Name: Date: 1/16 Period: Unit 3 Newton s 3 Laws Essential Questions: How do forces affect motion? What can you conclude about net force on an object when you don t observe it accelerate? When a mosquito collides with a windshield, which feels more force the mosquito or the windshield? Objectives Explain Newton s 3 Laws of Motion. Cite observed evidence for each law of motion. Apply Newton s 3 Laws of Motion to real-life situations. Slide 2 1
Newton s 1 st Law: When F net = 0 When no external net force acts on an object. this object will either stay at rest or continue moving at constant velocity. F T Puck sliding on ice (almost frictionless) F N v F net = 0 F net = 0 Slide 3 Newton s 1 st Law: What does it mean? When no external net force acts on an object. No net force is caused by another object this object will either stay at rest or continue moving at constant velocity. At rest F T Constant velocity (speed and direction) v F N Slide 4 2
Newton s 1 st Law: True in reverse too! When an object either stays at rest or continues moving at constant velocity If we don t see a change in velocity v No acceleration a. no external net force acts on this object. Then F net =0 Slide 5 Newton s 1 st Law is also called Law of Inertia Inertia means Resistance to change in velocity OR Resistance to acceleration Inertia is the same as mass in kg or g Explain the motion of the crash dummy s head after the crash using Newton s 1 st Law Slide 6 3
Question 1 A B If Abe pushes the box with the applied force shown in (A), and then reduces the applied force to the amount shown in (B), what can you conclude about the motion of the box? A. The box will gradually slow down to a stop B. The box will immediately stop moving C. The box will keep moving at a constant speed D. There is not enough information to decide Slide 7 Question 2 Inertia Compare a 2-kg iron brick to a 1-kg iron brick. Explain each answer. Does the 2-kg brick have Twice the inertia? Twice the mass? Twice the volume? (Hint: Think about density!) Twice the weight? (Hint: Think about Fg = mg!) Slide 8 4
Newton s 1 st Law What happens if Net Force is removed? Which path would a marble follow after exiting the circular track? Why do water drops come out of the washer in this pattern during spin cycle? A girl quickly pulls the wagon forward. Which way does the box fall? Explain. A car turns and hits a patch of ice. Draw the path of the car on the ice. Explain. Slide 9 Newton s 2 nd Law When F net is not 0 Newton s 2 nd Law is an equation Net Force F Net ma Mass Acceleration Find this equation on your MCAS formula sheet! Does it look the same? Also written like this, solving for a: a F Net m Slide 10 5
Newton s 2 nd Law What does it mean? Take notes to explain the key ideas of the simulation Slide 11 Question 2 What is the acceleration of a 2-kg mass, given the forces below? Free-Body Net Force Diagram 5N F net = F net 5 N 5N 10 N a F Net m Slide 12 6
Question 3 What is Net Force necessary to acceleration a 2-kg mass by 5 m/s 2? Net Force F net =? net FNet ma Slide 13 How can you test for evidence to support Newton s 1 st Law using PhET Acceleration? If F net = 0, a moving object continues moving at constant velocity (a = 0). 1. Turn off friction. 2. Apply a force to start an object moving, and let go. F net = 0 when you let go. 3. Observe if the object continues moving at constant velocity Constant speed Straight line (constant direction) Slide 14 7
How can you test for evidence to support Newton s 2 nd Law using PhET Acceleration? If F net is not 0, the object accelerates: 1. Turn off friction. a F Net 2. Apply a force of 50N, measure the acceleration. Double the original force, see if the acceleration doubles. Triple the original force, see if the acceleration triples. 3. Apply a constant force of 100 N to a crate, measure the acceleration. Double the original mass, see if the acceleration is ½ of original value. Triple the original mass, see if the acceleration is 1/3 of original value. m Net Net Slide 15 Newton s 3 rd Law Law of Action and Reaction for 2 objects Newton s 3 rd Law of Motion describes the forces two objects exert on each other: If you and your partner connect two spring scales and pull, what do both scales always read? Slide 16 8
Newton s 3 rd Law Action-Reaction forces do not cancel out Draw an FBD for A, and a separate FBD for B. Assume both stand on frictionless ice. Free-Body Diagram on A F N FN Free-Body Diagram on B FB on A FA on B A pushes on B with an equal and opposite force as B pushes on A Slide 17 Newton s 3 rd Law Action-Reaction forces do not cancel out Draw an FBD for A, and a separate FBD for B. Assume both stand on frictionless roller skates. Why does A have a greater speed than B after they push off? A B A B Slide 18 9
Newton s 3 rd Law Equal force on each mass; Smaller mass accelerates more A B A B Free-Body Diagram on A F N FN Free-Body Diagram on B FB on A FA on B A pushes on B with an equal and opposite force as B pushes on A Slide 19 Newton s 1 st Law What happens if Net Force is removed? Earth exerts a force on the apple. Does the apple exert a force on the Earth? Does the Earth accelerate toward the apple? A bug collides with a car windshield. Which feels a greater force, the bug or the car? Which feels a greater acceleration? A swimmer pushes back on the water. Which force pushes forward on the swimmer? Explain. Which force causes the jumper to accelerate up from the ground? Explain. Slide 20 10