Class 11 - Force and Motion I Chapter 5 - Friday September 17th Some particular forces Newton's third law Sample problems Reading: pages 87 thru 107 (chapter 5) in HRW Read and understand the sample problems Assigned problems from chapter 5: 24, 26, 34, 38, 44, 46, 52, 56 (due Sun. Sept. 26) Announcement: Physics tutoring center in NPB 1100 Mon/Wed Tues/Thurs 4,5,8 periods 4 to 8 periods
Review Newton's 1st law: If no force acts on a body, then the body's velocity cannot change; that is, it cannot accelerate. 1 Newton is that force required to accelerate our standardized mass (1 Kg) at a rate of 1 m.s -2. Mass is simply the characteristic of a body that relates a force on the body to the resulting acceleration Newton's 2nd law: F Free-body diagrams
Fnet Fi ma i 1 F S F F 3 2 F 2
1 F 3 F 0 a Some particular forces Gravity: During free fall
a g j F ma mg j Even when a mass is stationary on the surface of a table, gravity still acts downwards with a magnitude equal to mg. This leads to the concept of a normal or contact force: for the mass on the table to remain stationary, the table must exert an upward force on the mass so as to exactly balance the force due to gravity. N Fg ma Normal force
N Fg ma y N ma y Fg m a y g N Fg mg if a y 0 Weight (a force!): In the above example, the internal forces within the table supply the normal force, which is normal to the surface. If we hold the mass in a stationary state, we must supply the force. This is the sensation of weight, i.e. W Fg mg Newtons Friction and tension We will deal with friction next week (chapter 6). Friction is a force that acts parallel to a surface in the opposite direction to any motion.
A taut cord is said to be in a state of tension. If the body pulling on the cord does so with a force of 50 N, then the tension in the cord is 50 N. A taut cord pulls on objects at either end with equal and opposite force equal to the tension. Cords are massless, pulleys are massless and Newton's 3rd law When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. For every "action" force, there is always an equal and opposite "reaction" force; we call these a "third-law force pair." Newton's 3rd law When two bodies interact, the forces on the bodies from each other are always equal in
magnitude and opposite in direction. FBEg FBTn aB 0 FBTn FBTn FBEg FBEg Newton's 3rd law When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction.
FTEn FTEg FTEn ( FTEg FBEg ) FTBn FBEg Newton's 3rd law When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. Why doesn't the earth accelerate?
FETn FTEn FBEg FTEg Newton's 3rd law When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. FEBg FETg FBEg FTEg
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