Chapter 12: Forces and Motion Section 2: Newton’s 1 st and 2 nd 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 Inertia The 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. 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/s 2. But wait….In the equation a= F/ m …. How can we use m/s 2 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/s 2 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 m/s 2. What is the unbalanced force necessary to produce this acceleration of the lion and the stretcher?

Example Problem Given: Mass = 175kg Acceleration = 0.657m/s 2 Unknown: Force = ? Plug it in: F = 175kg x 0.657m/s 2 = 115N