Physical Science Chapter 4 Work and Energy Midterm 10/14/19
when a book is moved from a higher shelf to a lower shelf, what changes
(GPE) Gravitational Potential Energy
If you throw a 0.4 kg ball at a speed of 20 m/s, what is the kinetic energy from the balls motion?
122.5 J (KE = 1/2 5 multiplied by 7²)
A lever has a mechanical advantage of 5. How large would a force need to be to lift a rock with a mass of 100kg.
196 N
If you have a mass of 80 kg and you are standing on a platform 3.0 m above the ground, what is the gravitational potential energy between you and Earth relative to the ground?
2,352 J If GPE = mgh, then... GPE = 80 multiplied by 9.8 multiplied by 3
machine
A device that changes a force
simple machine
A machine that does work with only one movement
gravitational potential energy
Energy stored by objects due to their position above Earth's surface.
By applying a force of 50N, a pulley system can lift a box with a mass of 20 Kg. What is the mechanical advantage of the pulley system?
For a MA of 1 then 2 newtons would be required to lift 20 KG F = Mass Acceleration = 20* 10 = 200 Newtons so to lift the same mass with 50 newtons then the mechanical advantage must be 200/50 = 4 answer
If your weight is 500 N, and you are standing on a floor that is 20m above the ground, what is the gravitational potential energy between you and Earth, relative to the ground?
GPE = mgh w = mg GPE = wh GPE = ? w = 500 N h = 20 m GPE = 500 N x 20 m GPE = 10,000 J
gravitational potential energy equation
GPE=mgh
A 2.0 kg book is moved from a shelf that is 2.0 m off the ground to a shelf that is 1.5m off the ground. What is the change in GPE?
GPE=mgh=1.5kg * 10m/s * 2 m = 30 J 2.0kg * 10m/s * 1.5 m= 40 J 40 J -30 J =10 J change in GPE
A car moving at 30 m/s has 900 kJ of kinetic energy from its motion. What is the car's mass?
Given- mass=900kg Speed=30m/s Formula for Ek=1/2mv² Putting the value in the formula We get Ek=1/2×900×30 =13,500 joules
which statement is always true of work?
It involves transfer of energy.
Kinetic energy equation
KE=1/2mv^2
The muscles of a runner transforms chemical potential energy into what?
Kinetic Energy
An object has 600 J of kinetic energy from its motion and a speed of 10 m/s. What is its mass?
M = 2 x Ke / (divided by) v squared M = 2 X 600 / 10 squared = 12kg
mechanical advantage equation
MA= Fo/Fi
what is the efficiency of a machine if your work on the machine is 1,200j and the machines output work is 300J?
Percent efficientcy= Work (out)/Work (in) x 100% 1200/300 *100=400 J
What power is used by a machine to perform 800 J of work in 25s?
Power = energy/time During those 25 seconds, the machine is doing work at the rate of 800/25 = 32 watts. We don't know how much power the machine must consume in order to perform work at that rate, but we know it's more than 32 watts.
Energy
The ability to do work or cause change
The gravitational potential energy of a cucumber -Earth system changes when which factor changes? A) The cucumbers speed B) The cucumber's mass C) The cucumber's temperature D) The cucumber's length
The cucumber's mass
When moving a couch, you exert a force of 400 N and push it 4 m. How much work have you done?
W = FD W = 400 x 4 W =1600 Joules
How much work is needed to lift a 50 kg weight 3.0m?
Work = force x distance force = 50 kg * 9.81N gravitational distance = 3m 3m * 9.81N * 50kg = 1471.5 Joules of work
A combination of two or more simple machines is a
compound machine
The ratio of output force to input force is the ________of a machine.
efficiency
Efficiency equation
efficiency= Wo/Wi x 100
which cannot be done by a machine?
increase in work
System
is anything around around which you can imagine a boundary.
what happens to the mechanical energy of a book when you push it across a table at a constant speed?
kinetic and potential energy are all constant
What is the kinetic energy from the motion of 5.0kg object moving at 7.0m/s?
kinetic energy (KE) is equal to half of an object's mass (1/2*m) multiplied by the velocity squared. (1/2 * 5 kg) *7m/s =17.5 Joules
Power equation
p=w/t (power=work/time)
kinetic energy
the energy an object has due to its motion
mechanical energy
the energy associated with the motion and position of everyday objects
Potential Energy (PE)
the energy stored in an object because of its position or composition
chemical potential energy
the energy stored in the chemical bonds of a substance
Law of Conservation of Energy
the law that states that energy cannot be created or destroyed but can be changed from one form to another
elastic potential energy
the potential energy of an object that is stretched or compressed
Power
the rate at which work is done
mechanical advantage
the ratio of the output force to the input force
Work equation
work = force x distance
How much energy do you save per hour if you replace a 60 watt lightbulb with a 55 watt lightbulb?
you would save 60-55 = 5 watts per hour = 5 x 3600 = 18000 Joules
A car with a mass of 900 kg is traveling at a speed of 25 m/s. What is the kinetic energy from the car's motion?
𝐾𝐸=1/2𝑚𝑣2 KE = 281250 J