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The Law of Inertia

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Objects at rest remain at rest unless acted upon by an outside force. Objects in motion will remain in motion unless acted upon by an outside force. What causes the tire to stop rolling?

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Inertia is dependent on mass. The larger the mass, the more inertia it has. I’m not moving till you explain this report card!

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A force is a push or pull. Forces can change the motion of an object.

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Force is measured in Newtons. (appropriately named!) One Newton = the amount of force needed to accelerate 1 kilogram by 1 meter per second squared. 1 kg a = 1 m/s 2

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Weight – is a force. It is the force of gravity acting on an object’s mass. Weight (in newtons) = mass x acceleration due to gravity. example: 1 kg x 9.8 m/s 2 = 9.8 Newtons Calculate your weight in Newtons!

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Friction – is a force caused by objects touching that opposes motion, but is necessary. Friction helps go forward and stop Friction helps us stand Friction helps us walk and run

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Force is a vector quantity. 100 Newtons The length of the arrow shows the strength of the force. The direction shows the direction that the force is applied. Force vectors are added and subtracted just as velocity vectors are… The sum of the force vectors (the resultant) is called the net force.

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If the net force is zero, there is no movement. The system is at equilibrium. = The forces are balanced. An unbalanced force results in movement in the direction of the larger force.

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Consider the forces on this box: Fw = weight of the box Fn = Normal force (always acts perpendicular to the surface)

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If the box has a force applied to it, there are two additional forces. Fw = weight of the box Fn = Normal force (always acts perpendicular to the surface) Fa = Applied force Ff = Friction force

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Coefficient of Friction Each combination of surfaces that come in contact with each other have their own value for friction. This is called the coefficient of friction. The coefficient of friction ( ) can be calculated by: = Ff Fn

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Example problem involving friction: The box is placed on a smooth wooden table. A force of 14 N is necessary to to keep the box moving at a constant velocity. If the box has a weight of 40 N… 1.Draw the forces effecting the box. 2.What is the coefficient of friction?

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This 52 N saucer sled is pulled across a cement sidewalk with a force of 18 N. What is the coefficient of friction between the sled and the sidewalk?

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The 52 N saucer sled is now placed on snow and a 450 N boy sits on it. The coefficient of friction for the snow and metal sled is 0.012. What force is necessary to pull the sled at constant speed? (Hint: the applied force is equal, but opposite direction to the force of friction.)

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Newton’s second law explains the relationship between the net force on an object, it’s mass and the acceleration that results from the force:

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Let’s look at the relationships: If mass is constant, the larger the force on an object, the faster the objects accelerates. Large force = mass x large acceleration Small force = mass x small acceleration This is a direct proportion.

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force = small mass x large acceleration force = large mass x small acceleration Let’s look at the relationships: If the force is constant, the larger the mass of an object, the slower the object accelerates. This is an indirect or inverse proportion.

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Let’s look at the relationships: If the acceleration is constant, the larger the mass of an object, the stronger the force must be to change the motion. small force = small mass x acceleration large force = large mass x acceleration This is a direct proportion.

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Solving problems related to Newton’s second law: The basic equation is: Force = mass x acceleration or F = ma a = f / mm = f / a Try this: Rearrange the variables so that you can solve for acceleration or mass.

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For Practice: Example 1 How much force must a 30,000 kg jet plane develop in order to achieve an acceleration of 1.5 m/s/s?

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Example 2 What acceleration is produced by a force of 500 N applied to a 1000 kg car?

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Example 3 If a grocery cart is pushed with a force of 75 N, and the cart accelerates from rest at a rate of 1.5 m/s/s, what is the mass of the cart? What is the weight of the cart?

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