Force We will consider two kinds of forces Field Forces(Non-Contact): (action through"empty space Moon and earth Gravity Electricity Contact force: (physical contact between objects) This is the most familiar kind Kicking a ball I push on the desk The ground pushes on the chair On a microscopic level, all forces are non-contact Physics 121: Lecture 3, Pg 11
Physics 121: Lecture 3, Pg 11 Force … We will consider two kinds of forces Field Forces (Non-Contact): (action through “empty” space) Moon and Earth Gravity Electricity Contact force: (physical contact between objects) This is the most familiar kind. Kicking a ball I push on the desk. The ground pushes on the chair... On a microscopic level, all forces are non-contact
Action at a distance Gravity Physics 121: Lecture 3, Pg 12
Physics 121: Lecture 3, Pg 12 Action at a distance Gravity: next
Newton's law of universal gravitation Every particle in the Universe attracts every other particle with a force that is directly proportional the product of their masses and inversely proportional to the square of the distance between them F=Gm,m Constant of Universal gravitation G=6673×10N·m2/kg Physics 121: Lecture 3, Pg 13
Physics 121: Lecture 3, Pg 13 Newton’s Law of Universal Gravitation Every particle in the Universe attracts every other particle with a force that is directly proportional the product of their masses and inversely proportional to the square of the distance between them. Constant of Universal gravitation: 2 1 2 r m m F = G 11 2 2 G = 6.67310 N m k g − m1 m2 r12 F21 F12
Weight W Near the surface of the earth r≈RE( Earth radius) So the gravitation force between an object of mass m and the earth is simpl With the numbers for earth Physics 121: Lecture 3, Pg 14
Physics 121: Lecture 3, Pg 14 Weight W Near the surface of the Earth r ≈ RE (Earth radius) So the gravitation force between an object of mass m and the Earth is simply With the numbers for Earth:
Contact forces Objects in contact exert forces Convention :F means the force acting on a due to b So Fhead thumb means the force on the head due to the thumb head. thumb Physics 121: Lecture 3, Pg 15
Physics 121: Lecture 3, Pg 15 Contact forces: Objects in contact exert forces. Convention: Fa,b means “the force acting on a due to b”. So Fhead,thumb means “the force on the head due to the thumb”. Fhead,thumb next