Chapter 12: Forces and Motion

Section 2: Newton’s 1st and 2nd

Laws of Motion

Newton’s First Law - Inertia

• Newton’s First Law [A.K.A.: Law of Inertia] – An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.

• Seatbelts and other safety features are designed to counteract this effect.

Resisting Inertia

Why would a car seat be designed to go into a car

backwards?

Resisting Inertia

• Inertia is the reason that cars cannot stop instantaneously when the brakes are applied. It takes a few seconds to slow down because the inertia of the car continues to pull it forward as the breaks attempt to slow it down.

• So.. Inertia is the tendency of an object to resist a change in its motion.

Using Inertia

Inertia can be harnessed in many useful instances as well.Consider the birth of modern warfare [ballistics].

Using InertiaThe cannonballs are moving at a high velocity. Inertia is acting on their mass and helping them to move forward through the ship’s hull [and any other objects or people in their way].

If they can do this to a stone wall

imagine what they could do to an enemy soldier.

Newton’s Second Law - Force

• Newton’s Second Law – acceleration of an object is equal to the net force acting on it divided by the objects mass.

mFa /• Acceleration = Net Force

Mass

OR as its often seen as

• Force = mass x acceleration

F = m x a

• If you want to calculate the acceleration you plug in the numbers for force (100 N) and mass (50 kg), you find that the acceleration is 2 N/kg

• Notice that doubling the force by adding another dog doubles the acceleration (4 N/kg). Oppositely, doubling the mass to 100 kg would halve the acceleration to 2 N/kg

Units

• The SI unit of force is the Newton.

• We learned that the unit for acceleration is m/s2.

• But wait….In the equation a= F/ m …. How can we use m/s2 if F=newtons and mass=kg?

• In using the formula for Newton’s second law, its helpful to realize that the units N/kg and m/s2 are equivalent.

Example Problem

An automobile with a mass of 1000 kg accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

Example Problem

Given:

Mass = 1000 kg

Force = 4000 N

Unknown:

acceleration = ?

Plug it in:

a = F/m = 4000N/ 1000 kg = 4 N/kg

Example Problem

Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175kg. and the lion’s upward acceleration is 0.657 m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and the stretcher?

amf

Example Problem

Given:

Mass = 175kg

Acceleration = 0.657m/s2

Unknown:

Force = ?

Plug it in:

F = 175kg x 0.657m/s2 = 115N