Dynamics Notes.notebook October 22, 2018

Transcription

1 Dynamics 1

2 Kinematics Description of how objects move (what we just finished) Dynamics Description of why objects move Connection between force and motion Newton's Laws of Motion Force something capable of changing an objects state of motion 1. At rest 2. Moving with a constant velocity Net force (F Net ) determines if there is a change in motion F Net Result from adding all the forces Types of Forces Action at a Distance Contact Forces (Not physically touching) Applied Gravity Frictional Normal Tension Spring Electrical Magnetic Strong Nuclear Force Weak Nuclear Force Air Resistance 2

3 Inertia resistance an object has to a change in its state of motion also known as mass Mass (Amount of Stuff) Weight (Force of gravity) Metric English On Earth 2.2 lb = 1 kg (not true on planets with different acceleration due to gravity) Newton's First Law of Motion (Law of Inertia) In the absence of an unbalanced force a body at rest remains at rest a body in motion remains in motion with the same velocity The greater the Inertia the greater resistance to change Forces are balanced a = 0 m/s 2 Object at Rest v = 0 m/s Object in Motion v 0 Stay at Rest Stay in Motion with same magnitude and direction Examples Ball going in circle (Demo) Rock in outer space Groceries in back seat Pulling sheet out under dishes eywquog 3

4 Newton's 2nd Law of Motion the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force. units F Net = ma acceleration Net Force F Net = 6 N 10 kg acceleration =? 4

5 Weight force when acceleration equals g (acceleration due to gravity) F = ma Weight = mg Doc has a mass of 100 kg. What is his weight (N) on Earth? On the moon his weight is 170 N. What is value of g on the moon? Newton's 3rd Law Whenever an object exerts a force on a second object, the second object exerts an equal force in the opposite direction. or For every action there is an equal and opposite reaction. Examples jump in the air shoot bullet force Plate (Demo) YOjiOw 5

6 Determining F Net 10 N F Net (x) = F Net (y) = 10 N F Net (x) = F Net (y) = 15 N 15 N 10 N 5 N F Net (x) = F Net (y) = 6

7 Force Diagrams A 10.0 kg box is sitting on a table. 1. Draw a force diagram showing all forces. 2. Determine Fnet in each direction and calculate the magnitude of each of these forces. 3. Calculate the acceleration of the box. Recall Newton's Second Law F Net = ma (x) (y) Weight = force of gravity (F g ) Weight = mg F g = mg 7

8 Force Diagrams A 15.0 kg box is sitting on a table and now a 150 N vertically downward force is applied. 1. Draw a force diagram showing all forces. 2. Determine the Fnet in each direction (x) (y) 3. Calculate the magnitude of each of these forces. 4. Calculate the acceleration of the box. A 15.0 kg box is sitting on a table and now a 150 N horizontal force is applied. 1. Draw a force diagram showing all forces. 2. Determine the Fnet in each direction (x) (y) 3. Calculate the magnitude of each of these forces. 4. Calculate the acceleration of the box. 8

9 Bellwork 4/24 (This is page 9 of your notes!!) A 25kg box is sitting on a table. A vertical downward force of 120 N is applied. 1. Draw in arrows showing all the forces acting on this box. 2. Determine Fnet in each direction (x) (y) 3. Calculate the magnitude of each of these forces. 4. Calculate the acceleration of the box. 9

10 Force Diagrams A 15.0 kg box is sitting on a table and a 150 N horizontal force is pushed from the left side and a 30 N horizontal force is pushed on the right side. 1. Draw a force diagram showing all forces. 2. Determine Fnet in each direction (x) (y) 3. Calculate the magnitude of each of these forces. 4. Calculate the acceleration of the box. 10

11 Frictional Force (F f ) resistance to motion due to microscopic roughness. depends on interaction between the two surfaces (μ) is always parallel to surface proportional to normal force (changes with mass) Static Frictional Force (F fs ) frictional force when object is not moving value can change depending on applied force F fs = μsf n μs coefficient of static friction Kinetic Frictional Force (F fk ) frictional force when object is moving value is always the same regardless of applied force F fk = μkf n μk coefficient of kinetic friction μs > μk 11

12 F a 10.0 kg μs = 0.40 μk = Draw forces on box. 2. Write equations in the x and y direction. 3. Determine the Weight of the box. 4. Determine the Normal Force. 5. Determine the maximum Static Frictional Force. 6. Determine the Kinetic Frictional Force. 12

13 10.0 kg μs = 0.40 F a μk = 0.30 F a = 0 N F f = a = F a = 10 N F f = a = F a = 20 N F f = a = F a = 30 N F f = a = F a = 40 N F f = a = F a = 50 N F f = a = F a = 60 N F f = a = Static Friction Regime Kinetic Friction Regime F fs(max) = μsf n Frictional Force Threshold of motion F fk = μkf n Applied Force 13

14 A 10.0 kg box is sitting on a table and a 30 N force is applied horizontally to the right on the left side of the box. The coefficients of static and kinetic friction are 0.15 and 0.10 respectively. 1. Draw all forces acting on the box. 2. Write the force equations in the x and y direction. 3. Determine the normal force. 4. Does the box move? (Find Ffs) If so what is the value of the acceleration? (Find Ffk & then acceleration) 14