Presentation on theme: "1) According to Aristotle, the world was composed of 4 elements – earth, water, air, and fire. Each element had a natural place in a hierarchy. Earth,"— Presentation transcript:
1) According to Aristotle, the world was composed of 4 elements – earth, water, air, and fire. Each element had a natural place in a hierarchy. Earth, the heaviest, belonged to the lowest position. Water was next, then air, then fire. If any element is out of place it undergoes natural motion in a straight line back to its natural place.
2) According to Galileo, constant-speed straight-line motion was just as natural as a state of rest. The tendency of an object to remain at rest or to continue to move in a straight line at a steady speed is a property known as inertia. Galileo reasoned that in the absence of any motive or retarding force, a ball rolling on a level surface would neither speed up nor slow down. How long will this motion persist?
3) The velocity of an object remains constant unless an unbalanced force acts on the object. This is Newton’s 1 st law. The force of friction slows a sliding block. If a car moves at a constant speed on a level, straight highway, many forces act on the car but the net force acting on the car is zero! Can an object stop itself?
4) When we add two vectors, we draw the arrow that represents their sum from the tail of the first to the head of the second. If you walk 80 m N, turn right, then walk 60 m E, the magnitude of your displacement is 100 m and the direction of your displacement is 63 o NE or, equivalently, 37 o East of North. How do we compute the number 100 in this example?
5) The net force on an object is equal to its mass times its acceleration and points in the direction of the acceleration. This is Newton’s 2 nd law. We use the letters F, m, and a whence: F net = ma. Mass is the measure of inertia. The SI unit of mass is the kilogram or kg. The SI unit of force is the newton or N. What is a newton in terms of kg, m, and s?
6) Mass is an intrinsic property that does not depend on location. The gravitational force on an object is called its weight. We use the letters w, m, and g whence: w = mg. How much does a 50 kg object weigh and what are the SI units of weight?
7) If many forces act on a sled, the net force on the sled is the vector sum of all the forces acting on the sled. We identify the forces by drawing a free-body diagram. We represent the sled with a dot. The tail of each force vector starts on the dot. How many forces act on a book resting on a level table and what exerts each force?
8) As an object falls and speeds up its weight remains constant while air resistance increases. When air resistance equals weight the object stops accelerating. The speed where acceleration stops is called terminal speed. If you open a parachute at terminal speed, how does air resistance compare to weight and in what direction is acceleration?
9) When you push on a crate and it doesn’t move, the net force on the crate is zero. The frictional force is equal and opposite to your applied force. In this case the frictional force is called static friction. If the crate begins to slide the frictional force is called kinetic friction. How does kinetic friction compare to your applied force if velocity is constant?
10) If object A exerts a force on object B then object B exerts an equal and opposite force on object A. If the Earth pulls down on a ball with a force of 10 N, then the 2 nd force in the action-reaction pair is the ball pulling on the Earth. The ball pulls up on the Earth with a force of 10 N. They pull on each other! Why does the ball accelerate more than the Earth?