Forces: Equilibrium Examples

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1 Physics 101: Lecture 02 Forces: Equilibrium Examples oday s lecture will cover extbook Sections Phys 101 URL: Read the course web page! Physics 101: Lecture 2, Pg 1

2 Last Lecture Overview Newton s Laws of Motion» FIRS LAW: Inertia» SECOND LAW: F net = ma» HIRD LAW: Action/reaction pairs Gravity oday W = G M m æ Earth = m G M Earth 2 2 r Earth è ç r Earth mg Forces as Vectors = (near Earth s surface!) Free Body Diagrams to Determine F net» Draw coordinate axes, each direction is independent.» Identify/draw all force vectors Friction: kinetic f = m k N; static f m s N Contact Forces Springs and ension ö ø Physics 101: Lecture 2, Pg 2

3 Forces as Vectors Last lecture we calculated the force of gravity on a book (i.e. its WEIGH): Calculate the gravitational force on a 3 kg book held 1 meter above the surface of the earth. W = G M Earth m / r Earth 2 = (6.7x10-11 m 3 / (kg s 2 )) (6x10 24 kg) (3 kg)/ (6.4x ) 2 m 2 = 29.4 kg m/s 2 = 29.4 N We missed something: he direction! W is different than W Physics 101: Lecture 2, Pg 3

4 Forces as Vectors A quantity which has both magnitude and direction is called a VECOR; FORCES are VECORS Usually drawn as an arrow pointing in the proper direction, where the length indicates the magnitude W 1 W 2 = 2W 1 = W 1 + W 1 his is an example of VECOR ADDIION: to add vectors, you place them head to tail, and draw the RESULAN from the start of the first to the end of the last A B C + = Physics 101: Lecture 2, Pg 4

5 Another Example of a Force: ension ension in an Ideal String, : Direction is parallel to string (only pulls) Magnitude of tension is equal everywhere. Now we are ready to do some physics! QUESION: We suspend a mass m = 5 kg from the ceiling using a string. What is the tension in the string? m Physics 101: Lecture 2, Pg 5

6 Newton s 2 nd Law and Equilibrium Systems We suspend a mass m = 5 kg from the ceiling using a string. What is the tension in the string? Every single one of these problems is done the same way! Step 1: Draw a simple picture (called a Free Body Diagram), and label your axes! +y Step 2: Identify and draw all force vectors Step 3: Use your drawing to write down Newton s 2 nd law F Net = ma In equilibrium, everything is balanced! a = 0 - W = 0 = W = mg = (5 kg)*(9.8 m/s 2 ) = 49 N W -y Weight, W ension, Physics 101: Lecture 2, Pg 6

7 Checkpoint! What does scale 1 read? A) 225 N B) 550 N C) 1100 N = W = W he magnitude of tension in a ideal string is equal everywhere. Physics 101: Lecture 2, Pg 7

8 ension AC wo boxes are connected by a string over a frictionless pulley. In equilibrium, box 2 is lower than box 1. Compare the weight of the two boxes. A) Box 1 is heavier B) Box 2 is heavier C) hey have the same weight Step 1 Draw! Step 2 Forces! Step 3 Newton s 2 nd! F Net = m a 1) - m 1 g = 0 2) - m 2 g = 0 +y 1 m 1 g -y 2 +y m 2 g -y 1 2 Physics 101: Lecture 2, Pg 8

9 Another Force Example: Springs Force exerted by a spring is directly proportional to its displacement x (stretched or compressed). F spring = -k x Example: When a 5 kg mass is suspended from a spring, the spring stretches x 1 = 8 cm. If it is hung by two identical springs, they will stretch x 2 = A) 4 cm B) 8 cm C) 16 cm S 1 - W = 0 S 1 = W kx 1 = mg 1 Spring 2 Springs k = mg/x 1 = N/m S 1 + S 2 - W = 0 kx 2 + kx 2 = 2kx 2 = W = mg x 2 = mg/(2k) = (5kg)*(9.8m/s 2 )/ (2*612.5N) So: x 2 = 4 cm. S 1 +y Physics 101: Lecture 2, Pg 9 S 2 W -y

10 2 Dimensional Equilibrium! Calculate force of hand to keep a book sliding at constant speed (i.e. a = 0), if the mass of the book is 1 Kg, m s =.84 and m k =.75 We do exactly the same thing as before, except in both x and y directions! +y Step 1 Draw! Step 2 Forces! Step 3 Newton s 2 nd (F Net = ma)! reat x and y independently! Physics friction Normal -x +x W -y Hand F Net, y = N W = ma y = 0 F Net, x = H f = ma x = 0 his is what we want! Physics 101: Lecture 2, Pg 10

11 Calculate force of hand to keep the book sliding at a constant speed (i.e. a = 0), if the mass of the book is 1 Kg, m s =.84 and m k =.75. F Net, y = N W = 0 F Net, x = H f = 0 N = W H = f Magnitude of frictional force is proportional to the normal force and always opposes motion! f kinetic = m k N f static m s N m k coefficient of Kinetic (sliding) friction m s coefficient of Static (stationary) friction H = f = m k N = m k W = m k mg = (0.75)*(1 kg)*(9.8 m/s 2 ) H = 7.35 N Physics 101: Lecture 2, Pg 11

12 Forces in 2 Dimensions: Ramp Calculate tension in the rope necessary to keep the 5 kg block from sliding down a frictionless incline of 20 degrees. Step 1 - Draw! You should draw axes parallel and perpendicular to motion! Step 2 - Forces! N q W Weight is not in x or y direction! Need to DECOMPOSE it! Physics 101: Lecture 2, Pg 12

13 Vector Decomposition q W y N q W W x Now: Step 3 Newton s 2 nd! q Using rig: W y q W x Note that W Split W into COMPONENS parallel to axes W Wy Wx W x = W sin q W y = W cos q Physics 101: Lecture 2, Pg 13

14 Calculate force necessary to keep the 5 kg block from sliding down a frictionless incline of 20 degrees. N W x Now: Step 3 Newton s 2 nd! x direction: W y W x = W sin q W y = W cos q F net, x = ma x System is in equilibrium (a = 0)! F net, x = 0 W x - = 0 q = W x = W sin q mg sin q = (5kg)(9.8m/s 2 ) sin(20 o ) = 16.8 N Physics 101: Lecture 2, Pg 14

15 Normal Force AC What is the normal force of the ramp on the block? A) N > mg B) N = mg C) N < mg W x = W sin q W y = W cos q N W x y direction: F net, y = ma y Equilibrium (a = 0)! W y F net, y = 0 N - W y = 0 q Physics 101: Lecture 2, Pg 15

16 Summary Contact Force: ension Force parallel to string Always Pulls, tension equal everywhere Contact Force: Spring Can push or pull, force proportional to displacement F = k x Contact Force: Friction Static and kinetic Magnitude of frictional force is proportional to N wo Dimensional Examples Choose coordinate system; choose wisely! Analyze each direction independently Physics 101: Lecture 2, Pg 16

17 Force at Angle Example A person is pushing a 15 kg block across a floor with m k = 0.4 at a constant speed. If she is pushing down at an angle of 25 degrees, what is the magnitude of her force on the block? x- direction: F Net, x = ma x P x f = P cos(q) f = 0 P cos(q) m N = 0 N = P cos(q) / m y- direction: F Net, y = ma y N W P y = N W P sin(q) = 0 N mg P sin(q) = 0 q Combine: (P cos(q) / m) mg P sin(q) = 0 P ( cos(q) / m - sin(q)) = mg P = m g / ( cos(q)/m sin(q)) P = 80 N Pushing Friction q Normal Weight y x Physics 101: Lecture 2, Pg 17

18 ension Example: Determine the force exerted by the hand to suspend the 45 kg mass as shown in the picture. A) 220 N B) 440 N C) 660 N D) 880 N E) 1100 N y F Net = m a + W = 0 W x Remember the magnitude of the tension is the same everywhere along the rope! Physics 101: Lecture 2, Pg 18

19 ension AC II Determine the force exerted by the ceiling to suspend pulley holding the 45 kg mass as shown in the picture. y A) 220 N B) 440 N C) 660 N D) 880 N E) 1100 N F c x SF = m a F c - - = 0 Remember the magnitude of the tension is the same everywhere along the rope! Physics 101: Lecture 2, Pg 19

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