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

𝑣 1=0𝑣 2≠𝑐𝑜𝑛𝑠𝑡

4

Newton’s Third Law

•Forces exist always in pairs.

F12 F21

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

𝑔

�⃗�=𝑚�⃗�

�⃗�𝑔=𝑚�⃗�

𝐹𝑔𝐹𝑔

𝐹 ′

•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

𝑊=𝑚𝑔

SI unit: 1 N= 1 kg m/s2

9

Weight: dependence on the environment

No gravity

Mass is not a weight!!!

10

Normal force

𝑚𝑔

𝑁

•Normal force is the force acting to the object due to the deformation of the surface.

•Normal force is always perpendicular to the surface

𝑁 𝑁

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 life Free body diagram

𝑚𝑔

𝑇

x

y

∑ 𝐹=𝑚𝑎𝑇 −𝑚𝑔=0𝑇=𝑚𝑔

14

Free Body Diagram

𝑚𝑔

𝑁𝑓

x

y

𝑚𝑔co

s𝜃 𝑚𝑔sin 𝜃

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