# University Physics 226N/231N Old Dominion University. Newton s Laws and Forces Examples

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1 University Physics 226N/231N Old Dominion University Newton s Laws and Forces Examples Dr. Todd Satogata (ODU/Jefferson Lab) Wednesday, September Happy Birthday to Ivan Pavlov, Serena Williams, Olivia Newton-John, and Johnny Appleseed! Happy Banned Books Day, Love Note Day, and Street Dancing Day Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 1

2 Midterm Solutions Posted I ve graded midterms, distributing before and after class Class average was about 80 with a few outliers Please see me if you scored less than about Solutions and full statistics are posted on class website If you need a make-up, me separately for arrangements Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 2

3 Reminders Newton s three laws really boil down to two The net sum of all forces on an object produces acceleration F net = ma Any force on an object is always matched by an equal and opposite force on the object applying the force Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 3

4 Newton s Third Law Forces always come in pairs. If object A exerts a force on object B, then object B exerts an oppositely directed force of equal magnitude on A. Obsolete language: For every action there is an equal but opposite reaction. The two forces always act on different objects; they can t cancel each other. Example: Push on book of mass m 1 with force Note third-law pair F 12 and Third law is necessary for a consistent description of motion in Newtonian physics. F 21 F Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 4

5 Ponderable (10 minutes) ŷ ˆx F =5N 2kg 3kg 3kg box A box B box C Draw and label all the forces on all three boxes above Assume boxes A and B are touching Assume there is no friction with the table top Assume the force is constant What are the initial accelerations of all three boxes? What is the acceleration of all three boxes after boxes A and B have hit and move with box C? Do boxes A and B slow down (reduce velocity or speed) when they hit box C? Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 5

6 Newton s Second/Third Laws: Inclined Plane ŷ ˆx F g F n θ m =5kg θ = 20 F g = mg = 49 N A small ball of m=5 kg is on an inclined plane of 20 degrees What are all the forces acting on the ball? (no friction for now!) Note that forces are vectors: they have direction and magnitude! Two forces: gravity pointing down and push of plane pointing perpendicular to the surface of the plane. What are the components of the forces? One way to look at it is with the x,y axes shown above F g,x =0 F g,y = F g F n,x = F n sin θ F n,y = F n cos θ F n Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 6 θ F n sin θ F n cos θ

7 Newton s Second/Third Laws: Inclined Plane ŷ ˆx F g F n θ m =5kg θ = 20 F g = mg = 49 N A small ball of m=5 kg is on an inclined plane of 20 degrees What are the components of the forces? Another way to look at it is with axes parallel to and perpendicular to the plane This makes the final acceleration easier to calculate we know that the net force and acceleration are down the plane F n,x =0 F n,y = F n F g,x = F g sin θ F g,y = F g cos θ F g cos θ θ F g F g sin θ Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 7

8 Newton s Second/Third Laws: Inclined Plane ŷ ˆx F g F n θ m =5kg θ = 20 F g = mg = 49 N A small ball of m=5 kg is on an inclined plane of 20 degrees What is the normal force from the plane, F n? F g cos θ F g sin θ θ F g F g,x = F g sin θ F g,y = F g cos θ F n,x =0 F n,y = F n F net,y = F n F g cos θ =0 No acceleration perpendicular to the plane F n = F g cos θ = (49 N) cos(20 )= 46 N = F n Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 8

9 Newton s Second/Third Laws: Inclined Plane ŷ ˆx F g F n θ m =5kg θ = 20 F g = mg = 49 N A small ball of m=5 kg is on an inclined plane of 20 degrees What is the acceleration of the ball down the plane? F g cos θ θ F g sin θ F g F g,x = F g sin θ F n,x =0 F g,y = F g cos θ F n,y = F n F net,x = ma x = F g,x + F n,x = F g sin θ +0=mg sin(20 ) a x = g sin(20 )=(9.8 m/s 2 )(0.342) = 3.35 m/s 2 = a x a x Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 9

10 Spring Forces A stretched or compressed spring produces a force proportional to the stretch or compression from its equilibrium configuration: F sp = kx. The spring force is a restoring force because its direction is opposite that of the stretch or compression. Springs provide convenient devices for measuring force. k is in units of N/m Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 10

11 Ponderable ŷ ˆx F s F g F n θ m =5kg θ = 20 F g = mg = 49 N A small ball of m=5 kg is on an inclined plane of 20 degrees Now we put a spring (k=2 N/m) beneath it to support it The spring compresses until the force it exerts counteracts gravity s force on the ball How far does the spring compress? If we double the mass of the ball, how does the spring compression distance change? Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 11

12 A Typical Problem: What s the skier s acceleration? What s the force the snow exerts on the skier? Physical diagram: Free-body diagram: Newton s law: In components: x-component: mg sinθ = ma y-component: n mg cosθ = 0 Solve (with m = 65 kg and θ = 32 ) to get the answers: a = g sinθ =(9.8 m/s 2 )sin32 = 5.2 m/s 2 n = mg cosθ =(65 kg)(9.8 m/s 2 )cos32 = 540 N Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 12

13 Solve problems involving multiple objects by first identifying each object and all the forces on it. Draw a free-body diagram for each. Write Newton s law for each. Identify connections between the objects, which give common terms in the Newton s law equations. Solve the algebra. Multiple Objects Newton s law: In components: climber, y: T! m c g =!m c a Solution: rock, x: T = m r a rock, y: n! m r g = 0 m a = c g m c + m r Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 13

14 Circular Motion Problems involving circular motion are no different from other Newton s law problems the geometry is just more complicated. Identify the forces, draw a free-body diagram, write Newton s law. The magnitude of the centripetal force on an object of mass m in circular motion with radius r is the acceleration has magnitude v 2 /r and points toward the Newton s law: center of the circle. A ball whirling on a string: Free-body diagram: v = TL cos2! m In components: x :!!!T cos! = mv2 L cos! y :!!!T sin! " mg = 0 Solve for the ball s speed: = (mg / sin!)l cos2! m Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 14 = gl cos2! sin!

15 Loop-the-Loop! The two forces acting on the roller-coaster car are: gravity normal force Gravity is always downward, and the normal force is perpendicular to the track. At the position shown, the two forces are at right angles: The normal force acts perpendicular to the car s path, keeping its direction of motion changing. Gravity acts opposite the car s velocity, slowing the car. The net force is not toward the center Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 15

16 Loop-the-Loop! At the top of the loop, both forces are downward: Solving for v, we obtain For the car to stay in contact with the track, the normal force must be greater than zero. So the minimum speed is the speed that let the normal force get arbitrarily close to zero at the top of the loop. Then gravity alone provides the force that keeps the car in circular motion. Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 16

17 Loop-the-Loop! Therefore Newton s law has a single component, with the gravitational force mg providing the acceleration v 2 /r that holds the car in its circular path: Solving for the minimum speed at the loop top gives v = gr. Slower than this at the top, and the car will leave the track! Since this result is independent of mass, car and passengers will all remain on the track as long as v! gr. Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 17

18 Following slides for Friday Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 18

19 Friction Friction is a force that opposes the relative motion of two contacting surfaces. Static friction occurs when the surfaces aren t in motion; its magnitude is f s µ s n, where n is the normal force between the surfaces and µ s is the coefficient of static friction. Kinetic friction occurs between surfaces in motion; its magnitude is f k = µ k n. Friction is important in walking, driving and a host of other applications: Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 19

20 Solving Problems with Friction Problems with friction are like all other Newton s law problems. Identify the forces, draw a free-body diagram, write Newton s law. You ll need to relate the force components in two perpendicular directions, corresponding to the normal force and the frictional force. Example: A braking car: What s the acceleration? Newton s law: In components: x:!!!!µn = ma x y:!!!!mg + n = 0 Solve for a: y equation gives n = mg, so x equation gives a x =! µn m =!µg Prof. Satogata / Fall 2012 ODU University Physics 226N/231N 20

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