Acceleration
The cause of acceleration is force.
A=Change in Velocity/Time Interval
Kick a football and it neither remains at rest nor moves in a straight line.
Mass Resists Acceleration
The same force applied to twice as much mass results in only half the acceleration.
The acceleration is inversely proportional to the mass.
A~1/mass
Inversely
The two values change in opposite directions. As the denominator increases, the whole quantity decreases by the same factor.
The acceleration produced depends on the mass that is pushed.
Newton's 2nd Law
Describes the relationship among an object's mass, an object's acceleration, and the net force on an object.
A~net force/mass
The acceleration is equal to the net force divided by the mass.
If the net force acting on an object doubles, its acceleration is
Friction
Friction is a force and affects motion:
Friction acts on materials that are in contact with each other.
It always acts in a direction to oppose relative motion.
When two solid objects come into contact, the friction is mainly due to irregularities in the
Pressure
The amount of force per unit of area
When the force is perpendicular to the surface area
P=Force/Area of Application
Pressure is measured in newtons per square meter, or pascals (Pa). One newton per square meter is equal to one pascal.
The force of the bo
Terminal Speed
The speed at which the acceleration of a falling object is zero because friction balances the weight.
Sky divers reach terminal speed when air resistance equals weight.
Terminal Velocity
Terminal speed together with the direction of motion.
Falling and Air Resistance
A feather and a coin fall with equal accelerations in a vacuum, but very unequally in the presence of air.
When falling in air, the coin falls quickly while the feather flutters to the ground.
The force due to air resistance diminishes the net force actin
A car has a mass of 1000 kg. What is the acceleration produced by a force of 2000 N?
a= f/m= 2000 N/1000 kg= 2 m/s^2
How much force, or thrust, must a 30,000-kg jet plane develop to achieve an acceleration of 1.5 m/s2?
Arrange Newton's second law to read:
force = mass � acceleration
F = ma
= (30,000 kg)(1.5 m/s2)
= 45,000 kg�m/s2
= 45,000 N
Two forces act on a book resting on a table: its weight and the support force from the table. Does a force of friction act as well?
No, not unless the book tends to slide or does slide across the table. Friction forces occur only when an object tends to slide or is sliding.
The weight of a 1-kg stone is 10 N at Earth's surface. Using Newton's second law, the acceleration of the stone is
a=f/m=weight/m=10N/1 kg=10 kg�m/s^2/1kg= 10 m/s^2=g
The weight of a 10-kg cannonball is 100 N at Earth's surface and the acceleration of the cannonball is
a=f/m=weight/m=100N/10kg=100 kg�m/s^2/10 kg= 10 m/s^2 =g
If a heavy person and a light person open their parachutes together at the same altitude and each wears the same size parachute, who will reach the ground first?
The heavy person will reach the ground first. Like a feather, the light person reaches terminal speed sooner, while the heavy person continues to accelerate until a greater terminal speed is reached.
An object will accelerate when
it is pushed or pulled with a net force.
When a net force acts on an object, its acceleration depends on the object's
mass.
A cart is pushed and undergoes a certain acceleration. Consider how the acceleration would compare if it were pushed with twice the net force while its mass increased by four. Then its acceleration would be
half.
Friction is a force like any other force and affects motion. Friction occurs in
solids sliding over one another.
fluids.
air.
ALL OF THESE
When you stand on one foot instead of two, the pressure you exert on the ground is
twice.
The reason a 20-kg rock falls no faster than a 10-kg rock in free fall is that
the force/mass ratio is the same.
Kevin and Suzanne go sky diving. Kevin is heavier than Suzanne, but both use the same size parachute. Kevin has a greater terminal speed compared with Suzanne because
he has to fall faster for air resistance to match his weight.
Distinguish between the relationship that defines acceleration and the relationship that states how it is produced.
Acceleration is the change of velocity over time. Acceleration is produced when a force is applied to a mass.
What is meant by the net force that acts on an object?
The net force is the sum of the forces acting on an object, the resultant of those forces.
Suppose a cart is being moved by a certain net force. If the net force is doubled, by how much does the cart's acceleration change?
The acceleration will double.
Suppose a cart is being moved by a certain net force. If a load is dumped into the cart so its mass is doubled, by how much does the acceleration change?
he acceleration will be cut in half.
Distinguish between the concepts directly proportional and inversely proportional . Support your statement with examples
Directly proportional quantities both increase or decrease together, such as force and acceleration. Inversely proportional quantities move in opposite directions: as one increases, the other decreases. Ex: mass and acceleration.
State Newton's Second Law in words and then in the form of an equation.
The acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass. F = m x a.
How much force does a 20, 000 kilogram rocket develop to accelerate 1 m/s^2?
F = m x a = 20,000 kg x 1 m/s2 = 20,000 N
What is the cause of friction and in what direction does it act with respect to the motion of a sliding object?
Friction is caused by the contact force between two objects and the irregularities between the surfaces of the objects. It always acts opposite to the direction of motion.
If the force of friction acting on a sliding crate is 100 N, how much force must be applied to maintain a constant velocity?
100 N must be applied to cancel out the frictional force.
What will be the net force acting on the crate?
Zero net force.
What will be the acceleration?
With zero net force, the object will not accelerate. It will move at a constant velocity and the acceleration will be zero.
Distinguish between force and pressure.
Force is a push or a pull and is measured in Newtons. Pressure is force per unit area and is measured in N/m2 or Pascals. Pressure is how much force per square meter, for example.
Which produces more pressure on the ground, a person standing up or the same person lying down?
The person standing up is putting all his/her weight on top of his feet. The person lying down is putting all his/her weight over the entire surface of his body touching the ground. The person standing up has more weight per unit area and so has more pres
The force of gravity is twice as great on a 2 kg rock as on a 1 kg rock. Why does the 2 kg rock not fall with twice the acceleration?
The two kilogram rock also has twice the mass/inertia as the one kg rock. It needs twice the force because it has twice the mass. The 2's cancel out.
Why do a coin and feather in a vacuum tube fall with the same acceleration?
Gravity pulls down on all objects with the same acceleration in the absence of air resistance. Both objects will acceleration downwards at 9.8 m/s^2.
Why do a coin and a feather fall with different accelerations in the presence of air?
Although gravity pulls down on both objects at the same rate - 9.8 m/s2 - the force of air resistance is greater on the feather than the coin because of its shape.
How much air resistance acts on a 100 N bag of nails that falls at its terminal speed?
If the bag of nails is falling at terminal speed/velocity, this means by definition it is not accelerating anymore. That means the force upwards (air resistance) must equal the force downwards (force of weight). So Fa = Fw = 100 N.
How do the air resistance and the weight of a falling objet compare when terminal speed is reached?
They must be equal so the net force on the falling object is zero. Then no acceleration happens.
All other things being equal, why does a heavy sky diver have a terminal speed greater than a light sky diver?
All other things being equal means that the two divers' profiles are the same. They both fall with the same amount of surface area exposed to the air and so the only thing determining fall rate is their weight. Since the heavier sky diver has more weight,
What can be done so that both terminal speeds are equal?
The lighter skydiver must change his falling profile and fall straight down with his arms by his sides to make up the difference and become more aerodynamic. The heavier skydiver can also increase his air resistance by spreading his arms and legs out.
What is the net force acting on a 25 N freely falling object?
The net force is 25 N downwards, which is the object's weight.
What is the net force when the object encounters 15 N of air resistance?
If the object encounters 15 N of air resistance his net force downwards would be 25 N - 15 N = 10 N.
When it falls fast enough to encounter 25 N of air resistance?
When the object reaches terminal velocity, 25 N - 25 N = 0 N.
Calculate the acceleration of a 2000-kg, single-engine airplane just before takeoff when the thrust of its engine is 500 N
F / m = (500 N) / (2000 kg) = 0.25 m/s2
Calculate the acceleration of a 300,000-kg jumbo jet just before takeoff when the thrust for each of its four engines is 30,000 N.
a = F / m = 4 (30,000 N) / (300,000 kg) = 0.4 m/s2
Calculate the acceleration if you push with a 20-N horizontal force on a 2-kg block on a horizontal friction-free air table.
a = F / m = (20 N) / (2 kg) = 10 m/s2
What acceleration occurs if the friction force is 4 N?
a = (20 n - 4 N) / (2 kg) = 8 m/s2
Calculate the horizontal force that must be applied to a 1-kg puck to make it accelerate on a horizontal friction-free air table with the same acceleration it would have if it were dropped and fell freely.
F = ma = (1 kg) (9.8 m/s2) = 9.8 N
Calculate the horizontal force that must be applied to produce an acceleration of 1.8 g for a 1.2-kg puck on a horizontal friction-free air table.
F = ma = (1.2 kg)(1.8)(9.8 m/s2) = 21.2 N
What is the difference between saying that one quantity is proportional to another and saying it is equal to another?
Proportional - a change in one quantity implies a corresponding change in the other; when one quantity is equal to
another, they are identical
If an object has no acceleration, can you conclude that no forces are exerted on it? Explain and draw a free body diagram to illustrate an example.
No, you can conclude no net force is acting on it
What is the acceleration of a rock at the top of its trajectory when thrown straight upward. Explain whether or not the answer is zero by using the equation a = F/m as a guide to your thinking.
Always g; a = F / M = Mg / M = G does not equal 0
A rocket fired from its launching pad not only picks up speed, but its acceleration also increases significantly as firing continues. Why is this so?(Hint: About 90% of the mass of a newly launched rocket is fuel.)
Since mass decreases as fuel is expended, acceleration increases
If you push horizontally on your book with a force of 1 N to make the book slide at constant velocity, how much is the force of friction on the book?
1 N in the opposite direction to the push
When blocking in football,why does a defending lineman often attempt to get his body under that of his opponent and push upward? What effect does this have on the friction force between the opposing lineman's feet and the ground?
To reduce ground contact and friction between the opponent's feet and the ground
Why does a sharp knife cut better than a dull knife?
Smaller cutting area produces a greater pressure for a given force
An aircraft gains speed during takeoff due to the constant thrust of its engines. When is the acceleration during takeoff the greatest - at the beginning of the run along the runway or just before the aircraft lifts into the air? Think, and then explain.
At the beginning of take off, when air drag is least
As a skydiver falls faster and faster through the air (before reaching terminal speed), does the net force on her increase, decrease, or remain unchanged? Does her acceleration increase, decrease, or remain unchanged?
Decreases; Decreases; resisted by air drag
After she jumps, a sky diver reaches terminal speed after 10 seconds. Does she gain more speed during the first second of fall or the ninth second of fall? Compared with the first second of fall, does she fall a greater or a lesser distance during the nin
Maximum gain in speed occurs during the first second of fall. Since her speed increases, she will be falling faster during the last second of fall, and therefore cover the greatest distance during this time
A regular tennis ball and another one filled with sand are dropped at the same time from the top of a high building. Your friend says that even though air resistance is present, both balls should hit the ground at the same time because they are the same s
The heavier ball falls through the air with greater acceleration.
If a 1-N net force accelerates a 1-kg mass at 1 m/s2, what is the acceleration caused by a net force of 2 N on a 2-kg mass?
F=1N M=1kg
1N=1kg x 1m/s^2 makes sense, right
so if F=2N and M=2kg, divide F by M
What is the acceleration of a 747 jumbo jet, mass 30 000 kg, in takeoff when the thrust for each of its four engines is 30 000 N?
m=30,000kg F=30,000
F/m=a therefore 30,000N/30,000kg=1 m/s^2
A certain force applied to a 2-kg mass accelerates the mass at 3 m/s2. How much acceleration will the same force produce on a 4-kg mass?
0
A horizontal force of 100 N is required to push a crate across a factory floor at a constant speed. What is the net force acting on the crate? What is the force of friction acting on the crate?
The net force acting on the crate is 0 N and the force of friction acting on the crate is 100 N. Why?
1. Any object moving at constant speed is neither accelerating or decelerating (acceleration = 0 m/s^2)
2. There is ZERO NET FORCE acting on an object wi
If a four-engine jet accelerates down the runway at 2 m/s2 and one of the jet engines fails, how much acceleration will the other three produce?
a=2m/s^2
F=ma
if mass is kept the same, than the rest depends on F and a. if the force is reduced by 1/4 than so will the acceleration. Therefore, the acceleration would be 1.5m/2^2
What will be the acceleration of a sky diver when air resistance is half the weight of the sky diver?
do a force diagram. (Fg =weight) the force balancing Fg is .5 Fg. the force summation equation would be Fg+Fa=ma. Fa being the air resistance which is opposite of Fg. so it would be Fg - .5Fg=ma, which is .5Fg=ma. Fg=mg and so .5mg=ma. cancel the m. .5g=a
If a loaded truck that can accelerate at 1 m/s2 loses its load and has three-fourths of the original mass, what acceleration can it attain from the same driving force?
F=ma
if F remains the same, but m decreases, a HAS to decrease. make up some numbers if this will help you. if it only has 3/4 of its mass, the a would have to make up for it.
for example, if F=10, m=10 and a=1
the final mass would be 7.5. divide F by m,