 To learn Newton’s Three Laws  Be able to provide example of the laws  Draw Free Body Diagrams correctly  Use Newton’s second law to solve for F, m, and a of a system  Apply the friction equation to solve for F f, µ or F N

 An object in motion stays in motion or an object at rest stays at rest unless acted upon by an outside force.  In layman’s terms, stuff will keep doing what it’s doing unless you mess with it.  Examples: ◦ A Demo… ◦ A tablecloth…tablecloth ◦ What happens if you turn your car sharply to the right? You go straight. The car moves out from under you.)

 F = ma  Acceleration of an object varies directly with the force applied and indirectly with the mass of the object.  Examples: ◦ Big kid and little kid on roller skates—each gets the same push ◦ Baseball—if I want the ball to go farther, what needs to happen? ◦ A demo:  2nd Law 2nd Law

 For every action, there is an equal and opposite reaction.  Examples: ◦ A fire extinguisher… A fire extinguisher… ◦ Canoes Canoes ◦ You sitting in your chair

 You push on the earth, and the earth pushes on you.  A car pushes on the road as the road pushes on the car.  This equal and opposite force upwards is called the normal force. FNFN  The normal force is why you don’t fall down when you sit in a chair.  Always directed 90° from plane of object

 The resulting force an object feels is a sum of all forces acting upon it.  The donkey moves as if there is one overall pull in one direction

 Shows the directions all the forces acting upon an object  Types of forces: ◦ Applied ◦ Gravity ◦ Normal ◦ Friction ◦ Drag—a type of friction ◦ Tension—rope holding attached to an object ◦ Spring—coming soon to a classroom near you

 Use a dot or box to represent your object  Draw each force as an arrow  Attach each arrow to the dot/box  Aim the tip of the arrow in the direction the force is acting

 What forces are acting on the “rollerskate”? Hint: they can be at an angle.  Rope (tension)  Friction  Gravity—sled’s mass plus Naomi  Normal

 The resultant force on an object is the sum of all forces on that object. ◦ ΣF x =ma x ◦ ΣF y =ma y  If the object is not accelerating (it can be moving) in a given direction the ΣF = 0.

 What condition must be met for the “rollerskate” to accelerate?

FNFN T mg 30° Notice F N is perpendicular to the plane.

FNFN T mg 30°

FNFN T mg 30° mgcos30 mgsin30

 A child holds a sled at rest on a frictionless, snow-covered hill. If the sled weighs 77.0 N, find the tension, T, exerted by the rope on the sled and the normal force exerted by the hill on the sled.

 Set your frame of reference.  The sled is at rest for ΣF x = 0 and ΣF y = 0.  Add up all the x direction forces. ΣF x = 0 0 = T - mgsin30 0 = T – 77(sin30) T = 38.5 N  Add up all the y direction forces. ΣF y = 0 0 = F N – mgcos30 F N = 77(cos30) F N = 66.7 N

 You can have an acceleration up or down the ramp.  If this happens, ΣF x = ma and ΣF y = ma for the system.  Hint: usually ΣF y = 0 because the object is not moving up and/or down on the plane of motion. (The sled is not bouncing down the hill.)

 The equation for friction is ◦ F f = force due to friciton ◦ µ = coefficient of friction  Two types: Static (not moving) and kinetic (moving) ◦ F N = force Normal  Friction always opposes motion!!!!!

FNFN T mg 30° You then rework your sum forces in the x direction to include the oppositional force of friction. FfFf

 Newton’s Three Laws ◦ Objects in motion (at rest) stay in motion (at rest) unless acted upon by an outside force ◦ F = ma ◦ For every action, there is an equal and opposite reaction.  ΣF x = ma and ΣF y = ma  F f = µF N

 A gust of wind blows an apple from a tree. As the apple falls, the force of gravity on the apple is 9.25 N downward, and the force of the wind on the apple is 1.05 N to the right. Find the magnitude and direction of the net external force on the apple.

 The net external force on the propeller of a 3.2 kg model airplane is 7 N forward. What is the acceleration of the plane?

 A 6.0 kg object undergoes an acceleration of 2 m/s 2. What is the magnitude of the net external force acting on it? If this same force is applied to a 4 kg object, what acceleration is produced?

 A catapult exerts 450,000 N on an airplane of 23,000 kg. It starts from rest and reacher 72 m/s at the end of 85 m. The engines are producing 300,000 N of thrust. What is the average friction on the plane during the launch?

 A dockworker loading crates on a ship finds that a 20-kg crate, initially at rest on a horizontal surface, requires a 75-N force to set it in motion. However, after the crate is in motion, a horizontal force of 60-N is required to keep it moving with a constant speed. Find the coefficients of static and kinetic friction between the crate and the dock.