Isaac Newton: 1600’s, England
Force: A push or pull between pairs of objects
Mass is a measure
of resistance to
acceleration
Mass (inertia) vs weight
How do astronauts know which chocolate milk carton
is empty?
Mass m: a measure of _________________________
Weight W =
Newton’s second law
The acceleration of an object is _________________ to the total
force on it, and __________________________ to the mass:
a=
F=
Unit of force: In terms of a 1 kg mass…
1 Newton = 1 kg *1 m/s2
(1 Newton = 0.2248 lb)
Newton’s second law
Normal force N: perpendicular
to surface
Friction force f: parallel
Newton’s second law
Mary has a mass of 40 kg (weight = 392 N), and stands on a
scale in the elevator. The elevator accelerates downward at 2
m/s2. What is the scale reading?
(draw FBD)
Forces are vectors! Always draw a free-body diagram
Free-body diagrams
1. Draw the object of interest alone (you can combine several objects
as a “single object” if they have the same a).
2. Draw forces on the object exerted by other, external things
3. Label each force with a different symbol
4. Choose a positive direction (usually in the direction of a) for each
component
5. Draw the vector a on the diagram, but not on the object (it’s not a
force)
6. For the FBD you can write
SFx = max and SFy = may
(m is the mass of the object in the diagram)
Newton’s second law
Kinetic and static friction
Kinetic and static friction
Static friction
Static friction exactly cancels sideways force, until….
Static friction
The 70 kg block is at rest. The static coefficient of friction
is 0.6. What is the acceleration if you push with:
a) 200 N
b) 400 N
c) 600 N
P1. A monkey starts to slide down a rope. As it speeds up,
it tightens its grip, until it slides at a constant velocity
down the rope. Which of these choices is true as it slides
at constant speed?
a) The gravitational force is equal (in magnitude) to the
frictional force.
b) The gravitational force is greater equal (in magnitude)
than the frictional force.
c) The gravitational force is less than the frictional force
(in magnitude).
P1. A monkey starts to slide down a rope. As it speeds up,
it tightens its grip, until it slides at a constant velocity
down the rope. Which of these choices is true as it starts
to speed up?
a) The gravitational force is equal (in magnitude) to the
frictional force.
b) The gravitational force is greater equal (in magnitude)
than the frictional force.
c) The gravitational force is less than the frictional force
(in magnitude).
Ping-pong ball cannon
A 2-gram (0.002 kg) ping-pong ball is pushed 3 meters along an
evacuated tube by a constant force of 40 N from air behind it.
What is the final velocity?
Forces on object in equilibrium
100N
100N
Find the weight of the hanging mass, given 100 N tension in the right rope.
P2 Knowing that the T on the right rope is 100 N find T2.
a) 0 to 40 N
b) 40 to 80 N
c) 80 to 120 N
d) 120 to 160 N
e) more than 160
Forces on object in equilibrium
T2
T2
Find the tensions if the weight (W) of the boy is known.
P3 Write two equations that you could solve to find T1 and T2.
a) I did my best and checked with a classmate
b) I did my best alone
Lecture 5, Second law and friction
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Basic concepts:
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Basic problems, skills:
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Advanced concepts:

Advanced problems, skills:
◦ mass as resistance to acceleration
◦ weight vs. mass
◦ breaking static friction
◦ F = ma in one dimension
◦ friction from normal force
◦ free body diagrams
◦ effect of initial vy
◦ F = ma for both x and y directions
◦ Normal force often does not equal the weight
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◦
◦
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Normal force from free body diagram
Using symbols instead of numbers in setting up problems and doing algebra
Equilibrium with ropes and cables at angles
Solving 2 equations simultaneously for two unknowns