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Science
Physics
Physical Science ch. 1-3
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Gravity
Terms in this set (69)
a different scaffold that weighs 400 N supports two painters, one weighing 500 N and the other weighing 400 N. The reading in the left-hand scale is 800 N. What is the reading in the right hand scale (#44 pg 33)
500N
A horizontal force of 120 N is required to push a book case across a floor at a constant velocity
a)what is the net force acting on the bookcase?
b) How much is the friction force that acts on the sliding bookcase?
c) how much friction force acts on the bookcase when it is at rest on a horizontal surface without being pushed?
a) net force= 0 N
b) Friction force= 120 N
c) friction force= 0, no applied force means no friction
Gravitational force on the Moon is merely 1/6 the gravitational force on Earth. What would be the weight of a 10 kg object on the moon and on the earth?
What would it's mass be on the moon and on the earth?
weight on moon: 16 1/3 N
weight on earth: 98 N
mass on moon: 10 kg
mass on earth:10 kg
A monkey hangs stationary at the end of a vertical vine. What two forces act on the monkey? Which force, if either, is greater?
2 forces: 1. weight/ gravity pulling down 2. support force pulling up
- they are equal to each other which is why the monkey is stationary
Nellie hangs at rest from the ends of the rope over a pulley (one rope on each side of the pulley and one hand on each rope hanging, scale is on ONE of the ropes) How does the reading on the scale compare with her weight?
the scale reading is half of her weight
Place a heavy book on a table, and the table pushes up on the book. A friend reasons that the table can't push upward on the book because if it did, the book would rise above the table. What do you say to your stupid friend? Why does this upward push not cause the book to rise from the table?
The upward push from the table does not cause the book to rise because it is at equillibrium with the downward gravitational force/ weight of the book which allows it to stay in place and not go up.
a vase sits at rest on a table. If the action force is earth pulling downward on the vase, the reaction force is the:
vase pulling upward on the earth
A Mack truck and a Mini Cooper traveling at the same speed collide head-on. The collision force is greatest on the:
A. Mini cooper
B. Mack truck
C. same on both
C
A freight train rolls along a track with considerable momentum. If it rolls at the same speed but has twice as much mass, its momentum is:
doubled
to catch a fast- moving softball with your bare hand, you extend your hand forward just before the catch and then let the ball ride backward with your hand. Doing this reduces the catching force because the:
A. relative vel. is less
B. relative velocity is more
C. Time of catch is increased
D. Time of the catch is decreased
E. none
C. time increased
What is the speed of a 4 kg ball with a momentum of 12 kg *m/s?
3 m/s
Show that Leslies avg speed is 10 km/h when whe runs to the store 5 km away in 30 min.
- how fast is this in units of m/s
speed= dist/time= 5km/30 min (60 min/1hr)= 300km/30 hr= 10 km/hr
- 2.77777 m/s or about 3 m/s
Lillian rides her bicycle along a straight road at an average velocity (v)
a)write an equation showing the distance she travels in time t
b)if lillians average speed is 7.5 m/s for a time of 5.0 min, show that she travels a distance of 2250m.
a)d=vt
b)d= 7.5*300=2250m
A school bus slows to a stop with an average acceleration of -2.0m/s^2. Show that it takes 5.0 s for the bus to slow from 10.0 m/s to a stop.
Acceleration= change in speed/ time
-2.0m/s^2=(0-10 m/s)/time(sec)
t=(-10m/s)/ -2m/s^2
t=5.0 s
An Airplane starting from rest at one end of a runway accelerates uniformly at 4.0 m/s^2 for 15s before takeoff.
a) what is its takeoff speed?
b) show that the plane travels along the runway a distance of 450 m before takeoff.
a)60 m/s
b)Avg vel: 0+60/2= 30m/s
d=vt
d= 30 m/s*15s= 450m
Suppose that a freely falling object were somehow equipped with a speedometer. By how much would its speed readings increase with each second of fall?
it would increase by about 10m/s or 9.8 m/s
Consider a freely falling object dropped from rest. What is its acceleration at the end of 5 s? At the end of 10 s? Defend your answer and distinguish between velocity and acceleration.
Its acceleration after 5 sec= 10 m/s^2
Its acceleration after 10 sec is also 10 m/s^2
because acc. is the change in vel/ the interval of time; so as the object picks up velocity, more time goes by and since it is in freefall, when you divide the velocity by the time it continues to give a ratio of a constant acceleration= 10 m/s^2
Velocity (formula and units)
V=change in distance/ change in time
m/s
Acceleration (formula and units)
change in velocity/ change in time
m/s^2
Force:
(formula and units)
F=m*a
Newton (N)= 1 kg*m/s^2
or could be lbs
Power(formula and units)
Work/time
J/s=Watt (W)
Work = (formula and units)
Force* distance
J= N* m
Impulse(formula and units)
Force* time
N
s= kg
m/s^2
s=>kg
m/s
momentum: (formula and units)
p= mv (mass *vel)
kg*m/s
KE (formula and units)
1/2 mv^2
J
gravitational PE (formula and units)
PE= mgh
J
conservation in laymen's terms
as one thing decreases the other increases
describe the conservation of energy in terms of PE and KE at top, halfway point and bottom
top: PE= mgh & KE= 0
halfway: PE= 1/2 mgh & KE= 1/2 mgh
bottom: PE= 0 & KE = mgh
what is mechanical energy? (not thinking of friction, is it constant?)
Kinetic energy and potential energy
- without friction, it is constant (KE+ PE= constant)
describe terminal velocity:
when something reaches it's terminal velocity, it's acceleration =0 because upward air resistance is equal to the downward acceleration
Momentum conservation:
If two things are traveling toward eachother at one m/s (opposite toward eachother) and the collide and bounce the other way...1. what is their momentum before? 2. What is their momentum after?3. what is this collision called
1.before (going opposite ways)= 0
2. after (going away in opp. directions)= 0
3. Elastic collision
( in a collision the momentum of individual obects can change, but overall the system, remains constant)
what are 3 types of machines:
draw them
1. lever
2. pulley
3. ramp
formula for machines
fD= Fd
(less force, but more distance for input)= (more force, but less distance output)
Newton's 1st law:
object at rest remains at rest unless an outside force acts upon it
Object in motion stays in motion unless an outside force acts upon it
-AKA" law of inertia"
--when an object is moving in a uniform motion it has no net force acting on it
--when there is not force acting on an object, it will stay at rest or maintain its constant speed in a straight line
Newton's 3rd Law:
When an object exerts a force on a second object, the second object exerts an equal but oppositely directed force on the first object (action-reaction pair)
conservation of energy
if no work is done on an object, it's energy remains constant (as it loses PE, it gains KE, so total energy remains constant)
KE= PE sooo....
mgh= 1/2 mv^2
Galileo:
introduced inertia, the property of objects to resist changes in motion (if you have a lot of inertia, more resistance to change)... mass = measure of inertia
- created scientific method
- sun= center of universe (condemned by church)
Aristotle:
object always wants to be at rest, naturally comes to rest (natural state), forced to be in motion(violent state)
- taught that the earth was center of the universe= church adopted this
factors to make acceleration change:
if an object's speed is increasing/ decreasing or if it is changing direction
difference between mass and weight
gravity acts on weight (force) rather than measure of how much stuff you have (inertia)
-mass is scalar, weight is a force
difference between velocity and speed
direction with velocity! (can be -/+)
weightlessness/ freefall=
only force acting on object= gravity.
Copernicus
proposed sun was center of universe
equilibrium
net force on an object equals zero
types of forces:
-gravitational
-support
-friction: surface (static: sticking; kinetic: sliding), fluid (air resistance- depends on shape of object; viscosity: stickiness)
-tnesion
vector
magnitude (speed) and direction
-use with velocity
scalar
Magnitude only
- use with speed temperature mass, distance, etc.
can an object have a constant speed with a changing velocity?
yes, think of a race car (changing direction)
when finding change in something like velocity do you do v(i)-v(f) or v(f)-v(i)
v(f)-v(i)
Newton's second law:
when the net force on an object is not zero, it's velocity will change.
F= m*a
Forces are vectors so their _______ is just as important as their ______
direction; magnitude
Impulse definition:
in order to change an object's momentum by a fixed amount (e.g. stop a moving truck, you can apply a small force for a long time or a large force for a short time)
- f
T= F
t
direction and momentum
the direction of an object's momentum is the direction of its velocity
- since velocity is a vector, momentum is also a vector
---must be added as vectors
conservation of momentum
tif no external forces acting on a system of particles, then the total momentum of the system is constant
show examples of momentum in:
1. 2-dimensional collision
2. an explosion
1. one object moving at vel V with mass M hitting another object at rest (with mass m)
before collision p= MV
hit and both go in opposite directions with different vel.
after collision p= Mv1+mv2= MV
2. before explosion p=0
after explosion p= Mv1+mv2=0 (pieces going in opposite directions)
2 things enter where work is done:
1. application of force
2. movement of something by that force IN THE DIRECTION THE FORCE ACTS (if you push against a wall and it remains stationary- no work done)
2 categories of work:
1. work done against another force
2. work done to change the speed of an object
work is done in lifting a barbell. How much work is done in lifting a twice as heavy barbell the same distance?
twice as much
you do work when pushing a cart. if you push the cart twice as far with the same constant force, then the work you do is?
twice as much
If a 0.43 kg ball is rolling along a level surface, how much work is done by gravity when the ball rolls for 3 meters.
gravity is not doing work on the ball because it's pushing down and the ball is going sideways
power definition
measure of how fast work is done
a job can be done slowly or quickly. both may require the same amount of work, but different amounts of
A. energy
B. momentum
C. power
D. impulse
C. power
potential energy definition (due to position)
stored energy due to position shape or state. In its stored state, energy has the potential for doing work
-Examples: drawn bow/ stretched rubber band (elastic PE), box raised on top of ledge
Gravitational potential energy
the amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in raising it.
disapative force:
energy disapears into heat
does a car hoisted for repairs in a service station have increased potential energy relative to the floor?
Yes
the work done in braking a moving car to a stop is the force of tire friction x stopping distance. If the initial speed of the car is doubled the stopping distance is....
increased by a factor of 4 (because work= total change in KE)
In an ideal pulley system, a woman lifts a 100- N crate by pulling a rope downward with a force of 25 N. For every one- meter length of rope she pulls downward, the crate rises..... (in cm)
25 cm
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