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Force and Newton’s Laws
Vocabulary
•____________- A push or pull on an object.
•Net Force – vector____________of all forces
•Equilibrium:
–the state in which the net force on an object is ____________ or ____________
–no change in velocity
Free Body Diagrams
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FBD’s are a neat way to show the forces acting on an object.
The object is drawn as a ______ or a ______, and then vectors are drawn to show the forces acting on it.
Can you draw the FBD for you sitting on your chair?
Draw the FBD for your friend pushing you in a rolling chair.
The picture at the bottom shows a snowman pulling a sled. Draw a free-body diagram for this sled. The
magnitudes of the forces acting on the sled are 60N by the string, 130N from gravity, and 90N upward
by the ground.
1st Law of Motion - (Law of Inertia)
 An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless
acted upon by an unbalanced force.
 Translation: Objects____________doing what they are doing, in a ____________line.
 Inertia is a measurement of ______________ TO CHANGE
 It is measured in terms of ____________. The greater the ____________, the greater the inertia.
 What is Mass?
 Mass is the amount of ____________ a body/object contains
 Bigger does not equal more mass. ________ is means size.
2nd Law
 The net force of an object is equal to the product of its mass and acceleration, or F=ma.
 So, a little easier to read would be: ____________
 F = _________
m = __________
a = ___________
 Forces are measured in ____________
o N = (kg)(m/s2).
 What does F = ma mean?
o Force is ____________ ____________to mass and acceleration.
 If you double the mass, you ____________the force. If you double the acceleration, you ____________
the force.
 What if you double the mass and the acceleration?
 (2m)(2a) = 4F
o Doubling the mass and the acceleration ____________ the force.
o So . . . what if you decrease the mass by half? How much force would the object have now?
 What does F = ma say?
o F = ma basically means that the force of an object comes from its mass and its acceleration.
o Something very ____________ (high mass) that’s changing speed very slowly (low
acceleration), like a glacier, can still have ____________ force.
o Something very ____________ (low mass) that’s changing speed very quickly (high
acceleration), like a bullet, can still have a great force. Something very small changing speed
very slowly will have a very weak force.
Gravity causes weight
 The ____________ of an object is caused by the force of attraction between the earth and objects on the
surface of the earth.
 How much does a person with a mass of 70.0 kg weigh on the earth?
Let’s practice
You have run out of gas and have to push your car. Your car weighs 560 N and you push with a force of 395 N,
and your friend helps with 275 N. What is the acceleration of the car?
3rd Law
• For every action, there is an ____________ and ____________ reaction.
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What does this mean?
o For every force acting on an object, there is an equal force acting in the opposite direction.
o Right now, gravity is pulling you ____________in your seat, but Newton’s Third Law says your
seat is pushing up against you with ______ ______. This is why you are not moving.
o There is a ____________ force acting on you– gravity pulling down, your seat pushing up.
o Put really simply: F = F
Think about it . . .
o What happens if you are standing on a skateboard or a slippery floor and push against a wall?
You slide in the opposite direction (away from the wall), because you pushed on the wall but the
wall pushed back on you with equal and opposite force.
o Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the
rock exerts an equal force back on your toe. The harder you hit your toe against it, the more
force the rock exerts back on your toe (and the more your toe hurts).
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It requires two forces:
o _____________ Force
o ______________ Force
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Does it matter which force we call the ________ and which is the _________?
Forces always occur in ________. Therefore, it ________ matter which is the action and the reaction, as
long as you can identify both.
Forces
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Categories of forces
o ____________
o Non-Contact
Contact
o Normal
o ____________
o Tension
o Air Resistance
o ____________
Non-Contact
o Electrical
o ____________
o Nuclear
The Normal Force
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The normal force (FN) is one component of the force that a __________ exerts on an object with which
it is in __________
Simply, the component that is ____________ to the surface.
is __________ and __________ to the weight of an object if the object is on a horizontal surface.
The Fn is not always __________ .
The Fn can be found by __________ (where Θ is the angle between the contact surface and horizontal)
What is Friction?
 is a force __________ the relative motion of two surfaces in __________ with each other.
 is the parallel component of the force on an object that is in contact with a surface.
 is a force that __________ motion from occurring.
What causes friction?
 There are two factors which affect friction between two surfaces:
o Kind of surfaces in contact (__________ or __________)
o Amount of force __________ the surfaces together.
 The rougher the surface and the stronger the force between the surfaces, the __________ the amount of
friction.
Static Friction
 force that acts on objects that are __________ moving
 is the force that __________ the start of motion.
With no movement,
__________
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 Has a range of 0 – Fs,max
Kinetic Friction
 Friction force that __________ the direction of __________ of an object as it slides over a surface.
 Once motion has begun, the static friction force is __________ a concern.
 Instead, kinetic friction is important.
 Kinetic friction is __________ to movement of an already moving object.
Fluid Friction
 Friction force that opposes the motion of an object through a fluid.
Frictional Forces
 The frictional force will __________ depending on the surface interactions.
 Ex. Carpet vs tile
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Friction can be calculated by using the __________ __ __________.
Use the basic formula of:
o Ff = μFn
It can be rearranged to:
o μk = (Fk/Fn) and μs = (Fs/Fn)
o Ff = ______________ μ = _____________ Fn = ______________
A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion.
Find the coefficient of static friction between the crate and the floor.
Force of Tension
 Tension commonly means the tendency of a rope to be pulled apart.
 Tension is typically the __________ everywhere in a rope because the rope is treated as __________.
If it isn’t (and is vertical) you must account for the mass of the rope.
Terminal Velocity
 The velocity at which a __________ __________ object ceases to accelerate.
 T.V. is the constant velocity of a falling object when the force of gravity __________ the force of air
resistance
Spring Force
 Play safely!!
Electric Forces
 forces that act between charged objects or particles such as protons and electrons
Magnetic Forces
 Forces that act on certain metals, poles of magnets, and on moving charges
Nuclear Forces
 Nuclear forces act over very short distances and are what hold the particles of an atomic nucleus
together.
1) A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the
coefficient of static friction between the crate and the floor.
Quick Review (Net Force)
2) Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. There is a frictional
force of 11N, a normal force of 18N and gravitational force of 22N. What is the net force?
On your own!! (Net Force)
3) A student is designing a support to keep a tree upright. Two wires have been attached to the tree and placed
at right angles to each other. One wire exerts a force of 30N on the tree; the other wire exerts a 40N force.
Determine where the third wire should be placed and how much force is should have so that the net force acting
on the tree is zero.
4) A stationary, flying kite is acted on by a force of 9.8N downward. The wind exerts a force of 45N at an angle
of 50° above the horizontal. Find the force that the string exerts on the kite.
Putting it all together!!
5) A student attaches a rope to a 20kg box of books. He pulls with a force of 90N at an angle of 30° with the
horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.50. Find the acceleration of
the box.