Relative Motion You are swimming across a 50m wide river in which the current moves at 1 m/s with respect to the shore. Your swimming speed is 2 m/s with respect to the water. You swim across in such a way that your path is a straight perpendicular line across the river. – How many seconds does it take you to get across? 2m/s 1m/s50m a) b) c) d)

What Causes Acceleration? Group 1 Go on the internet and find newtons laws. Physicsclassroom.com might be good. Be prepare to discuss them. Group 2 Devise an experiment to test what is important in horizontal acceleration. You have a bowling ball, and tennis ball as equipment. (don't drop the bowling ball) List the things that effect the acceleration as you push a ball. Group 3 Discuss moving the contents of your house what would you least like to move and why? Group 4 Push against the wall. Tell me all the things that are pushing and being pushed. (Their are more than you think.)

The Laws of Motion Isaac Newton ( ) published Principia Mathematica in In this work, he proposed three “laws” of motion: Law 1: An object at rest tends to stay at rest and object in motion tends to stay in motion. FFa Law 2: For any object, F NET =  F = ma FF Law 3: Forces occur in pairs: F A,B = - F B,A (For every action there is an equal and opposite reaction.)

Force We have an idea of what a force is from everyday life. Physicist must be precise. – A force is that which causes a body to accelerate. (See Newton’s Second Law) – A Force is a push or a pull. – A Force has magnitude & direction (vector). – Adding forces is like adding vectors FF1FF1 FF2FF2 a FF1FF1 FF2FF2 a F F NET Fa F NET = ma

Force … We will consider two kinds of forces Contact force: (physical contact between objects) – This is the most familiar kind. – Kicking a ball – I push on the desk. – The ground pushes on the chair... Field Forces (Non-Contact): (action through “empty” space) – Moon and Earth – Gravity – Electricity On a microscopic level, all forces are non-contact (Hun????)

Contact forces: Objects in contact exert forces. Convention: F a,b means “the force acting on a due to b”. So F head,thumb means “the force on the head due to the thumb”. F F head,thumb

An Example F B,E = - m B g EARTH F E,B = m B g Consider the forces on an object undergoing projectile motion F B,E = - m B g F E,B = m B g

Action at a distance Gravity: We’ll come back to this later. Just remember you don’t have to touch to push

Mass We have an idea of what mass is from everyday life. Physicist must be precise. – mass (for this class) is a quantity that specifies how much inertia an object has. (See Newton’s First Law) Mass is an inherent property of an object. Mass and weight are different quantities. weight is a force.

Newton’s First Law inertial reference frame An object subject to no external forces moves with a constant velocity if viewed from an inertial reference frame. – If no forces act, there is no acceleration. The above statement can be thought of as the definition of inertial reference frames. – An IRF is a reference frame that is not accelerating (or rotating) with respect to the “fixed stars”. – If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector!

Newton’s Second Law The acceleration of an object is directly proportional to the net force acting upon it. The constant of proportionality is the mass. Units  The units of force are kg m/s 2 = Newtons (N)  The English unit of force is Pounds (lbs) Ex: weight is a force W=mg

Mass vs. Weight An astronaut on Earth kicks a bowling ball and hurts his foot. A year later, the same astronaut kicks a bowling ball on the moon with the same force. His foot hurts... (a) more (b) less (c) the same Ouch!

Mass vs. Weight THE SAME!!!! If I stand on earth and hit myself in the head with a hammer it hurts. If I do the same thing in space I think you can imagine that it would still hurt. Ouch!

Mass vs. Weight Wow! That’s light However the weights of the bowling ball and the astronaut are less: Thus it would be easier for the astronaut to pick up the bowling ball on the Moon than on the Earth. W = m g Moon g Moon < g Earth

Newton’s Second Law... Components of F = ma : F X = ma X F Y = ma Y Suppose we know m and F X, we can solve for a X and apply the things we learned about kinematics over the last few weeks:

Example: Pushing a Box on Ice. A skater is pushing a heavy box (mass m = 100 kg) across a sheet of ice (horizontal & frictionless). He applies a force of 50N in the i direction. If the box starts at rest, what is its speed v after being pushed a distance d=10m ? F v = 0 m a i

Force and acceleration A force F acting on a mass m 1 results in an acceleration a 1. The same force acting on a different mass m 2 results in an acceleration a 2 = 2a 1. If m 1 and m 2 are glued together and the same force F acts on this combination, what is the resulting acceleration? (a) (b) (c) (a) 2/3 a 1 (b) 3/2 a 1 (c) 3/4 a 1 Fa1a1 m1m1 Fa 2 = 2a 1 m2m2 Fa = ? m1m1 m2m2

Newton’s Second Law A constant force is exerted on a cart that is initially at rest on an air table. The force acts for a short period of time and gives the cart a certain final speed. Air Track Cart Force For a second shot, I can apply a force only half as large (I’m getting tired). To reach the same final speed, for how long must I apply the force ? A) 4 x as longB) 2 x as longC) Same time D) 1/2 as longE) 1/4 x as long

Talk about Quadratic and linear relationships

Newton’s 3 rd Law For every action there is an equal and opposite reaction

Newton’s Third Law A fly gets smushed onto the windshield of a speeding bus.  The force exerted by the bus on the fly is, A) greater than B) the same as C) less than that exerted by the fly on the bus.