Chapter 12: Forces and Motion Section 2: Newton’s 1 st and 2 nd Laws of Motion.
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Transcript of Chapter 12: Forces and Motion Section 2: Newton’s 1 st and 2 nd Laws of Motion.
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