Forces - Denton ISD [PDF]

Forces Newton’s Laws of Motion

Free Body Diagrams      

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In solving any problem, first draw a model. Circle the “system” Identify contact forces on the system. Identify Long-range forces on the system. Replace the object by a dot. Use arrows to represent forces (mag and direction) acting on the “dot”. Label the forces F with a subscript to identify the agent acting on the object. (Fg = Force due to earth’s mass)

Force and Motion 

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A force is a push or pull that gives energy to an object. In any interaction between an object and its surroundings, the object is considered the “system” and the surroundings that exert forces are the “environment”. A Force has magnitude AND direction so it is a vector quantity.

Force Types 

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Contact force- acts on an object by touching it. Long-range force: acts on an object without touching it. Object is in contact with a force “field” such as the force of gravity. All forces have “agents” that we can name. The agent is the cause of the force. If you can’t name it, it doesn’t exist!

Drawing Free Body Diagrams 

Draw a free body diagram and label the forces for: A book resting on a desk A child sitting on a swing (stationary) (same child when swing strings break) A ball held in your hand A plate being pushed across a table

Newton’s Second Law of Motion 

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Study forces acting on an object horizontally by minimizing friction (ice, wheels, air table…) (GRAVITY does NOT act horizontally) A FORCE is a push or pull that gives Energy to an object. What can that energy do? Change the motion of the object. What do we call a “change in motion of an object?”………….ACCELERATION! Larger the force, the greater the acceleration.

F

a

How does acceleration depend on the object? Acceleration is directly proportional to the NET FORCE on an object… (More push, more acceleration…)  What if the MASS changes for that force? If mass increases, then acceleration is less and vice versa for a decrease in mass. 

What about multiple forces? 

Forces are vectors so their tail is on the dot and they point in the direction of the force. (Remember free body diagram?) F1

F2 Fnet

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The total force on an object is the vector sum of all forces exerted on the object. This is the NET FORCE.

Sum it all up 

Acceleration is proportional to the net force on an object and inversely proportional to the mass of the object. So,

a 

Fnet mass

This is Newton’s Second Law of Motion.

Problem solving  

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Identify all of the forces acting on the obj. Draw a free-body diagram showing direction and magnitude of each force on the system Add the force vectors to find Fnet. Use Newton’s second law to calculate acceleration. Use kinematics to find velocity and position of the object. What does an unbalanced force do?

Units: What’s up, fig Newton? 

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One unit of force causes a 1-kg mass to accelerate at 1m/s2. F = m x a therefore a unit of force is derived from a kg x m/s2. This is a Newton (N)

F

a

m

Your turn to Practice 

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Two horizontal forces, 225N and 165N are exerted in opposite directions on a crate with a mass of 5 kg. What is the net horizontal force on the crate? What is the net vertical force on the crate? What is the acceleration of the crate?

Solution

-165N

FN

Ff

225N FA

Fg

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Fnet = 225 N – 165 N = 60 N toward the 225 N force. Net vertical force = weight – support force = zero. (Not moving up or down) Acceleration = Fnet / mass a = 60 N / 5 kg a = 12 m/s2

Really your turn       

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Open your book to page 122. Do Practice Problems # 4, 5, & 6 Read p 122-123 about Newton’s First Law Why is this the law of balanced forces? What is inertia? What is equilibrium? Pay attention to the misconceptions about forces on p 125. Do Ch 6 Rev #s 1,2,3,20-23