WARM UP: 1. Calculate the acceleration of a bag of softball gear weighing 50 N if pulled with a force of 25 N. 2. Calculate the force of a cars tires on the road if the car has a mass of 2000 kg and is traveling at a constant velocity of 60 mph north Quiz TOMORROW! Free Body Diagrams and Net Force Terms used with Forces Net Force: the sum of all forces acting on an object. F Expressed in N. https://www.youtube.com/watch?v=YyJSlcIbd-s Terms used with Forces Balanced Forces net force on an object is zero, the object is in equilibriumthe forces are balanced. The object is either: 1. Not moving 2. Moving at a constant velocity Terms used with Forces UNBalanced Forces Whenever the forces do not cancel each other out. The object is ACCELERATING: 1. changing direction 2. changing speed Free Body Diagrams Show all forces acting on one object Think only about the objects acceleration Length of vector matters Be careful with constant velocity problems F grav On every diagram F normal supporting F applied F friction Always opposes motion direction of movement F tension Hanging objects F air Falling terminal velocity - gliding A physics book rests upon a level table. F grav F norm A physics book is pushed across the desk with a rightward acceleration. But does not move F grav F norm F applied F friction A physics book is pushed across the desk with a rightward acceleration. F grav F norm F applied F friction A physics book is pushed across the desk and moves with a constant velocity. F grav F norm F friction F applied A ball is moving upward after being kicked F grav An egg is free falling from a nest. F grav A car hits the brakes and is coasting F friction F grav F norm An elevator is rising at a constant velocity. F tens F grav Determining Net Force: Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. If a = 1.5 m/s 2 determine the net force and the mass of the car. Unfortunately for Vanessa, the wheels on her suitcase are not working. She pulls on the strap in an effort to budge it from rest and drag it to the curbside check-in desk. Use force values to determine the net force, the mass and the acceleration of the suitcase. The values of the individual forces are: F grav = F norm = 207 N F tens = 182 N F frict = 166 N. Which one(s) of the following force diagrams depict an object moving to the right with a constant speed? List all that apply. Newtons 1 st law He is providing a pushing force. Constant velocity Newtons 1 st law There is an equal and opposite friction force. Constant velocity Applied force friction Remember when the forces are balanced (net force is zero) he travels at constant velocity. Constant velocity Applied force friction Newtons 2nd law Now lets imagine what happens if he pedals faster. Applied force friction Newtons 2nd law His velocity changes (goes faster). He accelerates! Applied force friction acceleration Remember from that acceleration is rate of change of velocity. In other words acceleration = (change in velocity)/time Newtons 2nd law Now imagine what happens if he stops pedalling. friction Newtons 2nd law He slows down (decellerates). This is a negative acceleration. But he is still moving forward. friction Newtons 2nd law So when the net force is greater than zero, an object accelerates (changes velocity) applied force friction Mrs. Rices Porsche An example We can use the diagram to determine his acceleration? Applied force (100 N) Friction (60 N) Mass of cyclist and bike = 100 kg An example Net force = 100 60 = 40 N F NET = ma 40 = 100a a = 0.4 m/s 2 Pushing force (100 N) Friction (60 N) Mass of cyclist and bike = 100 kg