2Force Force a push or a pull force of table g Causes object to start or stop moving or change directionforce of tableg
3Newton’s 1st LawGreeks thought that the natural state of an object was at rest.ex. A ball stops rollingwithout friction an object would never stop!Newton’s 1st an object in motion tends to stay in motion; an object at rest tends to stay at rest
4Newton’s 1st LawInertia property of matter that resists a change in motionAn object with great mass has high inertia
5Newton’s 2nd Law f = m x a A 50N B 100N FA = 1000 kg x 0.05 m/s/s FA = FB = 2000 kg x m/s/sFB =100N
6Newton’s 2nd Law Force = mass x acceleration An object will only accelerate if there is an unbalanced force
7Newton’s 3rd LawFor every action there is and equal and opposite reactionThe astronaut pushes on the rock and the rock pushes on the astronaut
8Newton’s 3rd LawFor every action there is and equal and opposite reactionA bird pushes down on the air and the air pushes up on the bird
9Newton’s 3rd LawNote: Equal and opposite forces does not result in equal acceleration if the masses are different.
10Newton’s Laws1st Law: (inertia: objects tend to do what they are doing)cannon ball will rest until a force is put on itball will roll straight until ramp puts a force on it2nd Law: (f = m x a)greater force put on ball accelerates it moregreater mass of ball but greater force on water3rd Law: (every action has an equal but opposite reaction)ball moves right, cannon recoils leftball move down, water splashes upNewton's Laws - YouTube
11Gravityforce of handgWith the upward force of the floor equilibrium is attained and there is no motionWithout the upward force of the hand there is not equilibrium of forces and motion occurs
12Gravity: air resistance Opposes downward motion of falling objects.Larger surface areas increases air resistance
18Free Body DiagramsA force diagram, which is also known as a free body diagram, is a sketch in which all the force vectors acting on an object are drawn with their initial points at the location of the object.
19Opposition to Motion razor's edge Friction a force that opposes motionCaused by rough surfaces of all materialsrazor's edge
203 Types of Friction1. Sliding when solid objects grind over each otherpuck and ice2. Rolling wheels spinning on an axleskateboards eventually roll to a stop3. Fluid liquids or gases slow the motion of a solidwind resistanceoil a squeaky hingepushes a surfer
21Free-body diagramsFree-body diagrams are used to show the relative magnitude and direction of all forces acting on an object.
22This diagram shows four forces acting upon an object This diagram shows four forces acting upon an object. There aren’t always four forces, For example, there could be one, two, or three forces.
23Problem 1A book is at rest on a table top. Diagram the forces acting on the book.In this diagram, there are normal and gravitational forces on the book.
24Problem 2An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.
25Gravity is the only force acting on the egg as it falls.
26Problem 3A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this situation looks like…
27Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.
28Problem 4A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see what this one looks like.
29Note the larger applied force arrow pointing to the right since the book is accelerating to the right. Friction force opposes the direction of motion. The force due to gravity and normal forces are balanced.
30Problem 5A skydiver is descending with a constant velocity. Consider air resistance. Draw a free-body diagram.
31Gravity pulls down on the skydiver, while air resistance pushes up as she falls.
32Problem 6A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram.
33The applied force arrow points to the right and is larger than the frictional force since the object is accelerating. Since the sled is on the ground, the normal and gravitational force are balanced.
34Problem 7A football is moving upwards toward its peak after having been booted by the punter. Draw a free-body diagram. (Neglect air friction)
35The force of gravity is the only force described. (no air resistance).
36Problem 8A car runs out of gas and is coasting down a hill.
37The car is coasting down the hill, there is dragging friction of the road (left pointing arrow) as well as gravity and normal forces, but no applied force.
38Net ForceNow let’s take a look at what happens when unbalanced forces do not become completely balanced (or cancelled) by other individual forces.An unbalanced forces exists when the vertical and horizontal forces do not cancel each other out.
39Example 1Notice the upward force of 1200 Newtons (N) is more than gravity (800 N). The net force is 400 N up.
40Example 2Notice that while the normal force and gravitation forces are balanced (each are 50 N) the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left.
41Another way to look at balances and unbalanced forces
42Balanced or unbalanced? Accelerating – UnbalancedConstant velocity –balanceddeccelerating – Unbalanced