force. 4.3 Newton’s Second Law of Motion Newton’s First Law This is a special case of the 2nd Law. If the net force equals 0, there is no acceleration and the velocity of the object will not change. If it is at rest, it will stay at rest. F 0 In this case the object is in equilibrium. •Static equilibrium – object is at rest for a finite period •Dynamic equilibrium –object is moving at constant velocity. Graphical Interpretation of Newton’s Second Law • Newton’s 2nd Law is the equation of a line with a 0 value for the y-intercept (in this case the aintercept!). F a m a F The following graphs plot acceleration vs force for different objects. Which object has the greatest mass? A B a a F F C D a a F A. B. C. D. F Problem-Solving Strategy for Newton’s Law Problems Problem-Solving Strategy: Equilibrium Problems Fx 0 Fy 0 If the net force is equal to zero, must the object be at rest? The next example shows the answer to this: Newton’s 1st Law: Towing a car up a hill A car with a weight (not mass) of 15,000 N is being towed up a 20° frictionless slope at constant velocity. The tow rope is parallel to the slope surface. What is the tension on the tow rope? Why is this a Newton’s 1st Law problem (ΣF = 0) and not a Newton’s 2nd Law problem (ΣF = ma)? Draw a free body diagram for this problem. Newton’s 1st Law: Towing a car up a hill A car with a weight (not mass) of 15,000 N is being towed up a 20° frictionless slope at constant velocity. The tow rope is parallel to the slope surface. What is the tension on the tow rope? What part of the story indicates that ΣF = 0? Newton’s 1st Law: Towing a car up a hill Once the pictorial representation is completed, use Newton’s 1st Law in component form: T and n have only one nonzero component, but FG has non-zero components in both x and y directions. How do we write the components of FG in terms of θ ? Newton’s 1st Law: Towing a car up a hill Solving the 2nd equation tells us that n = FG cos θ, which is not necessary information for this problem. Problem-Solving Strategy for Newton’s 2nd Law Problems 1. Use the problem-solving strategy outlined for Newton’s 1st Law problems to draw the free body diagram and determine known quantities. 2. Use Newton’s Law in component form to find the values for any individual forces and/or the acceleration. 3. If necessary, the object’s trajectory (time, velocity, position, acceleration) can be determined by using the equations of kinematics. 4. Reverse # 2 and 3 if necessary. Newton’s 2nd Law – Speed of a Towed Car •Draw a free-body diagram. Find the acceleration using Newton’s second law in component form. •Draw a pictorial representation and Find the velocity using one of the kinematic equations. Speed of a towed car – pictorial representation of motion, motion diagram and free body diagram Speed of a towed car Use the free body diagram to write Newton’s 2nd Law in component form: ΣFy = may = 0 (no change in speed up or down) ΣFy = n – FG , or n=FG (not pertinent in this problem) ΣFx = max T – f = max Speed of a towed car Speed of a towed car Apparent Weight – What the scale says (even if there is no scale!) • The value the scale reads when scale and object are accelerating or being acted upon by some force other than gravity (e.g the buoyant force in water) • Not equal to the true weight (FG = mg) • Apparent weight is either a normal force (step on scale) or tension force (hanging scale) • An object inside an accelerating elevator has the same acceleration as the elevator. • Acceleration is not a force so don’t include directly on free body diagram. Typical FBD for an apparent weight problem n FG = mg, but n does not! Apparent Weight – What the scale would say, even if there is no scale It takes the elevator in a skyscraper 4.0 s to reach a constant speed of 10.0 m/s. A 60.0 kg passenger gets on at the ground floor. What is her apparent weight during those 4 seconds? Use the problem-solving strategy (repeated in next slide). Problem-Solving Strategy for Newton’s Law Problems Problem-Solving Strategy for Newton’s 2nd Law Problems 1. Use Newton’s Law in component form to find the values for any individual forces and/or the acceleration. 2. If necessary, the object’s trajectory (time, velocity, position, acceleration) can be determined by using the equations of kinematics. 3. Reverse # 1 and 2 if necessary (Hint: it is!) Mass – a quantitative measure of inertia • An object’s mass (m) is a measure of the amount of matter that it contains. • The SI unit of mass is the kilogram (kg). The English unit of mass is the slug, which is virtually never used. • The mass of an object does not change unless you add or subtract matter from it. • The mass of an object is the same on any planet and in deep space. • Scales do not directly measure mass, except for a balance scale, which compares one mass to another. Weight • The weight of an object on or above the earth is the gravitational force that the earth exerts on the object (FG or W). • Since weight is a force, the SI unit of weight is the Newton (N); the English unit is the pound. • The weight of an object changes on other planets since the gravitational force of other planets are different than that of the earth. • A scale does not directly measure weight; it measures the normal force or (hanging scale) the tension force. In most situations, this force measured by the scale is numerically equal to the weight force. Apparent weight or “what the scale says” • In accelerating reference frames (e.g. the ever-popular elevator with a scale), the scale reading will differ from the true weight. This is called apparent weight. • Metric scales assume measurement on earth and are calibrated to read in mass units (kg). American scales read in English force units (lb). It’s a culture thing. 4.7 The Gravitational Force Newton’s Law of Universal Gravitation • Every particle in the universe exerts an attractive force on every other particle. • A particle is a piece of matter, small enough in size to be regarded as having no volume. In practice we use the particle model even for larger bodies • The Law of Universal Gravitation is an example of an inverse square law. The force between the two bodies is inversely proportional to the square of the distance between them. m1m2 FG 2 r For two particles that have masses m1 and m2 and are separated by a distance r, the force has a magnitude given by: F1 2 m1m2 F21 G 2 r G 6.673 10 11 N m kg 2 2 F1-2 and F2-1 are equal in magnitude but point in opposite directions If m1 has a mass of 12 kg and m2 has a mass of 25 kg and the two objects are 1.2 meters apart, what is the magnitude of the gravitational force between them? F1 2 m1m2 F21 G 2 r G 6.673 1011 N m 2 kg 2 Ratio Reasoning with Inverse Square Laws Two masses, separated by a distance of r experience a gravitational attraction, F. Now the distance is increased by a factor of 3. By what factor does the magnitude of the force change (what is the ratio F2 )? F1 a. 1/3 c. 9 b. 3 d. 1/9 The figure shows the moon being attracted by the earth. The earth is also attracted by the moon. Compared to FEonM (the force shown), FMonE is: A. much smaller and in the same direction. B. much smaller and in the opposite direction. C. the same size and same direction. D. the same size and opposite direction. earth moon FEonM 4.7 The Gravitational Force Definition of Weight The weight of an object on or above the earth is the gravitational force that the earth exerts on the object. The weight always acts downwards, toward the center of the earth. On or above another astronomical body, the weight is the gravitational force exerted on the object by that body. SI Unit of Weight: newton (N) W G W G M Em r 2 M Em RE 2 If the object is close (within 100 miles) to the earth, this becomes: Define the quantity g, for all the unchanging entities: ME g G 2 RE Calculate g G 6.673 1011 N m 2 kg 2 ME = 5.98 x 1024 kg RE = 6.38 x 106 m (don’t forget to square!) The Gravitational Force W G M Em RE 2 ME g G 2 RE W mg g = 9.8 m/s2 on Earth 1 kg of mass x g = 9.8 N, where N = kg m/s2 Ratio Reasoning A space traveler weighs 540.0 N on earth. What will he weigh on another planet whose radius is twice that of earth and whose mass is 3 times that of earth? Friction Kinetic Friction Experiments show that the kinetic friction force is nearly constant and proportional to the magnitude of the normal force. where the proportionality constant μk is called the coefficient of kinetic friction. Static Friction The box is in static equilibrium, so the static friction must exactly balance the pushing force: Static friction • An object remains at rest as long as fs < fs max • The object slips when fs = fs max • A static friction force fs > fs max is not physically possible. • fs max >fk for the same surfaces where the proportionality constant μs is called the coefficient of static friction. A model of friction • “ motion” indicates motion relative to the two surfaces • the max value static friction, fs max occurs at the very instant the object begins to move (which often means 1 nanosecond before, for problem-solving purposes. • for any given materials, μs > μk Determine which of the frictional forces is the largest. The box and the floor are identical in all situations. Which of these have the same frictional force? A. none B. a and d C. c,d,e, D. c,d Example Problem w tilted axis A 75-kg snowboarder starts down a 50-m high (not long!), 100 slope with μk = 0.06. Assume he has just started moving, but his starting velocity is essentially zero. What is his speed at the bottom? • Draw the free body diagram (fbd) to determine the forces. • Use Newton’s Law in component form. • Determine the acceleration. • Use kinematics to solve for the final speed. • Start with the fbd. Example Dynamics Problem w tilted axis A 75-kg snowboarder starts down a 50-m high, 100 slope, with μk = 0.06. What is his speed at the bottom? Write Newton’s Law for y-axis values. Is this 2nd Law, or 1st Law (a=0)? There is one force along the + y axis and one force with a negative y component. n fk FG Find: v1 uk = .006 Example Dynamics Problem w tilted axis A 75-kg snowboarder starts down a 50-m high, 100 slope, with μk = 0.06. What is his speed at the bottom? Write Newton’s Law for y-axis values. Is this 2nd Law, or 1st Law (a=0)? n fk FG ΣFy = n – mg cos θ = may But no acceleration along y axis means the net force is 0 therefore n = mg cos θ Find: v1 uk = .006 Example Dynamics Problem w tilted axis A 75-kg snowboarder starts down a 50-m high, 100 slope, with μk = 0.06. What is his speed at the bottom? n ΣFx = mg sin θ – fk = max where fk = uk n fk and we know that n = mg cos θ; therefore mg sin θ – ukmg cos θ = max You didn’t even need to know the mass; it cancels out! plug n chug: a = 1.12 m/s2 FG Find: v1 uk = .006 n = mg cos θ Example Dynamics Problem A 75-kg snowboarder starts down a 50-m high, 100 slope on a frictionless board. What is his speed at the bottom? Couldn’t find a picture of a snowboarder…. but the idea is the same Find v1 a, =1.12 m/s2 Example Dynamics Problem A 75-kg snowboarder starts down a 50-m high, 100 slope on a frictionless board. What is his speed at the bottom? Time is not important. Use the mantras of the Indian Princess to calculate the length (hypotenuse) of the slope. That’s our x1. Find v1 Then use the appropriate kinematic equation v1 = 25.4 m/s a, =1.12 m/s2 Reference Frames A reference frame is a coordinate system that allows description of time and position relative to some “observer”. • The origin moves (or stays at rest) in the same way as the observer does. • If a person is sitting still in a moving train, the description of the person's motion depends on the chosen frame of reference. • If the frame of reference is the train (i.e origin on the train), the person is considered to be not moving IN THAT FRAME OF REFERENCE • if the frame of reference is the Earth, (i.e. origin on the Earth), the person is considered to be moving IN THAT FRAME OF REFERENCE • The “observer” doesn’t have to be a person Inertial Reference Frames • An inertial reference frame is one where the origin is either at rest (the Earth, for our purposes) or moving at constant velocity. • Newton’s 1st Laws are valid in an inertial reference frame. • Newton’s 1st Law appears to be broken if one is looking at motion from a non-inertial (i.e. accelerating) reference frame. • The truck is accelerating to the right. Imagine 2 frames of reference; that of the truck driver and that of a pedestrian. Reference frames, revisited The truck is accelerating to the right. The truck driver thinks the box in the back is not moving, because its not sliding toward the back of the truck. The ground-based observer sees the box getting faster, just like the truck. Which observer is in an inertial reference frame? A. The truck driver B. The ground-based guy C. The box (if it could observe) D. None of the above. Free body diagram of the box In a free-body diagram of the box, there is a: a. static friction vector, pointing right b. static friction vector, pointing left c. kinetic friction vector, pointing right d. kinetic friction vector, pointing left e. no friction vector, the box is stationary in the truck Make sure the cargo doesn’t slide • A box is in the back of a flatbed truck What is the maximum acceleration the truck can have without the box sliding? Coefficients of friction between truck and box are μs = 0.40 and μk = 0.20. To solve this problem: • Is the object of interest the box, truck, or both? • Do the forces on the object of interest include static friction, kinetic friction, both, or neither? EOC 115 – Hanging a picture A person is trying to judge whether a picture of mass 1.10 kg is properly positioned by pressing it against a wall. The pressing (normal) force is perpendicular to the wall. The coefficient of static friction between picture and wall is 0.660. What is the minimum amount of pressing force required? Draw a freebody diagram. In which direction is the normal force in this problem? Does it have anything to do with the weight of the picture? Push the crate A dockworker pushes a 50-kg crate with a downward force of 300.0 N, at an angle of 30 degrees below horizontal. The coefficient of kinetic friction is 0.30. A. What is the normal force? B. What is the frictional force? C. What is the acceleration? • Draw a freebody diagram of the object of interest (will it have tilted axes?) • Apply Newton’s Law in component form. Push the crate - Freebody diagram Knowns m = 50 kg Fpush = 300N θ = 30˚ μk = 0.30 n fk θ Find n, fk ,a Fpush Fg Push the crate (problem 1 from Exploration 4) A. ΣFy = may n - FG - Fp sin 30˚ = 0 this solves for n = 640N B. fk = μk |n| = 192N C. ΣFx = max Fp cos 30˚ - μk |n| = max This will give us ax which is a = 1.36 m/s/s Keep it up! Problem 3 from Exploration 4 A man pushes with a 100 N force on a 10- kg crate, which is sliding down the wall (despite his efforts). The pushing force is directed 53 degrees above horizontal. The coefficient of kinetic friction is 0.60. Determine the magnitude of the friction force between wall and crate. Draw a freebody diagram. In which direction is the normal force in this problem? Use Newton’s law in the appropriate direction to determine the normal force.