Download AP Physics – Newton`s Laws – Force and Motion Types of Forces

AP Physics – Newton’s Laws – Force and Motion
Types of Forces
Contact forces:
Sample Problem: Three forces act upon a 3.0 kg
body moving at constant velocity. F1 = (4i – 6j +k) N and F2
= (i – 2j - 8k) N. Find F3.
Field forces:
Equilibrium
Definition:
Sample Problem: Two forces, F1 = (4i – 6j + k)
N and F2 = (i – 2j - 8k) N, act upon a body of
mass 3.0 kg. No other forces act upon the body
at this time. What do you know must be true?
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
Define Inertia:
Sample Problem: A tug-of-war team ties a rope
to a tree and pulls hard horizontally to create a
tension of 30,000 N in the rope. Suppose the
team pulls equally hard when, instead of a tree,
the other end of the rope is being pulled by
another tug-of-war team such that no movement
occurs. What is the tension in the rope in the
second case?
Mass:
Weight:
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AP Physics – Newton’s Laws – Force and Motion
Problem: determine the normal force acting on a 5.0 kg
box sitting on a flat table.
1st or 2nd Law Problems
1. Identify the system to be analyzed. This
may be only a part of a more
complicated system. This is really the
most important step.
2. Select a reference frame or coordinate
system, stationary or moving, but not
accelerating, within which to analyze
your system.
3. Identify all forces acting on the system.
Ignore the others. This is best done by
drawing a Free Body Diagram!
4. Set up F = ma equations for each
dimension. Solve for relevant
unknowns.
5. Use kinematics or calculus where
necessary to calculate more about the
motion.
Problem: Now determine the normal force acting on a
5.0 kg box sitting on a flat table.
Sample Problem: A 5.00-g bullet leaves the
muzzle of a rifle with a speed of 320 m/s. The
bullet is accelerated by expanding gases while it
travels down the 0.820 m long barrel. Assuming
constant acceleration and negligible friction,
what is the force on the bullet?
Problem: Now determine the normal force acting on a
this 5.0 kg box sitting on a ramp at angle =30o.
Sample Problem: A 3.00 kg mass undergoes an
acceleration given by a = (2.50i + 4.10j) m/s2.
Find the net force F and its magnitude.
Normal force
Definition:
Always calculate the normal force by
considering all forces that have components
perpendicular to a surface.
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AP Physics – Newton’s Laws – Force and Motion
Sample problem: derive a formula for acceleration,
assuming the table is frictionless.
Tension
Definition:
Why can we depict tension as pulling in opposite
directions on two different connected objects?
Problem: What is the tension is the cable attached to a 5,000
kg elevator that starts on the ground floor at rest and
accelerates upward, reaching a speed of 3.0 m/s in 2 seconds?
Sample problem: derive a formula for the tension T in the
Sample problem: derive a formula for acceleration, assuming the
table is frictionless
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AP Physics – Newton’s Laws – Force and Motion
Atwood machine
Definition:
Friction
Definition:
Problem: For the Atwood machine shown, derive
an equation which can be used to find g, the
gravitational acceleration, from a measured value of
What is static friction?
What is kinetic friction?
Problem: How high up the frictionless ramp will the block slide?
Problem: Calculate acceleration of the 5 kg block. Table and pulley are
magic and frictionless. Calculate tension in the string.
Why is it disadvantageous for cars to skid to a
stop?
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AP Physics – Newton’s Laws – Force and Motion
Draw free body diagrams that include friction for a body
which is
Problem: Assume a coefficient of static friction
of 1.0 between tires and road and a coefficient of
kinetic friction of 0.80 between tires and road.
How far would a car travel down a 15o incline
after the driver applies the brakes if it skids to a
stop? Assume a speed before brakes are applied is 26 m/s.
Sample problem: A 1.00 kg book is held against a wall by
pressing it against the wall with a force of 50.00 N. What
must be the minimum coefficient of friction between the book
and the wall, such that the book does not slide down the wall?
Centripetal Force
Definition:
Examples:
Problem: Assume a coefficient of static friction
of 1.0 between tires and road. What is the
minimum length of time it would take to
accelerate a car from 0 to 60 mph?
How to begin centripetal force problems:
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AP Physics – Newton’s Laws – Force and Motion
Problem: Find the minimum safe turning radius
for a car traveling at 60 mph on a flat roadway,
assuming a coefficient of static friction of 0.70.
Problem: You swing a 0.25-kg rock in a vertical
circle on a 0.80 m long rope at 2.0 Hz. What is
the tension in the rope a) at the top and b) at the
bottom of your swing?
Problem: Derive the expression for the period
best banking angle of a roadway given the radius
of curvature and the likely speed of the vehicles.
Problem: A 40.0 kg child sits in a swing
supported by 3.00 m long chains. If the tension
in each chain at the lowest point is 350 N, find a)
the child’s speed at the lowest point and b) the
force exerted by the seat on the child at the
lowest point.
Non-uniform Circular Motion
Characteristics:
Problem: A 900-kg automobile is traveling
along a hilly road. If it is to remain with its
wheels on the road, what is the maximum speed
it can have as it tops a hill with a radius of
curvature of 20.0 m?
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