Download Unit 8 Student Notes

Newton’s Laws of Motion
Motion is relative. When discussing motion, we mean motion relative to something else. When we say a space shuttle moves at
30,000 kilometers per hour, we mean relative to the Earth below. When we say a racing car reaches a speed of 300 kilometers per
hour, we mean relative to the track. Unless stated otherwise, all speeds discussed in this class are relative to the surface of the Earth.
Newton’s 3rd Law
Newton’s 3rd Law of Motion – For every action force there is an equal and opposite reaction force.
Force – A push or a pull that causes an object to move, change speed, change direction, or stop.
Newton’s 3rd Law of Motion – For every action force there is an equal and opposite reaction force.
When you lean against a wall, you exert a force on the
wall. At the same time the wall exerts an equal and
opposite force on you. That’s why you don’t topple
over.
The balloon recoils from the escaping air and climbs upward.
The rocket recoils and raises from the “molecular cannonballs” it fires.
Newton’s 2nd Law
Newton’s 2nd Law of Motion – An object will accelerate (proportionately to the unbalanced force) in the direction in which the
unbalanced force is applied.
 The greater the force the greater the acceleration. We say that the acceleration produced is directly proportional to the net force.
Acceleration ~ net force
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The more massive the object, the greater the unbalanced force that is needed to cause the object to accelerate.
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Mass is also a measure of how much material an object contains. Mass depends on the number and kinds of atoms making up
the object. It does not change with location!
Do not confuse mass and volume. Volume is a measure of space. It is measured in units such as cubic centimeters, cubic
meters, or liters. Mass is measured in kilograms. If an object has a large mass, it may or may not have a large volume. For
example, equal size bags of cotton and rocks may have equal volumes, but very unequal masses.
Mathematically stated F = ma
DIRECTLY PROPORTIONAL
INVERSELY PROPORTIONAL
The greater the mass, the greater
the force needed for a given
acceleration.
1. If you push on a shopping cart it will accelerate. If you apply four times the net force, how much greater will the acceleration be?
2. Suppose you apply the same amount of force to two carts, one cart with a mass of 4 kg, and the other with a mass of 8 kg. Which
cart will accelerate more? How much greater will the acceleration be?
To cause an acceleration (change an object’s velocity) a force is required. The longer a constant force is applied the greater the
acceleration. Likewise the greater the force that is applied will also have a greater acceleration.
A large change in velocity in a long time requires a small force.
A large change in velocity in a short time requires a large force.
How much force is applied to the Golf Ball? (Big or Small)
1. Long-range cannons have long barrels. The longer the barrel, the greater the velocity of the emerging cannonball or shell. Why?
The force exerted against the recoiling cannon is just as great as the force that drives the cannonball along the barrel. Why, then, does
the cannonball undergo more acceleration than the cannon?
Newton’s 1st Law of Motion
Newton’s 1 Law of Motion – an object will remain at rest or will remain in motion, traveling in a straight line and a constant speed,
until an unbalanced force acts on it.
st
Force – A push or a pull that causes an object to move, change speed, or stop.
 We measure all forces in a unit called NEWTONS. 1 Newton = ¼ pound.
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The key word is remain. An object will remain doing whatever it is already doing unless it is forced to do something
else.
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The tendency to remain at rest or in motion is called Inertia.
1. Inertia is caused by mass. More mass = more inertia.
Momentum

Inertia in motion is called Momentum.
1. Momentum = Mass x Velocity
2. During a collision momentum cannot be lost.
3. Law of Conservation of Momentum states that:
 In the absence of an external force, the momentum of a system remains unchanged.
 Momentum cannot be lost in a collision but it can be transferred from one object to another.
Which has more Inertia, a heavy truck at rest or a skateboard going 15 miles/hour?
Which has more momentum?
Impulse
Impulse - The product of the force acting on an object and the time during which it acts.
 If you want to get maximum increase in velocity of something, you not only apply the greatest
force you can, you also extend the time of application as much as possible.
The boulder, unfortunately,
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We can calculate impulse by rearranging Newton’s 2nd Law.
 Newton’s 2nd Law is Force = Mass x Acceleration
 Acceleration is the change in velocity over time (v/t)
So if we rearrange Newton’s 2 nd Law as follows we can calculate impulse:
Force x time = (mass x velocity)
Consider that mass does not change with location we can consider it a constant.
Therefore: Force x Time = Δ Velocity
has more momentum than
the runner.
Whether you hit the wall or the haystack, your momentum decreases the same–for you come to rest. According to the
impulse-momentum relationship, the same change in momentum means the same impulse. The same impulse doesn’t mean
the same force, or the same time–it means the same product of force and time.
In both cases the boxer’s jaw provides an impulse that
reduces the momentum of the punch. (a) The boxer is
moving away when the glove hits, thereby extending the
time of contact. This means the force is less than if the
boxer had not moved. (b) The boxer is moving into the
glove, thereby lessening the time of contact. This means
that the force is greater than if the boxer had not moved.
1.
If the boxer instead moves into the punch and shortens the contact time by half, by how much is the force increased?
2. Explain the statement: “It is not the fall that kills someone, but rather the sudden stop at the end.”
3. In terms of the amount of work, impulse, momentum, and energy for the total fall, compare
someone who skydives with a parachute to someone who skydives without a parachute.
Law of Conservation of Momentum in Collisions
Net momentum before collision = net momentum after collision.
Elastic Collisions - When objects collide without being permanently deformed and without generating heat.
Elastic collisions of equally massive balls. (a) A
green ball strikes a yellow ball at rest. (b) A headon collision. (c) A collision of balls moving in the
same direction. In each case, momentum is
transferred from one ball to the other.
Inelastic Collisions - when the colliding objects become distorted and generate heat during the collision.
Inelastic collisions. The net
momentum of the trucks
before and after collision is
the same.
Projectile Motion
A tossed stone, a cannonball, or any object projected by any means that continues in motion is called
a projectile. A thrown stone falls beneath the straight line it would follow with no gravity. The
stone curves as it falls. Interestingly, this familiar curve is the result of two kinds of motion
occurring at the same time. One kind is the straight-down vertical motion. The other is the horizontal
motion of constant velocity. Both occur simultaneously. As the stone moves horizontally, it also
falls straight downward–beneath the place it would be if there were no gravity.
The curved path of a projectile is the result of constant motion horizontally and accelerated motion
vertically under the influence of gravity.
A typical projectile path shows velocity vectors and their components. Notice that the horizontal
component remains the same at all points. That’s because no horizontal force exists to change this
component of velocity (assuming negligible air drag). The vertical component, however, changes
because of the vertical influence of gravity.
Suppose a cannon fires a cannonball so fast that its curved path matches the curvature of the Earth. Then without air drag, it would be
an Earth satellite! The same would be true if you could throw a stone fast enough. Any satellite is simply a projectile moving fast
enough to fall continually around the Earth.
What do we call a projectile that moves fast enough to travel a horizontal distance of 8 kilometers during 1 second? We call it a
satellite. Neglecting air drag, it would follow the curvature of the Earth. A little thought tells you that this speed is 8 kilometers per
second. If this doesn’t seem fast, convert it to kilometers per hour and you get an impressive 29,000 kilometers per hour (18,000
mi/h). Fast, indeed!
At this speed, atmospheric friction would incinerate the projectile. This happens to grains of sand and other meteorites that graze the
Earth’s atmosphere, burn up, and appear as “falling stars.” That is why satellites like the space shuttles are launched to altitudes higher
than 150 kilometers–to be above the atmosphere.
It is a common misconception that satellites orbiting at high altitudes are free from gravity. Nothing could be farther from the truth.
The force of gravity on a satellite 150 kilometers above the Earth’s surface is nearly as great as at the surface. If there were no gravity,
motion would be along a straight-line path instead of curving around the Earth. High altitude puts the satellite beyond the Earth’s
atmosphere, but not beyond Earth’s gravity. Earth gravity goes on forever, getting weaker with distance, but never reaching zero.
Centripetal Acceleration
Centripetal Acceleration is the acceleration toward the center of a curved or circular path. An unbalanced force must act on the object
for the object to curve. The force that accelerates an object toward the center of a circle or curved path is called a Centripetal Force.