Department of Physics and Applied Physics , F2010, Lecture 7 Physics I LECTURE 7 9/27/10

Department of Physics and Applied Physics , F2010, Lecture 7 Exam Prep Problem A 2kg mass sits on a frictionless table, travelling with an initial velocity at t=0. A constant Force of is applied to this mass at t=0. –A) (5pts) What is the acceleration of the mass for t>0s? –B) (5pts) At what time does the y component of velocity go to 0? –C) (5pts) At what time does the x component of velocity go to 0? –D) (10pts) What is the displacement of the mass after 5s?

Department of Physics and Applied Physics , F2010, Lecture 7 Exam Prep Problem A 2kg mass sits on a frictionless table, travelling with an initial velocity at t=0. A constant Force of is applied to this mass at t=0. –A) (5pts) What is the acceleration of the mass for t>0s?

Department of Physics and Applied Physics , F2010, Lecture 7 Exam Prep Problem A 2kg mass sits on a frictionless table, travelling with an initial velocity at t=0. A constant Force of is applied to this mass at t=0. –B) (5pts) At what time does the y component of velocity go to 0?

Department of Physics and Applied Physics , F2010, Lecture 7 Exam Prep Problem A 2kg mass sits on a frictionless table, travelling with an initial velocity at t=0. A constant Force of is applied to this mass at t=0. –C) (5pts) At what time does the x component of velocity go to 0?

Department of Physics and Applied Physics , F2010, Lecture 7 Exam Prep Problem A 2kg mass sits on a frictionless table, travelling with an initial velocity at t=0. A constant Force of is applied to this mass at t=0. –D) (10pts) What is the displacement of the mass after 5s?

Department of Physics and Applied Physics , F2010, Lecture 7 Force Review Problem Superman must stop a 4x10 5 kg train traveling at 80m/s in 320m. What Force must he exert to do this?

Department of Physics and Applied Physics , F2010, Lecture 7 Outline Force of Gravity and Normal Force Free Body Diagrams Problem Solving What do we know? –Units –Kinematic equations –Freely falling objects –Vectors –Kinematics + Vectors = Vector Kinematics –Relative motion –Projectile motion –Newton’s Laws

Department of Physics and Applied Physics , F2010, Lecture 7 We know that, ignoring air resistance, all objects dropped near the surface of the Earth will accelerate with Force of Gravity If something is accelerating, that means there must be a force on it. In this case, that Force is the gravitational force. Magnitude of this force is known as weight.

Department of Physics and Applied Physics , F2010, Lecture 7 Example Problem I(a) A upwards Force is exerted on a mass of 2kg. What is the acceleration of the mass if the Force is A) 10N? 2kg

Department of Physics and Applied Physics , F2010, Lecture 7 Example Problem I(b) A upwards Force is exerted on a mass of 2kg. What is the acceleration of the mass if the Force is B) 19.6N? 2kg

Department of Physics and Applied Physics , F2010, Lecture 7 Example Problem I(c) A upwards Force is exerted on a mass of 2kg. What is the acceleration of the mass if the Force is C) 40N? 2kg

Department of Physics and Applied Physics , F2010, Lecture 7 Force of Gravity This force is always acting on objects at the surface of the Earth. Why don’t we see everything accelerating downwards towards the center of the Earth? There is another force acting on the object. And since the acceleration of the resting object is 0, this force must be equal to the Force of gravity.

Department of Physics and Applied Physics , F2010, Lecture 7 The Normal Force The Normal Force acts perpendicular to the surface exerting it. Normal=perpendicular. The Normal Force is NOT always vertical. It is perpendicular to the surface. If the surface is not horizontal, the Normal Force isn’t vertical.

Department of Physics and Applied Physics , F2010, Lecture 7 Example II(a) For a mass M at rest on a flat surface, we can determine the Normal Force for 3 different situations: A) Box on flat surface

Department of Physics and Applied Physics , F2010, Lecture 7 Example II(b) For a mass M at rest on a flat surface, we can determine the Normal Force for 3 different situations: B) Box on flat surface, hand pushing down -F H

Department of Physics and Applied Physics , F2010, Lecture 7 Example II(c) For a mass M at rest on a flat surface, we can determine the Normal Force for 3 different situations: C) Box on flat surface, hand pulling up +F H <mg

Department of Physics and Applied Physics , F2010, Lecture 7 Solving Force Problems FREE BODY DIAGRAMS –If we are interested in the motion of an object, then we need to know the NET FORCE acting on that object!!

Department of Physics and Applied Physics , F2010, Lecture 7 My kids are fighting over a 0.5kg toy resting on a table (xy plane). My son pulls with a force: and my daughter with a force of: Find the net Force on the object, and its acceleration. Example Problem III Draw Free Body Diagram Find Net Force Find Acceleration

Department of Physics and Applied Physics , F2010, Lecture 7 Comments on Example Problem III What happened to the Normal Force and the Force of gravity in this problem? What about friction, air resistance, etc.?

Department of Physics and Applied Physics , F2010, Lecture 7 Example IV Two boxes are held together by a cable and pulled by a string on a frictionless table. A) Find acceleration of each box B) Tension in cord

Department of Physics and Applied Physics , F2010, Lecture 7 Example IV Choose coordinate system Free Body Diagrams for each box

Department of Physics and Applied Physics , F2010, Lecture 7 Example IV How do we handle the connecting (massless) cord?

Department of Physics and Applied Physics , F2010, Lecture 7 Example IV Sum Forces

Department of Physics and Applied Physics , F2010, Lecture 7 Example IV Solve system of equations

Department of Physics and Applied Physics , F2010, Lecture 7 Free Body Diagrams For any object subject to a Force, we can always draw a free body diagram. When we choose coordinate system, we want to choose carefully…

Department of Physics and Applied Physics , F2010, Lecture 7 Boxes on Inclines Now suppose we have a box of mass M on a frictionless inclined plane… –Coordinate system θ

Department of Physics and Applied Physics , F2010, Lecture 7 Boxes on Inclines Now suppose we have a box of mass M on a frictionless inclined plane… –Free Body Diagram θ

Department of Physics and Applied Physics , F2010, Lecture 7 Boxes on Inclines Divide Forces into x, y components θ Φ

Department of Physics and Applied Physics , F2010, Lecture 7 Boxes on Inclines What is Φ?

Department of Physics and Applied Physics , F2010, Lecture 7 Boxes on Inclines Divide Forces into x, y components θ θ

Department of Physics and Applied Physics , F2010, Lecture 7 Example V A 10 kg box sits on a frictionless inclined plane (θ=30º). –What is the Normal Force on the box? –What is the box’s acceleration? Draw diagram and choose coordinate system

Department of Physics and Applied Physics , F2010, Lecture 7 Example V A 10 kg box sits on a frictionless inclined plane (θ=30º) Free body diagram Divide Forces into x, y

Department of Physics and Applied Physics , F2010, Lecture 7 Example V A 10 kg box sits on a frictionless inclined plane (θ=30º) Solve for acceleration

Department of Physics and Applied Physics , F2010, Lecture 7 Problem Solving approach Draw Diagram Choose coordinate system Draw Free Body Diagram Determine Force components Write equations of motion Solve