Kin 216: Newton’s Laws

Define kinetics

Explains what causes, alters and prevents movements 

Explaining why kinematics happen, factors responsible for motion

Describe Newton’s first law

Desire for a system to remain in its static state

Can an object be in motion if no external forces act on it?

Yes. Key idea from Newton’s 1st law of motion, also known as the Law of Inertia

Object in motion will remain in motion at a constant velocity (in a straight line and at constant speed)

Object at rest will remain at rest, unless acted upon by a net external force

Describe Law of inertia

Newton’s 1st law of motion

Objects remain in a state of constant velocity. System’s desire to remain in the same state, resist external influences.

How is inertia quantified

Quantified by how much mass/matter system is made of

Relationship between inertia and mass

The amount of inertia in a system is proportional to it’s mass

Less mass = less external force needed to change objects state

Describe momentum. Formula and UoM

The property (strength or force) that a moving object has due to its mass and its motion

M = m x v

UoM = kg m/s

Operational definition of momentum

The effect a moving system has on another system

Bigger person → more mass → bigger velocity → bigger momentum

eg: A running back = big + moves fast

Describe the conservation of momentum

Newton’s 1st law can be rephrased as conservation of momentum: in the absence of an external force every system remains in a constant state

Constant state = not changing

p1 = p2 since velocity doesn’t change, and mass remains the same 

Describe the law of momentum

Newton’s 2nd law is the Law of Momentum

It’s not about calculating momentum, it’s about change in momentum

Desire to change the object’s state of motion/momentum

In order to change momentum of a system, you need to apply a force over a period of time (impulse)

What does the sign in front of momentum mean

sign in front of momentum only means direction

Describe impulse. UoM and formula

Force applied over a period of time. The thing that changes momentum is impulse

Impulse = Force x time

UoM = Ns

Impulse = Ft = mv2 - mv1

What are we changing in a system’s momentum via impulse

Rigid body mechanics means the mass will remain the same. So when we apply impulse, we are only change the velocity of the system

How is Newton’s 2nd Law: Law of Momentum and Law of Acceleration. Explain without numbers

An object that is moving has a certain momentum. If we wish to change it’s momentum we need to apply force on it over a period of time. That’s an impulse. 
Because mass is fixed, the impulse only changes the velocity of the object. Change in velocity is known as acceleration. This is why it’s both the Law of Momentum and Law of Acceleration

Newton’s 3rd Law

Law of Action-Reaction

Forces come in pairs

For every force applied on one object by another, the second object applies a force of equal magnitude but in the opposite direction

What force(s) is responsible for movement of the system

Reaction force, an external force, is responsible for movement in the system

Does force create motion? Newtonian explanation

No, force doesn’t cause motion. it causes a change in motion. if something is in motion, it can remain in motion (constant vel) until an external force acts on it

Is it possible to blow your own sail?

No, because the reaction force (which is equal and opposite in direction) isn’t large enough to create motion/overcome inertia. It’s an internal force. According to the first law, you need an external force to create entire body movements/overcome inertia

For the blowing your own sail question, what if we applied 2nd law ie. got a smaller boat and bigger fan

No, still it would be internal forces. They would be counteracted by the boats reaction forces.

only external forces can cause entire body movements

Example of conservation of momentum in sports

I kick a soccer ball. I transfer my momentum to the soccer ball which causes it to go into motion

Example of impulse in sports

A soccer ball is stationary

I kick it which applies a force over time on it, which changes it’s velocity (mass stays the same). i changed it’s momentum by changing it’s velocity

Example of Newton’s 2nd law in sports

Law of momentum/acceleration
A moving soccer ball is kicked. The impulse (force over time to change momentum) changes it velocity so now it’s moving with greater momentum. 
The greater momentum means change in velocity which means acceleration. 

What are the forces when you’re simply standing in place

You have static friction where the grooves and bumps are interlocking heavily.

at that point the external forces acting on you are gravity and ground reaction forces