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Newton’s Laws Review Quiz Newton’s Laws • What is Newton’s second law (in words) • Acceleration is proportional to force and inversely proportional to mass Newton’s Laws • What is Newton’s Second Law, mathematically? • F=ma Newton’s Laws • What is Newton’s First law (in words) • An object in motion (or at rest) stays in motion (or at rest) unless acted upon by an outside force • Why cant you lift yourself off the ground by puling up on your belt? • A: because there is no outside force internal forces do not cause motion Newton’s Laws • What does proportional mean? Newton’s Laws • What are the two factors that determine how much friction there is between two objects? • Normal force, coefficient of friction of the materials Newton’s Laws • Does a larger block with the same mass have more, less or the same friction • same - surface area is not in the calculations, does not matter Newton’s Laws • What is pressure? • Amount of force per unit area Newton’s Laws • What are the units of pressure?MKS? English? • Pascals = 1newton/meter2 • Pounds/in2 = psi Ramp Problem • A 12 Kg block sits on a 40 degree ramp with a • • • • • • • coefficient of sliding friction of .4 Find: Fwt Fax Fn Ff Fnet a *challenge - what µ so that it does not slide Pressure • A 140kg skater stands on one skate blade that is 10 cm x 1 cm. What is the pressure on the blade? Newtons Laws • • • • • • Accelerations are produced by a. forces. b. velocities. c. accelerations. d. masses. e. none of the above Newton’s Laws • • • • • The SI unit for force is equal to: a) kilogram b) kilogram-meter/s2 c) pound d) slug • and is called the ____________ Newton’s Laws • • • • • If an object is at accelerating, we know that a) Fnet = 0 b) It is falling c) there is a net force on it d) there is no friction Newton’s Laws • If the net force on an object is zero it must • • • • be: a) at rest b) have friction c) accelerating d) be at a constant velocity or at rest • A 20 kg box is on a surface that has a µ=.25 It is accelerating at 2m/s2. • Draw a force diagram • What is the applied force? • How far will it go in 2 seconds? • When a woman stands with two feet on a scale, the scale reads 500 N. When she lifts one foot, the scale reads • a. less than 500 N. • b. more than 500 N. • c. 500 N. • What is the difference between weight and mass? Which stays the same on the moon vs earth? • When a woman stands with two feet on a scale, • • • • the scale reads 500 N. When she lifts one foot, the pressure on her foot a. Stays the same b. Decreases c. Doubles D. Cannot be determined • Suppose the force of friction on a sliding object is 13 N. The force needed to maintain a constant velocity is • a. more than 13 N. • b. 13 N • c. less than 13N. • A 1800g block is pulled by a 1200 gram hanging mass. µ = .2 What is the acceleration of the cart? • What is terminal velocity? Draw a force diagram of an object at terminal velocity. • Why do a coin and a feather fall with the same acceleration if there is no air?
