Do Now: A boy pushes a 5Kg box across a floor with a force of 40N. If the force of friction is 10N find the acceleration of the box. (Draw a force diagram)

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Do Now: A boy pushes a 5Kg box across a floor with a force of 40N. If the force of friction is 10N find the acceleration of the box. (Draw a force diagram) 10N40N Net force = =ma A = 30/5 a= 6m/s 2 N mg

Newton’s Third Law: For every action (force) there is an equal and opposite reaction (force). Forces always come in pairs:

Examples of Force Pairs

The glove pushes the ball the ball pushes on the glove. The force of the bat on the ball = the force of the ball on the bat. More Examples of Force Pairs

Check Your Understanding 1. While driving down the road, a firefly strikes the windshield of a bus. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater?

4. In the top picture (below), Kent is pulling upon a rope which is attached to a wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 Newtons. Kent is pulling... a. with more force when the rope is attached to the wall. b. with more force when the rope is attached to the elephant. c. the same force in each case.

3. A recoil is the result of action-reaction force pairs. The acceleration of the recoiling rifle is... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet.

Review : Mass and Weight If a astronaut measures his weight on the moon would it be the same as earth? What if he measures his mass using a balance?

Friction Friction is a force that opposes motion. When friction works, an object slows down and heat is produced. Friction is caused by the attractive force between matter. Three types of friction are sliding, rolling and fluid.

Sliding Friction Caused by the roughness of the surfaces and the molecular attraction between surfaces. »depends on the materials involved and the normal force Maximum Friction = Normal force x coefficient of friction F =  x N =   x m x g

Coefficient of Friction Symbol is  measures the surface roughness and attractive forces.   is usually less than one.   has no units.  Friction is independent of surface area

Static Friction Vs Kinetic Friction Every surface has two coefficients of friction: Static and a Kinetic before an objects starts sliding use static. If an object is moving use kinetic.   Static >  kinetic

Problem solving strategies 1.Draw a diagram, show forces as arrows and label. 2.Determine if you are given the mass or the weight (Normal force). Normal force (weight) is in Newtons Mass is in Kg (Weight) F g = m x 9.8m/s 2 3.Determine if the object is moving (kinetic) or at rest (static friction)

Sample Problem: A 15 N wooden box is pulled across a wooden surface at constant speed. What is the force of friction? Pull Kinetic friction a = 0 mg Normal f =  x N F =.3 X 15 = 4.5N

What is the magnitude of the force needed to keep a 60.-newton rubber block moving across level, dry asphalt in a straight line at a constant speed of 2.0 meters per second? (1) 40. N (3) 60. N (2) 51 N (4) 120 N

1.0

Skid marks What is the force of friction on a 2000Kg car? How fast was he going if the skid marks are 20m?

A steel 5kg block sits on the steel floor of a cargo ship. The ship begins to move. A)What is the normal force? B)What is the maximum amount of friction between the block and the floor? C)Will the block stay put if the ship accelerates at 4 m/s 2 ? Static friction mg Normal acceleration

Rolling Friction Much less than sliding friction. Caused by molecular attractions between the wheels and the ground. Axles also have a small amount of sliding friction. Molecular attraction Axles sliding

Fluid Friction and Air Friction Fluid Friction: depends on the material ex: walking through water is harder than walking in air  Fluid Friction is usually much less than rolling or sliding friction.  Shape and velocity determines how much resistance force is expected.  A streamlined shape has least resistance.

Why does an object have a terminal velocity? Terminal velocity is the maximum velocity during free fall.

Calculate the Maximum force of Kinetic and Static Friction For a 10N wooden block on a wooden floor: Friction =  x Normal F s = 10x.42 = 4.2N F k = 10 x.3 = 3N For a 100N steel block on a steel surface: F s = 100x.74 = 74N F k = 100 x.57 = 57N

Newton = kg m/s 2 N / kg = __kg m/s 2 _ kg = m/s 2