v1chap5(4) - Chapter 5 Forces and Motion II 5.1 5.1.1 The...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 5 Forces and Motion II 5.1 The Important Stuf 5.1.1 Friction Forces Forces which are known collectively as “friction forces” are all around us in daily life. In elementary physics we discuss the friction force as it occurs between two objects whose surfaces are in contact and which slide against one another. If in such a situation, a body is not moving while an applied force F acts on it, then static ±riction forces are opposing the applied force, resulting in zero net force. Empirically, one ±nds that this force can have a maximum value given by: f max s = μ s N (5.1) where μ s is the coe²cient o± static ±riction for the two surfaces and N is the normal (perpendicular) force between the two surfaces. If one object is in motion relative to the other one (i.e. it is sliding on the surface) then there is a force of kinetic ±riction between the two objects. The direction of this force is such as to oppose the sliding motion and its magnitude is given by f k = μ k N (5.2) where again N is the normal force between the two objects and μ k is the coe²cient o± kinetic ±riction for the two surfaces. 5.1.2 Uni±orm Circular Motion Revisited Recall the result given in Chapter 3: When an object is in uniform circular motion, moving in a circle of radius r with speed v , the acceleration is directed toward the center of the circle and has magnitude a cent = v 2 r . 99
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
100 CHAPTER 5. FORCES AND MOTION II Therefore, by Newton’s Second Law of Motion, the net force on this object must also be directed toward the center of the circle and have magnitude F cent = mv 2 r . (5.3) Such a force is called a centripetal force , as indicated in this equation. 5.1.3 Newton’s Law of Gravity (Optional for Calculus–Based) The force of gravity is one of the fundamental forces in nature. Although in our Frst physics examples we only dealt with the fact that the earth pulls downward on all masses, in fact all masses exert an attractive gravitational force on each other, but for most objects the force is so small that we can ignore it. Newton’s Law of Gravity says that for two masses m 1 and m 2 separated by a distance r , the magnitude of the (attractive) gravitational force is F = G m 1 m 2 r 2 where G = 6 . 67 × 10 - 11 N · m 2 kg 2 (5.4) While the law as given really applies to point (i.e. small) masses, it can be used for spherical masses as long as we take r to be the distance between the centers of the two masses. 5.2 Worked Examples 5.2.1 Friction Forces 1. An ice skater moving at 12 m s coasts to a halt in 95m on an ice surface. What is the coe±cient of (kinetic) friction between ice and skates? [Ser4 5-51] The information which we are given about the skater’s (one-dimensional) motion is shown in ±ig. 5.1(a). We know that the skater’s notion is one of constant acceleration so we can use the results in Chapter 2. In particular we know the initial and Fnal velocities of the skater: v 0 = 12 m s v = 0
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 28

v1chap5(4) - Chapter 5 Forces and Motion II 5.1 5.1.1 The...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online