Lecture 12

2 Newton’s first Law If there are no external forces, the object does not change its velocity and direction. LAW of INERTIA!!!

3 Newton’s Second Law Acceleration Frame A

4 Newton’s Third Law Forces exist always in pairs. F 12 F 21 These forces are called action-reaction forces The action and reaction forces act on different objects

5 Examples When you push the wall with force F then the wall pushes you with the same force

6 Action-reaction

7 Summary: Newton’s third Law Interacting objects exert equal forces upon each other These are called action reaction forces Every action receives an equal but opposite reaction

8 The Gravitational Force - Weight Weight and mass are NOT the same: Mass is an intrinsic property of the object determined by the type and amount of atoms The weight is the measure of the gravitational force and therefore dependence on the conditions how the force is measured SI unit: 1 N= 1 kg m/s 2

9 Weight: dependence on the environment No gravity Mass is not a weight!!!

10 Normal force Normal force is the force acting to the object due to the deformation of the surface. Normal force is always perpendicular to the surface

11 Friction Friction is the resistance to the motion due to the interaction of the object with the environment Friction is always proportional to the normal force The direction of frictional force is always opposite to the direction of motion N

12 Tension Tension is the force transmitted through a “rope” from one end to the other. An ideal cord has zero mass, does not stretch, and the tension is the same throughout the cord.

13 Free Body Diagram Identify all the forces acting on the object Choose an appropriate coordinate system Idealize (simplify) the system Real lifeFree body diagram x y

14 Free Body Diagram x y

15 Weight: case a=0 y Chose the coordinate system Apply Newton’s second equation From Newton’s third equation

16 Weight: case a>0 y The object become heaver

17 Weight: case a<0 y The object become lighter For a=g the object becomes weightless