Exam 2 (chapters 4-6)

Any time you're analyzing two different objects, there has to be an

action reaction pair

superposition of forces

any # of forces applied at a point on a body have the same effect as a single force equal to the vector sum of forces

buoyancy force on a swimmer is one half of an action reaction pair. What is the other half?

downward force that the swimmer exerts on the water *not the weight because that is the earth exerting a force on the swimmer so that reaction is the swimmer exerting a force on the earth, and the weight is acting on the same body as the buoyancy force (swimmer).

twice the mass, same speed, ________ the kinetic energy

twice

Net force=

vector sum of all forces

power

time rate of doing work 1 watt = 1J/s

contact forces

normal force frictional force tension

1N=

1kg*m/s^2

air surrounding an airplane in flight exerts a drag force opposite to airplanes motion, when an airbus is flying in a straight line at a constant altitude at 250 m/s, what is the rate at which the drag force does work on it?

-432000hp airliner has constant horizontal velocity so the net horizontal force on it must be zero hence the backwards drag force must have the same magnitude as the forward force due to the combined thrust of the four engines, this means that the combined thrust does positive work, and its rate is 4 (108000) so drag force is equal and opposite

fluid resistance

-acting on a body, depends on the speed of the body -falling body reaches its terminal speed when the resisting force equals the weight of the body -terminal speed = drag force= weight

Newton's third law

-action reaction pairs - Faonb=-Fbona -when 2 bodies interact, they exert forces that are equal and opposite on each other -these pairs never act on the same body -action reaction forces are always antiparallel

kinetic energy

-amount of work required to accelerate particle from rest to speed x -amount of work required to bring a moving particle to rest -joules

inertial frame of reference

-constant velocity, a=0 -if one frame is inertial, then every other frame moving relative to it with constant velocity is also inertial

Newton's second law

-intertial properties of a body are characterized by its mass -F=ma

if gravity was 19.6m/s2, (double), compared to earth, would it be easier, harder or just as easy to walk around? easy or harder to catch a ball moving horizontally at 12 m/s

-it would take twice the effort to walk because she is double the weight -ball would be just as easy to catch because it's mass is the same as it is on earth so the horizonal force exerted to stop the ball is the same

You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. The force acting on you that causes you to fall forward is:

-no forces are acting on you, just inertia

work and energy with varying forces

-when force is parallel to displacement but is changing in magnitude, then work is varying. -on a graph of force v displacement, the total work done is represented by the area under the curve during the interval X2-X1

Newton's first law

-when the sum of all forces= zero, the body is in equilibrium and there is zero acceleration -object at rest stays at rest, object in motion stays in motion -newton's first law does not apply when motion is referenced to an accelerating reference frame

In which situation is there a net force of zero acting on the body? 1. airplane flying due north at a steady 120m/s and constant altitude 2. a car driving straight up a hill with a 3degree slope at a constant 90 km/hr 3. a hawk circling at a constant 20 km/hr at a constant height of 15m 4. a box with a slick frictionless surface in the back of a truck as the truck accelerates forward on a level road at 5 m/s?

1, 2 and 4 3, path is a circle so it's accelerating because it's constantly changing direction

A traffic light of weight w hangs from two lightweight cables, one on each side of the light. Each cable hangs at a 45 degree angle from the horizontal. What is the tension in each cable? 1. w/2 2. w/sqrt2 3. w 4. wsqrt2 5. 2w

2. w/sqrt2

Rank the following situations in order of the magnitude of the object's acceleration, from lowest to highest. Any cases that have the same magnitude of acceleration? 1. 2kg object acted on by a 2N net force 2. 2kg object acted on by an 8N net force 3. 8kg object acted on by a 2 N net force 4. 8 kg object acted on by a 8 N net force

3 1,4 2

rank the following bodies in order of their kinetic energies from least to greatest 1. a 2 kg body moving at 5 m/s 2. a 1 kg body initially at rest then had 30 J of work done on it 3. a 1 kg body initially moving at 4 m/s then had 20 J of work done on it 4. a 2kg body initially moving at 10 m/s and then did 80 J of work on another body

4 1 3 2 *just looking at final KE

same mass, twice the speed, _________ the kineic energy

4 times

Consider the situation shown in Figure 5.5 in your textbook where a cart (weight w1) is attached by a lightweight cable to a bucket (weight w2) as shown. The ramp is frictionless. The pulley is frictionless and does not rotate. The difference now is that when released, the cart accelerates up the ramp and the bucket accelerates downward. How does the cable tension T compare to w2?

T is less than W2

You are trying to lift a cinder block of mass m up to the roof of your house by tying a massless rope to the block and pulling the cinder block upward at a constant speed. The tension T the rope exerts on the block is_____

T= mg

a box of mass m is pulled with a constant acceleration along a horizontal frictionless floor by a wire that makes an angle of 15degrees above the horizontal. If T is the tension in this wire, then

T> ma

work occurs when,

a constant force acts on a particle that undergoes displacement W=f*s*costheta

if the sum of the forces is not zero, the body _______

accelerates

direction of net force is the same as the direction of_____

acceleration

You are driving a car and hit a mosquito, which has a greater magnitude, the force of car on mosquito, or force of mosquito on car?

by newton's third law, the forces are equal, but the acceleration on the mosquito is extremely large whereas the acceleration on the car is extremely small because of the mass differences.

a satellite in a small radius orbit moves at a higher speed than a satellite in an orbit of a large radius, what can you conclude about the earth's gravitational attraction for the satellite? 1.increases with increasing distance from earth 2. same at all distances from earth 3. decreases with increasing distance from earth 4. not enough info

decreases with inc. distance from earth

acceleration is ______ proportional to force and __________proportional to mass

directly inversely

An elevator is being lifted at a constant speed by a steel cable attached to an electric motor. There is no air resistance, nor is there any friction between the elevator and the walls of the elevator shaft. The upward force exerted on the elevator by the cable is...

equal to the force of gravity

If you swing a bucket of water fast enough in a vertical circle, at the highest point the water does not spill out because an outward force balances the pull of gravity on the water. T/F?

false

four classes of fundamental forces

gravitational electromagnetic strong interaction (hold nucleus together) weak interaction (radio carbon decay)

dynamics of circular motion

if a particle is in uniform circular motion, both its acceleration and the net force are directed toward the center of the circle

work is path independent or dependent?

independent of path

when a force varies during a displacement, the work done is an _________

integral area under the curve on a graph of force vs. displacement

action reaction forces are _____ forces that don't affect the motion of overall system

internal forces

in uniform circular motion, acceleration is directed toward ______

inward, toward the center of circle

normal force on a body

is always equal to the weight

when total work = 0 the object

is at rest or at a constant speed

the vector sum of a body in equlibrium

is zero

friction of a box on a rough surfaced flat bed truck climbing a hill and the box is sliding toward the back of the truck

kinetic

no relative motion, then magnitude of force of static friction is less than or equal to______

max static friction

static friction

max static friction is about equal to magnitude of normal force x coefficient of static friction -usually coefficient of SF is larger than coefficient of KF

A cable attached to a car holds the car at rest on a frictionless ramp that makes an angle alpha with respect to the horizontal. The ramp exerts a normal force on the car. How does the magnitude n of the normal force compare to the weight w of the car?

n is less than the weight

In Figure 5.12 a toboggan of weight w (including the passengers) slides down a hill of angle? alpha at a constant speed. Which statement about the normal force on the toboggan (magnitude n) is correct? 1. n=w 2. n > w 3. n<w

n is less than weight

Consider a block that is moving to the right on a smooth horizontal surface. You apply a force F on the block that is directed to the left. The work done by the force F on the block is

negative

if an object is moving up an inclined plane, then both the x and y components of the gravitational force are ____________

negative

if an elevator is being lowered at a constant speed, the cable is doing ________ work on the elevator and the elevator does _____ work on the elevator

negative positive

does kinetic energy depend on direction of motion?

no

friction of a box on a rough surfaced flat bed of a truck that is moving at a constant velocity on a straight, level road (box remains in place)

no friction *not accelerating and no other force is acting parallel to horizontal

Friction of a box at rest on a rough horizontal surface

no friction *not accelerating and no other force is acting parallel to horizontal

If an elevator is being lifted at a constant speed, the cable lifting it does __________ work on the elevator, and the elevator does _______ work on the cable

positive negative

same mass, same speed, different direction of motion= ______ the kinetic energy

same

work is a ________ quantity

scalar

when total work is < 0 the object

slows down

When the total work on an object moving through a displacement x is negative, then the speed of the object at the end of the displacement x is

smaller relative to the speed at the beginning

when total work is > 0, the object

speeds up

friction of box on the rough surfaced flat bed of a truck that is speeding up on a straight road and box remains steady

static

friction of a box at rest on a rough tilted surface

static *would normally slide so static friction prevents this

A woman pulls on a 6kg crate which is connected to a 4kg crate, compared to the 6kg crate, the 4 kg crate is...

subjected to a smaller net force and has same acceleration

inertia

tendency of a body to keep moving once its in motion

A sled moves on essentially frictionless ice. It is attached by a rope to a vertical post set in the ice. Once given a push, the sled moves around the post at constant speed in a circle of radius R (same situation as in Fig. 5.31(a) in your textbook). If the rope breaks,

the sled will move in a straight line

a ball sits at rest on a horizontal tabletop. according to newton's third law, the reaction force to the weight of the ball is...

the upward force that the ball exerts on the earth

long range force

weight/ magnetism

negaitve work

when force and displacement are in opposite directions

positive work

when force and displacement are in same direction

zero work

when force is perpendicular to displacement

Work energy theorem

when forces act on a particle while it undergoes displacement, KE changes by an abount equal to the total work done by all the forces

spring has a natural length, and ______ is done to stretch and compress it

work

sometimes a force can be broken into its_______

x and y components (an x and y component of acceleration)

if the speed of a conical pendulum remains constant as it travels around a circle, then over one complete circle, how much work does the tension force F do on the bob? 1. positive 2. negative 3. zero over one complete circle, how much work does the weight do on the bob? 1. positive 2. negative 3. zero

zero and zero * at any point during the pendulum bob's motion, both tension and weight act perpendicular to the motion

an electron moves in a straight line toward the east with a constant speed of 8*10^7 m/s. it has electric, magnetic and gravitational forces acting on it. During a 1m displacement, the total work done on the electron is....

zero, a=0 fo the net force is zero, so total work done by all of the forces is zero, even if individual forces do non zero work

A stalled car is being pushed up a hill at constant velocity by three people. The net force on the car is:

zero, because constant velocity