Introduction to Mechanics Dynamics Forces Newton s Laws

Transcription

1 Introduction to Mechanics Dynamics Forces Newton s Laws Lana heridan De Anza College Feb 14, 2018

2 Last time relative motion review projectiles and relative motion

3 Relative Motion and Projectiles A science student is riding on a flatcar of a train traveling along a straight, horizontal track at a constant speed of 10.0 m/s. The student throws a ball into the air along a path that he judges to make an initial angle of 60.0 with the horizontal and to be in line with the track. The student s professor, who is standing on the ground nearby, observes the ball to rise vertically. How high does she see the ball rise? 1 erway & Jewett, 9th ed, #53, pg106.

4 Relative Motion and Projectiles A science student is riding on a flatcar of a train traveling along a straight, horizontal track at a constant speed of 10.0 m/s. The student throws a ball into the air along a path that he judges to make an initial angle of 60.0 with the horizontal and to be in line with the track. The student s professor, who is standing on the ground nearby, observes the ball to rise vertically. How high does she see the ball rise? h = v 2 i sin 2 θ 2g or v 2 f,y = v 2 i,y 2gh 1 erway & Jewett, 9th ed, #53, pg106.

5 Relative Motion and Projectiles A science student is riding on a flatcar of a train traveling along a straight, horizontal track at a constant speed of 10.0 m/s. The student throws a ball into the air along a path that he judges to make an initial angle of 60.0 with the horizontal and to be in line with the track. The student s professor, who is standing on the ground nearby, observes the ball to rise vertically. How high does she see the ball rise? h = v 2 i sin 2 θ 2g or v 2 f,y = v 2 i,y 2gh h = 15.3 m 1 erway & Jewett, 9th ed, #53, pg106.

6 Overview forces net force

7 Forces Up until now we have predicted the motion of objects from knowledge of their motional quantities, eg. their initial velocities, accelerations, etc. We did not consider what the causes of this motion might be. We now will think about that.

8 Forces Up until now we have predicted the motion of objects from knowledge of their motional quantities, eg. their initial velocities, accelerations, etc. We did not consider what the causes of this motion might be. We now will think about that. We will understand forces as the cause of changes in the motion of objects.

9 Forces Up until now we have predicted the motion of objects from knowledge of their motional quantities, eg. their initial velocities, accelerations, etc. We did not consider what the causes of this motion might be. We now will think about that. We will understand forces as the cause of changes in the motion of objects. Forces are a push or pull that an object experiences because of an interaction. Forces are vectors.

10 Forces Two types of forces contact forces another object came into contact with the object field forces a kind of interaction between objects without them touching each other

11 Forces Force type examples: The Laws of Motion f orce the Contact forces xed t. a b c Field forces m M q Q Iron N d e f orbit around the Earth. This change in velocity is caused by the gravitational force exerted by the Earth on the Moon. When 1 erway a coiled & Jewett, spring Physics is pulled, foras cientists in Figure and 5.1a, Engineers. the spring stretches. When a

12 Forces are Vectors We typically draw them like this 1 : The block is the object that experiences the forces. There are two forces here, N and W, they are drawn as arrows to indicate their direction. 1 Figure from

13 Forces are Vectors A downward force F 1 elongates the spring 1.00 cm. A downward force F 2 elongates the spring 2.00 cm. When F 1 and F 2 are applied together in the same direction, the spring elongates by 3.00 cm. When F 1 is downward and F 2 is horizontal, the combination of the two forces elongates the spring by 2.24 cm F 1 u F 2 F a F 1 b F 2 c F 1 F 2 d Figure of a forc scale. and its direction is u 5 tan 21 (20.500) Because forces have been experimentally verified to behave as vectors, you must use the rules of vector addition to obtain 1 the net force on an object. Figure from erway & Jewett.

14 Net Force 5.2 Newton s First Law and Inertial Frames Net Force F 1 F 2 When and are applied together in the same direction, the spring elongates by 3.00 cm. the vector sum of all forces acting on an object. F 1 F 2 F net = When is downward and F i is horizontal, the combination i of the two forces elongates the spring by 2.24 cm F 1 u F 2 F In the diagram F = F 1 + F 2.

15 elongates by Net 3.00 Force cm. of the two forces elongates the spring by 2.24 cm F 1 u F 2 F c In the diagram F = F 1 + F 2. F 1 The magnitude of F is Figure 5.2 The vector F 2 F = F 2 of a force is tested with a 1 + F 2 2 = = 2.23 N d The direction of F is 0.500) Because θ = forces tan 1 have (F been experiectors, you must use the rules of vector addition 1 /F 2 ) = 26.6 to scale.

16 the eave Figure Rank the four cases, A, B, C, and D, in order of the magnitude of the puck s acceleration, starting with the smallest. Indicate ties with an equal sign. A hockey puck is acted on by one or more forces, as shown. What is the net force on each puck? Net Force Question 3 N 5 N 7 N 3 N A B t 3 N 3 N 3 N t C In case C, assume that the forces make an angle of 60 to each other. FIGURE 5 19 Conceptual Exercise 10 1 Figure from Walker, Physics, page. D

17 els Net Using Force Newton s and econd Equilibrium Law 121 What is the net force on this lamp? exerted by des no help- T they act on exerted by rium model. a b F g ith the paris Newton s Figure 5.7 (a) A lamp suspended from a ceiling by a chain of negligible mass. (b) The forces acting on the lamp are the gravi-

18 els Net Using Force Newton s and econd Equilibrium Law 121 What is the net force on this lamp? T exerted by des no helpthey act on exerted by rium model. When the net force on aan object is zero: b F g ith the paris Newton s Figure 5.7 (a) A lamp F i = 0sus- F net = pended from a ceiling i by a chain we say that the of object negligible is in equilibrium. mass. (b) The forces acting on the lamp are the gravi-

19 ummary forces net force Homework Walker Physics: Ch 5, onward from page 138. Questions: 1, 3, 5 (wait to do)