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Physics: Forces and Newton’s Laws

Physics: Forces and Newton’s Laws. 10/29/2014 Objective: I can describe the difference between mass and weight. Force. What is a force? In physics, a force is “Any action that has the ability to change motion.” We can simplify that to “a push or a pull”. Two Types of Forces.

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Physics: Forces and Newton’s Laws

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  1. Physics: Forces and Newton’s Laws 10/29/2014 Objective: I can describe the difference between mass and weight

  2. Force • What is a force? • In physics, a force is “Any action that has the ability to change motion.” • We can simplify that to “a push or a pull”.

  3. Two Types of Forces Contact Forces Field Forces • An object from the outside touches a system and exerts a force on it. • If I push on the wall, I’m exerting a contact force against it. • Field forces are exerted without contact. • Gravity is a common example. So is magnetism

  4. Force, mass, and acceleration • Acceleration is proportional to force (a∝ F) • Acceleration is inversely proportional to mass (a∝ 1/m) • Combining these two, we get an equation: a = F/m • Mass is defined as the amount of matter in an object • When mass is constant, if force increases, acceleration also increases • When force is constant, if mass increases, acceleration decreases

  5. Force, mass, and acceleration • This is how Newton developed his Second Law of Motion: • “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.”

  6. Force, mass, and acceleration • We often just express this in equation form: a = F/m, or more often, F = ma • The force on an object is equal to the product of its mass and its acceleration • Note that force is a vector; its direction is the same as the acceleration.

  7. Force, mass, and acceleration • Our unit for force is the Newton. 1 Newton is the force it takes to give a 1 kg mass an acceleration of 1 m/s2 • So, 1 N = 1 kg*m/s2 • If we have a 500 kg mass with an acceleration of 3 m/s2, the force causing it to move must be F = (500 kg)(3 m/s2) = 1500 N • If we want our answer to be in Newtons, we need mass in kg and acceleration in m/s2

  8. Force of Gravity • Since we know the acceleration due to gravity on Earth, we can calculate the force of gravity on any mass. This is the same thing as its weight. • For example, if we have a 200 kg mass: • F = ma = (200. kg)(9.80 m/s2) = 1960 N • Likewise, if we knew the force of gravity on an object, we could work backwards to find its mass.

  9. Weight Lab • First, answer the pre-lab questions #1-5 at the top of the page • Then, follow the procedure written on the bottom half of the page. • Note: There are only three scales. You will need to share. • When you finish, answer the questions on the back side, including the conclusion about the relationship between mass, weight, and force (this can probably be done in 2-3 sentences)

  10. Mass Lab • First, answer the pre-lab questions #1-7 at the top of the page • Then, follow the procedure written on the bottom half of the page. • Measure the mass of each object using an electronic balance • Measure the weight of each object using a spring scale. • NOTE: Instead of a metal block for the second object, use one of the metal ramps from our bulls eye lab

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