Mass & Weight. weight a force acting on a body due to the gravitational attraction pulling that body to another. NOT constant.

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Oswald Foster
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1 Mass & Weight mass how much stuff a body has. Doesn t change. Is responsible for the inertial properties of a body. The greater the mass, the greater the force required to achieve some acceleration: Fnet = m a weight a force acting on a body due to the gravitational attraction pulling that body to another. NOT constant. w = mg Since mass doesn t change, inertia properties don t change. Typeset by FoilTEX 1

2 Clicker Question Consider a person standing in an elevator that is accelerating upward. The upward normal force N exerted by the elevator floor on the person is A. larger than B. identical to C. smaller than the downward weight w of the person. Typeset by FoilTEX 2

3 Free Body Diagrams g m θ Typeset by FoilTEX 3

4 Free Body Diagrams y x F n m F g θ Typeset by FoilTEX 4

5 Newton s 2nd law A ball is hanging from a long string that is tied to the ceiling of a train car that is travelling eastward on horizontal tracks at a constant velocity. An observer inside the train car sees the ball hanging motionless. Draw a clearly labeled free-body diagram for the ball. Is the net force on the ball zero? v Typeset by FoilTEX 5

6 Newton s 2nd law A ball is hanging from a long string that is tied to the ceiling of a train car that is travelling eastward on horizontal tracks and speeding up. An observer inside the train car sees the ball hanging motionless. Draw a clearly labeled free-body diagram for the ball. Is the net force on the ball zero? a v Typeset by FoilTEX 6

7 Example equilibrium A large wrecking ball is held in place by 2 light steel cables. If the mass m of the wrecking ball is 4090 kg, what is A. The tension T B in the cable that makes an angle of 40 with the verticle? B. The tension T A in the horizontal cable? T A T B 40 Typeset by FoilTEX 7

8 Example apparent weight A 550 N physics student stands on a bathroom scale in an elevator. As the elevator starts moving, the scale reads 450 N. 1. Find the acceleration of the elevator (magnitude and direction). 2. What is the acceleration if the scale reads 670 N? 3. If the scale reads zero, should the student worry? Typeset by FoilTEX 8

9 Newton s 3rd Law F AonB = F BonA If object A exerts a force on object B, then object B exerts an oppositely directed force of equal magnitude on A. Old school lyrics: for every action there is an equal and opposite reaction, (sometimes called action-reaction pair, but there is no cause and effect). 3rd law pair always involves forces on two different objects 3rd law pairs are never on the same free body diagram Typeset by FoilTEX 9

10 Rules of thumb for Newton s laws Newton s 1st & 2nd laws consider all forces action a particular body (not forces exerted by that body). These forces are all on a single free body diagram. Newton s 3rd law deals with forces acting on different bodies. The forces will never be on the same free body diagram. Typeset by FoilTEX 10

11 Clicker Question Consider a car at rest. We can conclude that the downward gravitational pull of the earth on the car and the upward contact force of the earth are equal and opposite because A. the two forces form an interaction pair. B. the net force on the car is zero. C. neither of the above. Typeset by FoilTEX 11

12 Clicker Question Consider a horse pulling a buggy. Is the following statement true? The weight of the horse and the normal force exerted by the ground on the horse constitute an interaction pair that are always equal and opposite according to Newton s third law. A. yes B. no Typeset by FoilTEX 12

13 A locomotive pulls a series of wagons. Which of the following is correct? A. The train moves forward because the locomotive pulls forward slightly harder on the wagons than the wagons pull backward on the locomotive. B. Because action always equals reaction, the locomotive cannot pull the wagon. The wagons pull backward just as hard as the locomotive pulls forward, so there is no motion. C. The locomotive gets the wagons to move by giving them a tug during which the force on the wagons is momentarily greater than the force exerted by the wagons on the locomotive. D. The locomotive s force on the wagon is as strong as the force of the wagons on the locomotive, but the frictional force on the locomotive is forward and large while the backward frictional force on the wagons is small. E. The locomotive can pull wagons forward only if it weighs more than the wagons. Typeset by FoilTEX 13

Physics A - PHY 2048C

Physics A - PHY 2048C Mass & Weight, Force, and Friction 10/04/2017 My Office Hours: Thursday 2:00-3:00 PM 212 Keen Building Warm-up Questions 1 Did you read Chapters 6.1-6.6? 2 In your own words: What