unit 4 forces and the laws of motion. 4-1 forces force: a push or pull exerted on an object forces...
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Unit 4 FORCES AND THE LAWS OF MOTION
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4-1 Forces
• Force: A push or pull exerted on an object
• Forces cause a change in velocity (acceleration)
*cause a stationary object to move
*cause a moving object to stop
*cause an object to change direction
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• SI Unit for Force is the Newton• Newton: amount of force that will
accelerate a 1 kg mass by 1 m/s2
1 N = 1 kg·m/s2
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• Forces can act through contact or at a distance.
• Contact forces: result from physical contact between 2 objects (kicking a ball)
• Field forces: do not involve physical contact (example: gravity, magnetism)
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• Force is a vector quantity. (has magnitude and direction)
• Force diagram: shows all forces in a situation (objects are treated as points)
• Free-body diagram: shows only forces on a single object of interest.
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A force diagram (a) shows all forces acting in a situation
A free body diagram (b) shows only those forces acting on an object of interest (the car)
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4-2 Newton’s First Law of Motion
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4-2 Newton’s First Law
• Newton’s First Law (Law of Inertia)
• An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force.
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• Inertia: The tendency of an object to maintain its state of motion. (that is, a constant speed in a straight line)*If net external force is zero, then acceleration is
zero.
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• External force: a single force that acts on an object.• Net external force: vector sum of all the forces
acting on an object.• An object’s acceleration is the same as if the net
external force were the only force acting on the object.
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• Mass is a measure of inertia.– (The greater the mass, the less the body accelerates under an applied
force.)
• Objects in motion tend to stay in motion.
• Objects that are at rest or moving constantly tend to stay at rest and are at equilibrium. (net force must be zero)
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If no seat belt, no force to hold person when car collision occurs.
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Practice Problem 1• Steve is pulling a wagon with a force of 100 N directed at an angle of +40 to the horizontal.
a. Draw a free body diagram showing all the forces
b. Find the x and y components of this force (using trig).
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Practice Problem 2• A box is pulled to the right with a force of
150 N, to the left with a force of 225 N, upward with a force of 520 N and downward with a force of 321 N.
a. Draw a free body diagram showing the forces
b. Find the net external force in the x direction
c. Find the net external force in the y direction
d. Find the magnitude and direction of the net external force on the box
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4-3 Newton’s Second and Third Laws
• Newton’s Second Law:
• The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass.
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• Newton’s Third Law:
• For every action there is an equal and opposite reaction.
• When two forces interact with one another, the forces they mutually exert are called an action-reaction pair
• Field forces also exist in pairs.
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Practice Problem 3• A 6.0 kg object undergoes an acceleration of 2.0 m/s2
a) What is the magnitude of the net external force acting on it?
b) If the same force is applied to a 4.0 kg object, what will be the acceleration?
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Practice Problem 4• Identify the action-reaction pairs in the following situations
– Person takes a step– Snowball hits someone in the back– Baseball player catches a ball– Gust of wind strikes a window
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4-4 Everyday Forces• Weight (Fw or Fg): force acting on a mass toward center of Earth due to gravity.
• Normal force (FN): a force exerted by one object on another in a direction perpendicular to the surface of contact.
• Mass is a measure of the amount of matter in an object
• Weight is a measure of the force of gravity on an object.
• Because weight is a measure of force, we use Newtons to measure weight in Physics.
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Converting Mass to Weight
Remember: F=ma. • Therefore, to convert mass (kg) to weight
(Newtons), one must multiply the mass of an object by 9.8 m/s2.
• To convert weight to mass, one must divide the weight of an object by 9.8 m/s2.
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Converting Mass to WeightPractice Problem 5: A dog weighs 195 Newtons. What is its mass in kg?• 195 Newtons / 9.81 m/s2 = 19.9 kg
Practice Problem 6: Susan has a mass of 50.0 kg. What is her weight in Newtons?• 50.0 kg x 9.81 m/s2 = 491 Newtons.
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Tension and Compression• Tension is the magnitude of the pulling force exerted by a
string or rope on another object, and is the opposite of Compression
• When two objects connected by a rope are pulling in opposite directions, the pulling forces are added to determine tension
• Practice Problem 7. If RJ and Billy are playing tug of war with a rope and each is pulling with a force of 2000 Newtons, what is the tension on the rope?
• 2000 N + 2000 N = 4000 N
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Friction
• Force of friction (Ff): force that opposes motion.
• Caused by the attractive forces of two surfaces in contact
• Friction is necessary for life because we depend on it to hold things together.
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• Static friction: resistive force that keeps an object from moving when object is at rest.
• Kinetic friction: resistive force that opposes an object that is moving (slows it down)
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• Frictional forces (Ff) are proportional to normal force FN. (heavier an object is, the greater the friction)
• Friction depends on the surface.
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• The coefficient of friction is a ratio of forces.
µk = coefficient of kinetic friction
Fk = kinetic force
FN = normal forcen
kk F
F
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Coefficient of Static Friction: Is the ratio of the maximum value of the force of static friction to the normal force
µs = coefficient of static friction
Fs = static force
FN = normal forcen
ss F
F max,
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Practice Problem 8
• A 50 kg crate at rest on a horizontal floor requires a 150 N horizontal force to set it in motion. What is the coefficient of static friction between the crate and the floor?
31.0491
150
/81.950
1502
max,
N
N
smkg
N
F
F
n
ss
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• Air resistance is a form of friction.
• Free-falling objects experience air resistance.
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• If air resistance = weight of object then the object reaches terminal velocity
• When an object is at terminal velocity, there is: no net force, no acceleration, The object is at maximum constant velocity.