# Steps to Solving Newtons Laws Problems.

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1 Mathematical Analysis With Newtons Laws similar to projectiles (x y) isolation Steps to Solving Newtons Laws Problems. 1) FBD 2) Axis 3) Components 4) Fnet (x) (y) 5) Subs 1

2 Visual Samples F 4 1) F 3 F 1 F 2 F 5 F 6 2) A 20 N force is applied at an angle of 30 degrees below the horizontal to a box moving on a horizontal surface 2

3 1 A man pulls a 10 kg box with a horizontal force of 150 N. 50 N of friction act on the box. Determine the acceleration and the normal force 3

4 2 One Saturday evening you are out at a physics study session and your friend passes out from too much physics fun. You tie a rope to your 98 N friend and pull him accelerating at 0.7 m/s 2. You pull with a force of 10 N at 30 degrees up from the horizontal and your friend is dragged horizontally across the floor a) Determine the friction force acting b) Determine the normal force acting on him fk = 1.66 N Fn = 93 N 4

5 3 On Halloween, Wells slides a 981 N box of dynamite across your horizontal driveway with an initial speed of 10 m/s towards your house. The box slides 4 m to a stop and explodes destroying all of your favorite toys. Determine the acceleration and friction force acting on the box as wells laughs watching the box slide to annihilate your crappy toys. v i = 10 m/s v f = 0 m/s d = 4 m a = 12.5 m/s 2 fk = 1250 N 5

6 Workbook Newtons Laws Students Sheet Problems #1) Can an object be in equilibrium if only one force acts on it. #2) If an 800 kg car is traveling westward with a constant velocity of 20 m/s, what is the net external force acting on it? #3) A 190 gram dart is launched horizontally from a dart gun and has 2.1 N of friction acting on it while moving in the gun barrel. The acceleration of the dart is 3.2 m/s 2. Determine the average force of the gun pushing the dart out of the barrel. 2.7 N 6

7 #4) A 5 kg sack of potatoes slides across a flat floor with an initial velocity of 10 m/s right. It comes to a stop in 2 seconds. (a) What is the acceleration of the sack (b) What is the normal force (c) How much friction force acts on the sack a = 5 m/s 2 Fn = N fk = 25 N 7

8 #5) A 490 N box is pushed constantly by a 100N force directed at 25 degrees down from the horizontal and moves towards the right on a flat surface. It moves at a constant speed of 2 m/s. (a) What is the normal force (b) How much friction force acts on the box a) Fn = N b) fk = 90.6 N 8

9 #6) A 1000 kg elevator is lifted vertically by a force of N N of friction impede the motion of the elevator. What is the acceleration of the elevator? 1.19 m/s 2 9

10 #7) A N sled is pulled by a force of degrees above the horizontal and slides across the snow with 20 N of friction acting on it. (a) Determine the acceleration of the sled (b) Determine the normal force acting on the sled (c) How much force in a horizontal direction would need to be applied to make the sled move at a constant speed. a) 1.5 m/s 2 b) 197 N c) 30 N 10

11 Coefficient of Friction used as another way to find friction force Roughness Physics definition Variable / Unit Reference Table values Formula 11

12 More on Coeff of Friction How to use it A 100N box is sliding on a variety of surfaces, how much friction force do you get in each example? Constant nature of u / Roughness Static vs. Kinetic 12

13 Friction Problems 2 ways to find the friction force acting 1 A man pushes a 20 kg box with a force of 200N and it accelerates at 2 m/s 2. What is the coefficient of kinetic friction of the two surfaces?

14 2 75 kg joe bob runs on the floor and trips on a banana and goes sliding into a pile of dirt. The coefficient of kinetic friction is a) What is the friction force acting on the student b) What is the acceleration of the student fk = N a = 3.92 m/s 2 14

15 Workbook Newtons Laws Student Sheet Problem #8) A 675 N box is pulled with a 250 N force directed 30 deg above horizontal. The box moves at a constant speed. What is the coefficient of kinetic friction of these surfaces. fk = N Fn = 550 N u =

16 Example A 10 kg box is pushed with 200 N of force and just barely moves. It is then pushed with 150 N of force and moves at a constant speed. Determine u s & u k us = 2.04 uk =

17 Workbook Newtons Laws Student Sheet #9.) A 49 N box is pushed with a horizontal force of 30 N east on a surface with a coefficient of kinetic friction = a) Determine the friction force acting on the box b ) Determine the acceleration of the box? new a) fk = N b) a = 3.56 m/s 2 #10.) A 250 kg wood bed is on a wood floor. (a) How much horizontal force would need to be applied to start moving it? (b) How much force would need to be applied to move it at a constant speed? fs = 1030 N fk = 735 N 17

18 String Tension Notes Newtons Laws Student Sheet #16). Find the Tension in the ropes shown below that support a 1000 kg sign 18

19 Inclines Determine the magnitude of 'Fn' and also the acceleration of the box y m = 10 kg F n 20 N F 1 θ 60 F N x 19

20 Incline Notes 20

21 Newtons Laws Student Sheet 7 1.) Draw the x and y components of the weight for the inclines below 2.) Based on your drawings above, answer the following What happens to the Normal Force acting on a box when you increase the incline? Assuming that the box does not move, what happens to the friction force when you increase the incline? Which component of the weight causes the box to accelerate? (a) At which location is the component parallel to the ramp the largest (b) At which location is the acceleration the greatest (c) At which location is the acceleration the least (d) Is the speed of the block the greatest where the acceleration is the greatest 21

22 Finding Fgx and Fgy y m = 10 kg F 20 N F 1 θ 60 F N x 22

23 Incline Sample Problems a 2 kg steel box slides down a steel 30 degree incline. Determine the friction force acting on the box and the acceleration of the box on the incline Fgx = 17 Fgy = 9.81 fk = 5.59 M+N a = 5.7 m/s 2 23

24 Workbook Newtons Laws Student Sheet #1) A 20 kg box rests on a 15 degree incline and does not move. and the coefficient of static friction on the incline? 24

25 2) A 1 kg box slides down a 30 degree incline and accelerates at 1.2 m/s 2. Determine the friction force acting on the box and the coefficient of kinetic friction on this incline? new Fgy = 8.5 N Fgx = 4.9 N fk = 3.7 N u =

26 3) A 98.1 N box is pushed up a 60 degree incline with a force of N directed parallel to the incline surface. The coefficient of kinetic friction between the box and the ramp is 0.5. If the box starts from rest, how fast will it be moving when it reaches the top of the 5 m long incline? Determine the acceleration of the box new fgx = fgy = 49 fk = 24.5 N a = 5 m/s 2 26

27 Elevator Problems very brief 27

28 Workbook REGENTS PRACTICE 7 1.) A bird feeder with two birds has a total mass of 2.0 kilograms and is supported by wire as shown in the diagram. The force in the top wire is approximately 1) 10. N 2) 14 N 3) 20. N 4) 39 N 2.) A 50. kilogram woman wearing a seatbelt is traveling in a car that is moving with a velocity of +10. meters per second. In an emergency, the car is brought to a stop in 0.50 second. What force does the seatbelt exert on the woman so that she remains in her seat? 1) N 2) N 3) N 4) N 3.) In the diagram, as angle between the lawnmower handle and the horizontal increases, the horizontal component of F 1) decreases 2) increases 3) remains the same 28

29 4) A box initially at rest on a level floor is being acted on by a variable horizontal force, as shown in the diagram. Compared to the force required to start the box moving, the force required to keep it moving is 1) less 2) greater 3) the same 5) The table lists the coefficients of kinetic friction for four materials sliding over steel. A 10. kilogram block of each of the materials in the table is pulled horizontally across a steel floor at constant velocity. Which block would require the smallest applied force to keep it moving at constant velocity? 1) aluminum 2) brass 3) copper 4) steel 6) A cart rolls down an inclined place with constant speed as shown in the diagram. Which arrow represents the direction of the frictional force? 1) A 2) B 3) C 4) D 29

30 7) The diagram represents a 10. newton block sliding down a 30. incline at a constant speed. The force of friction on the block is approximately 1) 5.0 N 2) 10. N 3) 49 N 4) 98 N 8) Two forces are applied to a 2.0 kilogram block on a frictionless, horizontal surface, as shown in the diagram. The acceleration of the block is 1) 5.0 m/s2 to the right 2) 5.0 m/s2 to the left 3) 3.0 m/s2 to the right 4) 3.0 m/s2 to the left 9) A horizontal force is used to pull a 5.0 kilogram cart at a constant speed of 5.0 meters per second across the floor, as shown in the diagram. If the force of friction between the cart and the floor is 10. newtons, the magnitude of the horizontal force along the handle of the cart is 1) 5.0 N 2) 10. N 3) 25 N 4) 50. N 30

31 10.) In the diagram, surface B of the wooden block has the same texture as surface A, but twice the area of surface A. If force F is required to slide the block at constant speed across the table on surface A, approximately what force is required to slide the block at constant speed across the table on surface B? 1) F 2) 2F 3) ½ F 4) 4F 11.) A man weighs 900 newtons standing on a scale in a stationary elevator. If some time later the reading on the scale is 1200 newtons, the elevator must be moving with 1) constant acceleration downward 2) constant speed downward 3) constant acceleration upward 4) constant speed upward 12.) A book weighing 20. newtons slides at a constant velocity down a ramp inclined at 30. to the horizontal as shown in the diagram. What is the force of friction between the book and the ramp? 1) 10. N up the ramp 2) 17 N up the ramp 3) 10. N down the ramp 4) 17 N down the ramp 31

32 13.) The diagram shows a compressed spring between two carts initially at rest on a horizontal frictionless surface. Cart A has a mass of 2 kilograms and cart B has a mass of 1 kilogram. A string holds the carts together. What occurs when the string is cut and the carts move apart? 1) The magnitude of the acceleration of cart A is one half the magnitude of the acceleration of cart B. 2) The length of time that the force acts on cart A is twice the length of time the force acts on cart B. 3) The magnitude of the force exerted on cart A is one half the magnitude of the force exerted on cart B. 4) The magnitude of the impulse applied to cart A is twice the magnitude of the impulse applied to cart B. 14.) When a 12 newton horizontal force is applied to a box on a horizontal tabletop, the box remains at rest. The force of static friction acting on the box is 1) 0 N 2) between 0 N and 12 N 3) 12 N 4) greater than 12 N 15.) The diagram below shows a newton truck at rest on a hill that makes an angle of 8.0 with the horizontal. What is the component of the truck s weight parallel to the hill? (1) N (3) N (2) N (4) N 16.) A block weighing 10.0 newtons is on a ramp inclined at 30.0 to the horizontal. A 3.0 newton force of friction, F f, acts on the block as it is pulled up the ramp at constant velocity with force F, which is parallel to the ramp, as shown in the diagram below. What is the magnitude of force F? (1) 7.0 N (3) 10. N (2) 8.0 N (4) 13 N 32

33 The End 33

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