A force is a push or a pull.

Transcription

1 A force is a push or a pull. Contact forces arise from physical contact. Action at adistance forces do not require contact and include gravity and electrical forces. 1

2 Force is a vector [F]=[Newton]=[N] 2

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4 Inertia is the natural tendency of an object to resist change in motion (remain at rest or in motion at a constant speed along a straight line). The mass of an object is a quantitative measure of inertia. Mass is a scalar. SI Unit of Mass: kilogram (kg) [m] = [kg] 4

5 First Law of Motion (or Law of Inertia) Version 1: An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an unbalanced force. Version 2 of the 1st Law: If F net =0, then a=0 (v=constant) where F net vector sum of all the forces acting on an object 5

6 F net on a car that is moving at a constant speed How do you change the speed of the car? What actions do you perform? When the car is moving on a leveled road at a constant speed, what action(s) are you (the driver) performing? 6

7 Drawing Free Body Diagrams Forces you MUST Know and account for in the diagram Gravity, Normal, Friction,Tension, Applied Force 7

8 Gravity (Weight) due to Earth attractive force Earth exerts on an object points vertically down (toward center of Earth) all * free body diagrams MUST include gravitational force 8

9 Normal force with which a surface reacts to the "push" on it in math word "Normal" means "perpendicular" acts perpendicular to the surface 9

10 Tension (Force of Tension) force exerted by a rope/chain that is being pulled acts in the direction of a rope 10

11 Friction (Frictional Force) force that acts to retard/resist motion acts in the direction opposite to the direction of motion 11

12 Applied force any other force (push or pull) applied to the object 12

13 Please draw a free body diagram for each of the cases described below An object in free fall An object accelerating down (air resistance is present) An object moving down at terminal velocity 13

14 A ball suspended from a rope (v=0) A ball suspended from a rope moving down at a constant velocity A ball suspended from a rope accelerating down A ball suspended from a rope accelerating up 14

15 A box standing at a table (v=0) A box on a table sliding to the right at a constant speed A box on a table accelerating to the right 15

16 A box is standing still on an inclined plane 16

17 A box is accelerating down an inclined plane 17

18 Consider two boxes attached by a pulley (see picture below) Draw forces on each box when v=0 18

19 Consider two boxes attached by a pulley (see picture below) Draw forces on each box when the system is accelerating to the right/down) 19

20 Free body diagrams for four situations are shown below. For each situation, determine the net force acting upon the object. F N = 3N F f = 5N F = 5N F g = 3N F N = 3N F f = 5N F g = 3N F N = 20N F g = 20N F air = 40N F g = 25N 20

21 Free body diagrams for four situations are shown below. In each case, the net force is known. However, the magnitudes of some of the individual forces are not known. A B F = 50N F net = 0 N F g = 200N C F net = 900 N, up F g = 200N F N = 300 N F f = 80 N D F net = 60 N, left E F F f = 20 N G F net = 30 N, right E 21

22 A 2 kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction? 22

23 A 3 N force is needed to move a 2 kg object horizontally with a speed of 4 m/s. What is the force of friction acting on this object? 23

24 A 4.0 kg object is moving across a frictionless surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? A) 0 N B) 0.5 N C) 2.0 N D) 8.0 N E) depends on the speed. 24

25 Flip, an exhausted gymnast, hangs from a bar by both arms in an effort to catch his breath. If Flip has a mass of 65.0 kg, what is the tension in each of Flip s arms as he hangs in place? 25

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