Peoples Physics Book Version-3

http://www.ck12.org Chapter 5. Newton’s Laws

5.2 Newton’s Laws Explained

TheFirst Law is about inertia; objects at rest stay at rest unless acted upon and objects in motion continue
that motion in a straight line unless acted upon. Prior to Newton and Galileo, the prevailing view on motion was
still Aristotle’s. According to his theory the natural state of things is at rest; force is required to keep something
moving at a constant rate. This made sense to people throughout history because on earth, friction and air resistance
slow moving objects. When there is no air resistance (or other sources of friction), a situation approximated in space,
Newton’s first law is much more evident.

The “motion” Newton mentions in the Second Law is, in his language, the product of the mass and velocity of an
object — we call this quantity momentum — sothe Second Law is actually thefamous equation:

~F=∆(m~v)

∆t

=

m∆~v

∆t

=m~a [1]

That is, the acceleration experienced by an object will be proportional to the applied force and inversely proportional
to its mass. If there are multiple forces, they can be added as vectors and it is thenetforce that matters:

m~a=F~net=∑

i

~Fi Net force is the vector sum of all the forces

max=Fnet,x=∑

i

Fix Horizontal components add

may=Fnet,y=∑

i

Fiy As do vertical ones

When the net force on an object is zero, it is said to be intranslational equilibrium:

∑

i

~Fi= 0 Translational Equilibrium Condition [2]

Finally, theThird Law states that you can’t push someone or something without being pushed back. This
law is somewhat confusing: if to each applied force there is an equal and opposite force, why does anything ever
accelerate? The key is that the ’equal and opposite’ forces act on different objects. If I push a cart, the cart is in turn
pushing on me. However, I’m also pushing (and being pushed by)the earth, through my feet. Therefore, in the end,
the cart and I move in the same direction and the earth moves opposite us. The cart-person system experienced a net
force in one direction, while the earth experienced an equal and opposite force. According to Newton’ssecondlaw,
the acceleration objects experience due to applied forces is inversely proportional to their mass; clearly, the earth —
with its gigantic mass — doesn’t move very far compared to the cart and person.

Newton’s Laws Example

Question: Tom and Mary are standing on identical skateboards. Tom and Mary push off of each other and travel in
opposite directions.

a) If Tom(M)and Mary(m)have identical masses, who travels farther?