Transcript Forces and Newton`s Laws

ICP
“Forces and the Laws of
Motion”
Students who demonstrate understanding can:
Analyze a major global challenge to specify qualitative and
HS-ETS1quantitative criteria and constraints for solutions that account for
1.
societal needs and wants.
Design a solution to a complex real-world problem by breaking it
HS-ETS1down into smaller, more manageable problems that can be solved
2.
through engineering.
Evaluate a solution to a complex real-world problem based on
HS-ETS1- prioritized criteria and trade-offs that account for a range of
3.
constraints, including cost, safety, reliability, and aesthetics as
well as possible social, cultural, and environmental impacts.
Use a computer simulation to model the impact of proposed
HS-ETS1- solutions to a complex real-world problem with numerous criteria
4.
and constraints on interactions within and between systems
relevant to the problem.
What is a force?
A
force will change the state of motion of
an object, a push or pull.
 Newton’s second law defines a force as the
product of a body’s mass and acceleration.
 F = ma
Types of Forces

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
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1. Electric Forces -- forces caused by the interaction of electrons. Most
forces we deal with are electric. For example, when you push an object
you are creating electric forces between the electrons in your hand and the
electrons in the object. Mechanical and frictional forces are electrical
forces.
2. Gravitational forces -- The force of attraction between masses.
Gravitational forces are the weakest of all forces. But gravitational forces
act over very long distances.
3. Magnetic forces -- forces produced by moving electric charges.
Magnetic forces are closely related to electric forces but the relationship is
not completely understood at present.
4. Nuclear forces -- forces within the nucleus which hold particles
together. Nuclear forces are the strongest of the known forces but they act
over the shortest distances.
5. Weak Interaction forces -- the forces which is believed to cause atoms
to break apart.
Changes in Motion
 1.
A net force will change the state of
motion of an object.
 2. Forces can be exerted through long
distances.
 3. Forces always occur in pairs.
 4. In each pair of forces, one acts opposite
the other.
Units
System
Mass
Acceleration
Force
SI
kg
m/s2
N- kg m/s2
cgs
g
cm/s2
dyne-g cm/s2
Avoirdupois slug
ft/s2
lb-slug ft/s2
Forces are vectors, the product of a vector and a
scalar.
Newton's Laws of Motion
 Newton's
First Law of Motion ( Law of
Inertia )

An object at rest or in motion will
remain so until acted upon by an outside
(net) force.
 inertia - the resistance of a body to
change its state of motion.
Newton's Second Law of Motion
When an unbalanced force acts on an
object, the object will be accelerated. The
acceleration will vary directly with the
applied force and inversely with the mass of
the object.
 Equation:
F = ma

Sample Problem
A
5.0kg watermelon is pushed across a table. If
the acceleration of the watermelon is 4.0m/s2 to the
right, find the net external force exerted on the
watermelon.
Newton's Third Law of Motion
 For
every action there is an equal and
opposite reaction.
 This is the law of interaction.
 Page 103; 1-7
Everyday Forces
 Mass
depends on the amount of matter a body possesses.
 Weight depends on the distance an object is from the
center of the earth.
 A question for thought; If an object fell down a very deep
hole, is there a point at which the object would become
weightless?
 F = ma Fw = mg
Fw = weight, g = 9.8 m/s2 , etc.
Sample Problems
 Find
the weight of a 2 kg object.
 Find the mass of an object which weighs 49N.
Friction
 Friction
is the force which opposes the motion of
two surfaces in contact.
 Static friction is the force which opposes the start
of motion.
 Sliding friction is the force which opposes motion
in progress. Sliding friction is much less than static
friction.
Open Response
A
boat crosses a stream.
 A. Identify two forces that may act on the
boat.
 B. Using a vector diagram, describe the
effect of these forces acting on the boat.
Experiment
 My
Earthly Attraction
 1. Find your weight, or the weight of an 80Kg person, in
newtons, N. F = mg, g = 98.m/s/s.
 2. Find the Gravitational Force between you and a
person sitting one meter from you.

m1m2
 F= G ----------
r2
 3. Find the Gravitational Force between you and the
earth.
Experiment: Paper Tower
 You
will be given the following material:
1 sheet of 8 X 10 paper
50cm of tape
one ruler
one pencil
one pair scissors
 Purpose: Construct a tower of paper and tape of
maximum height.