Chapter 2: Lesson 3

Newton’s 2nd Law of Motion The acceleration of an object is equal to the net force exerted on the object divided by the object’s mass. Force = Acceleration = The acceleration due to gravity on earth = Mass x Acceleration Force Mass 9.8 m/s2

Three Forms of Newton’s 2nd Law If you want to find… And you know… Then the formula you would use is… Acceleration (a) Net Force (F) Mass (m) Net Force (F) and mass (m) Acceleration (a) and mass (m) Net Force (F) and acceleration (a)

Units for solving for Newton’s 2nd Law Identify the information you are given Look at the units Unit for Force – Remember: is a force due to If the problem asks you to solve for weight you need to identify the acceleration due to gravity for your given location. Unit for Acceleration – Unit for Mass – Use the appropriate formula Newton (N) Weight gravity m/s2 Kg or g

Practice Problem A man has a mass of 66kg on Earth. What is his weight?

Practice Problem Johnny hits the baseball with 100N of force. The baseball has a mass of 14.2kg. Identify the acceleration of the baseball.

Practice Problem A girl on roller skates with a mass of 55kg accelerates at a rate of 2m/s2. What is her force?

Practice Problem Richie went fishing with his dad. He felt a bite on his line and started reeling the fish in with a force of 201N. The fishing line was moving at an acceleration of 22m/s2. What was the mass of the fish that he reeled in?

An object at REST Balanced Force The force exerted by the is to Unbalanced Force The force exerted by gravity is than air resistance. HAND GREATER EQUAL THE FORCE OF GRAVITY

An object in MOTION Unbalanced forces cause objects to ACCELERATE Increase Speed Decrease Speed Change Direction

Part I: Acceleration Depends on Mass Acceleration as its mass decreases increases Acceleration as mass increases decreases Acceleration and mass are inversely related Example: You are pushing a shopping cart at the grocery store. At the beginning of your shopping trip, you exert a small force on the cart to accelerate it. (smaller mass = greater acceleration) Exert the same amount of force when the cart is full and the cart will not accelerate as much. (greater mass = smaller acceleration) http://www2.hawaii.edu/~kobatake/secondlaw4.html

Part II: Acceleration Depends on Force Acceleration as the force on it increases increases Acceleration as the force on it decreases decreases Acceleration and force are directly related Example: When pushing the full shopping cart, if you push harder (greater force), the cart will move faster. If you push the full shopping cart with less force, the cart will move slower. **The acceleration is always in the as the force applied. The shopping cart moved forward because the push was in a forward direction same direction

We know that objects with different masses accelerate to the ground However, because of the 2nd Law we know that they don’t hit the ground with the same at the same rate. force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

Newton’s second law explains why objects fall to Earth with the same acceleration (9.8 m/s) Less mass Less Gravitational force Less inertia = easier to move More mass More Gravitational force More inertia = harder to move

Constant Type of Relationship Force Mass Increases Acceleration ________ Mass ___________ Acceleration Increases Mass Force Increases Force ____________ Acceleration Decreases Acceleration Mass Decreases Decreases Inverse Decreases Increases Direct Decreases Increases Direct Decreases

Circular Motion Any motion in which an object is moving along a curved path. For example: A rider on a merry-go-round moves in a circle. This type of motion is called If you are in circular motion, your of motion is constantly This means you are constantly Circular motion direction changing accelerating

Centripetal Force A force that causes an object to move in a circular path If you are constantly accelerating there must be a force acting on you The force exerted is the and always points to the center of the circle. at all times centripetal force

All requires a Because the force acts toward the of the circular path, the acceleration must also be toward the CIRCULAR MOTION CENTRIPETAL FORCE CENTER CENTER