Summary : 4. Newton's Laws

Newton’s 1st law of motion (definition of inertia) :

Newton’s 2nd law of motion : net

d

d t

pF

Newton’s 3rd law (conservation of momentum) :

Normal force n : contact force acting normal to contact surface.

m in f = m a is the inertia mass.

Weight = force of gravity on mass:

mw g

0net const F v

m is gravitational mass

ab baF F

inertial frame only

Special Cases

The Fundamental Forces

The fundamental forces :

• Gravity: large scale phenomena

• Electroweak force

• Electromagnetic force: everyday phenomena

• Weak (nuclear) force

• Strong (nuclear) force

1

1025

1036

1038

Free Body (Force) Diagram

• Keep only features relevant to the dynamics.

• Replace all objects by points upon which forces act.

• Represent all forces by vectors.

1. Identify all forces on object 2. Skier reduces to dot

3. Draw forces as vectors on dot

Force from ground n

Gravity Fg

Free-body diagram

n

Fg

Example 4.4. Pushing Books2 books lie on frictionless horizontal surface.

You push with force F on book of mass m1 ,

which in turn pushes on book of mass m2.

What force does the 2nd book exert on the 1st ?

1 2m m

F

aAcceleration of books :

12 2mF aNet force on 2nd book :

21 12F F

2

1 2

m

m m

F

Force exerted by 2nd book on the 1st

2

1 2

m

m m

F

Normal force n : contact force acting normal to contact surface.

3rd law also applies to non-contact forces such as gravity.

Not a 3rd law pair

Normal force Fnet 0