MCAT Physics
A driver in a 2,000 kg SUV moving at 15 m/s crashes into a stopped car, causing it to be propelled forward and the truck to be slowed to 10 m/s. What is the work done on the SUV by the stopped car?
-1.2 x 10^5 J KEinitial = (0.5)(2 x 103 kg)(15 m/s)2 = (0.5)(2 x 103 kg)(225 m2/s2) = 2.25 x 105 J KEfinal = (0.5)(2 x 103 kg)(10 m/s)2 = (0.5)(2 x 103 kg)(100 m2/s2) = 1 x 105 J final- initial= -1.25 x 10^5 J
kinetic friction
Friction between moving surfaces force applied is greater than or equal to Fmax constant even as applied force increases -coefficient of kinetic friction xN -present if applied force drops to 0
static friction
Friction that acts on objects that are not moving force applied was less than or equal to Fmax
kinetic energy
KE=1/2mv^2 an object in motion
conservation of energy equation
KEi + PEi = KEf + PEf or 1/2mvi^2 + mghi = 1/2mvf^2 + mghf
mechanical advantage of incline plane
MA= incline length/ incline height
How much kinetic energy can a person gain if they are attached to a bungee cord that has been stretched 3 m and they are suddenly released? Assume the stiffness of the bungee cord is 5 N/m.
Use the elastic potential energy equation U=1/2 k x^2 =22.5 J this is how much Kinetic Energy a person can gain once they are released
Fmax
coefficient of static friction x normal force of object
conservative vs nonconservative forces
conservative: path independent - gravitational, electromagnetic, spring forces nonconservative: path dependent - friction and air resistance
In the presence of friction, the velocity of an object sliding along a uniform surface with no other forces acting on it will do what?
decrease linearly Newtons second law states that F=ma F/m is constant and the acceleration must be opposite the direction of movement and constant. Since velocity is a function of acceleration by v = a*t, a constant acceleration opposite the direction of movement leads to a linear decrease in velocity.
Friction coefficients and Normal force
different coefficients= different static forces more force needs to be applied to heavier objects
Law of conservation of energy
energy cannot be created or destroyed but can be changed from one form to another
Work
force applied over a distance. * productivity Joules= N x m kg x m^2/s^2 Work= Force x distance W=F x d cos(theta) cos (90)=0 example: if you are applying a forward force on a lawn mower, it is going to move but if you apply a downward force on a lawn mower. it is not going to move
Newton units
kg x m/s^2
two types of friction
static and kinetic
What kind of horizontal force acts between the powered tires of a car accelerating slowly on dry pavement without an incline, and the road? -gravity -kinetic friction -static friction -normal force
static force: the contact patch of the tire is not moving with respect to the road, rather rotation of the tire moves the contact patch, the tire is pushing the road backwards and the road is pushing the tire forwards kinetic is wrong: the tires themselves are in constant, static contact with the ground under normal operation
If a 7 kg object moving at some velocity collides with a 5 kg object, which then hits a third object, how will the amount of work done on the second and third object compare? Assume complete transfer of energy with each collision.
the amount of work done on the second and third objects will be equal According to the work-energy theorem, the amount of work done by an object is equal to the change in its kinetic energy. In other words, an object's kinetic energy will increase in direct proportion to the amount of work put in, and it will decrease proportionally to the amount of work the object expends. The heavier object will transfer all of its kinetic energy to the lighter object, which will in turn transfer all of its kinetic energy to the third object. The objects will move at different velocities if they have different masses, but the amount of total kinetic energy transferred each time remains the same.
What will happen to the gravitational potential energy of a red blood cell traveling through the arterial system down, away from the heart if its speed increases by half?
the gravitational energy will continue to decrease by some independent amount height from the ground is decreasing. height is directly proportional to gravitational PE. the amoiunt of decrease in gravitational PE is not dependent on the cell's velocity so it will decrease by some amount independent of the changes in velocity
You and your friends have decided to do some urban exploring. In the middle of an abandoned warehouse, you find a wooden crate on a horizontal surface. The crate is not moving. This crate is not moving because of what reason?
there is no net force acting on the box
mechanical advantage
using a mechanical apparatus to perform work using less force work input= work output Fin x d in = F out x d out so if you put in a smaller force over a greater distance, a greater force over a smaller distance will be generated must be larger than 1, this means that the input force must be smaller than the output force
power
work/time = J/s= watt system that performs work more quickly= more power
An aide is rigging a pulley system to elevate a patient's leg. The leg weighs 12 kg and the aide needs to raise it by 0.6 m. How much work does the aide need to put into the system in order to elevate the patient's leg?
7.2 x 10^1 J PEfinal = mgh = (12 kg)(10 m/s2)(0.6 m)= 72 J PEfinal - PEinital = 72 J - 0 J = 72 J
System with multiple masses
-designate reference point (0) -measure distances from reference point to each object (x1,x2, x3, etc. -masses =m1, m2, m3, etc.. -xcenter of mass= xcenter=m1x1+m2x2+m3x3/ m1+m2+m3 * careful with units!! convert to meters
What forces must be considered when observing a box on an incline plane?
-normal force -force of friction -force of gravity
What is the gravitational potential energy and the kinetic energy of a 1 g stone that is halfway down an 800 m drop off a cliff in an ideal system?
4 J of gravitational potential energy and 4 J of kinetic potential energy The potential energy at the top = (1x 10^-3) (10 m/s^2) (800)
Which of the following objects would NOT possess any elastic potential energy? A An incompressible liquid B A protein C A rubber ball D A taut string of an archery bow
A. an incompressible liquid to have elastic PE, there must be compression involved
What happens to the kinetic energy of an object when its velocity is tripled? A The kinetic energy increases by a factor of 3. B The kinetic energy increases by a factor of 6. C The kinetic energy increases by a factor of 9. D The kinetic energy increases by a factor of 4.5.
C. (3)^2=9
A 6 kg object, in a non-ideal system, is initially at rest and then falls from a shelf with a height of 250 m. What is the kinetic energy of that object shortly before impact?
PE=mgh =(6)(10)(25)= 15,000 but since it is an non-ideal system, some of the energy is lost to the environment so the KE will be less than 15,000 J
gravitational potential energy
PE=mgh PE is also known as U an object at rest
Work Energy theorem
The work done on an object equals the change in kinetic energy of the object In an ideal system, the magnitude of work done by one object on another does not change
elastic potential energy equation
U = 1/2 k x^2 U= elastic energy k is the spring constant delta x is the change in position
A muscle contraction creates 16 N of force to lift the arm 0.2 m. Which of the following correctly describes the work done on the arm by the muscles?
W=(F)(D) =16 x 0.2= 3.2 kg(m^2/S^2) positive work reflects work done on an object by its environment which corresponds to an increase in that objects energy so the 3.2 is positive
work in relation to volume and pressure
Work =Pressure x change in volume -piston examples (work being done by a gas on a piston) W=pascals x m^3 1 atm = 1 x 10^5 pascals -or work being done on container of gas to compress it -when there is no volume change, there is no work!!
The following are all types of ______________ energy -electric -elastic -mechanical -
potential energy
pressure vs volume graph
pressure on y axis volume on x axis the volume under the curve is work done
