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Write down everything you know about force.
Things you may want to include:
◦ Definition
◦ Formula
◦ SI Units
◦ Examples

Chapter 4

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A force is a push or pull
Forces are vectors
Represented by “
𝑭
”
 kg
 m
Has SI units of Newtons
N s
2
When all forces on an object are added, the resultant is called a net force.
A non-zero net force causes an acceleration

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𝐹 𝑔
: Force of gravity; weight – down towards center of Earth
𝐹
𝑁
: Normal force – perpendicular to surface object is on
𝐹 𝑓
: Friction – opposes motion when two surfaces are in contact with each other
𝐹
𝑇
𝐹
: Tension – in direction of string/rope/chain drag
: Drag – opposes motion when an object moves through a gas or liquid (i.e. air resistance)

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Vectors have a magnitude and direction
Vectors can be depicted by an arrow
When drawing multiple vectors, their arrow lengths should be proportional to the vector magnitudes

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Draw the object or a dot to represent the object.
Draw an arrow for each force on the object
◦ Arrows should start at the dot and point away
◦ Arrow lengths should be proportional to force magnitude. For unknown forces, estimate length
3.
4.
Label each arrow
Choose a direction to be positive and indicate it on the side of the diagram
◦ If object is moving, have positive direction be in direction of motion.
◦ For 2-D, show both positive directions.

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A book is sitting at rest on a table. Draw a free-body or force diagram for the book.

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A box is pushed along the floor at a constant speed. Draw a free body or force diagram for the box.

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A rope lifts a bucket at a constant speed.
Draw a free body or force diagram for the bucket.

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A student is pushing against a table, but not hard enough to move it. Draw a free body or force diagram for the table.

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Power Tower is a thrill ride at Cedar Point, the roller coaster capital of the world.
At the beginning of the ride, it accelerates the riders upwards.
Draw a free body diagram of a rider at the beginning of a
Power Tower ride.

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Same process as adding velocity vectors.
The vector sum is called the net force,
𝐹 𝑛𝑒𝑡

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A canoe is sitting on frictionless ice. Two horizontal forces, 273 N and 131 N, are exerted on the canoe in the same direction.
Draw a free body diagram and find the net horizontal force on the canoe.

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Now imagine the same two forces are acting in opposite directions. Draw a free body diagram and find the net horizontal force on the canoe.

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Al and Bob are playing tug-of-war (two teams pulling on a rope in opposite directions) against
Chris and Dan. Al pulls with a force of 127 N,
Bob pulls with a force of 93 N, Chris pulls with a force of 104 N, and Dan pulls with a force of
119 N. Draw a free-body diagram and calculate the net force on the rope.

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An object at rest will stay at rest unless acted upon by a non-zero net force. An object in motion will stay in motion unless acted upon by a non-zero net force.
The acceleration of an object is equal to the net force acting on the object divided by the object’s mass.
For every force, there is an equal and opposite force.

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The amount of matter in an object.
Represented by “ 𝑚
”
SI unit of kilograms (kg)

F net
 ma

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The driver of a car hits the accelerator and causes a net force of 3.00 x 10 3 N. If the car has a mass of 1.70 x 10 3 kg, what is the resulting acceleration?

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A chair is pushed horizontally with a force of
93 N, and experiences a friction force of 11 N.
If the chair accelerates at 1.7 m/s 2 , what is the mass of the chair?

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Free-Body Diagrams worksheet
◦ On website
◦ Due Friday
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You have the rest of class to either work on the worksheet or prepare for the projectile motion test.

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A 2.5 kg rock is falling off the edge of a cliff.
Gravity is pulling it down with a force of 24.5 N.
There is a drag force of 7.0 N. Draw a force or free-body diagram, and solve for the acceleration of the rock.

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Near the surface of Earth, 𝑔 = 9.8 𝑚/𝑠 2
An object in free fall will have this initial acceleration
The weight of an object is given by
𝐹 𝑔
= 𝑚𝑔

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A skydiver with a mass of 73 kg free-falls. At one point, he has an acceleration of 8.3 m/s 2 .
What is the force due to air resistance at this point?

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A rope is pulling a bucket of water out of a well. The bucket of water has a total mass of
25 kg. What must the tension force be in order to accelerate the bucket at 0.60 m/s 2 .

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Forces are balanced if there is zero net force
Can be for just 𝑥 direction, just 𝑦 direction, or both.
Results in zero acceleration ( direction
𝐹 net
= 𝑚𝑎
) in that
Zero acceleration  constant velocity
That velocity could be zero (no motion)

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Forces are unbalanced if there is a non-zero net force.
Arrows are different lengths.
Results in an acceleration in the direction of the larger force.

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If all forces are balanced (in both 𝑥 and 𝑦 directions), then the system is in equilibrium.
There is zero acceleration
Zero acceleration  constant velocity
That velocity could be zero (no motion)

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𝐹 𝑑𝑟𝑎𝑔
= −𝐹 directions) 𝑔
(Equal magnitudes, opposite
Zero net force  zero acceleration  constant velocity
The fastest an object can free-fall

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Forces Situations Worksheet
◦ On web page
◦ Due Tuesday

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A crane attached to a chain is being lowered.
If the crane has a mass of 105 kg and is accelerating downward at a rate of 2.45 m/s 2 , what is the tension force between the chain and crane?