Units and formulas
What is the formula for force?
What are the units?
Force = mass x acceleration
Force = work / distance
units = Newtons, N
A student pushes a cart with a force of 60 N across the floor for 5 m in the same direction as the push.
How much work is done on the cart?
W= F×d = 60N×5m = 300 J
A student uses a simple machine that lets them lift a 300 N load using only 100 N of input force.
What is the mechanical advantage of the machine?
Formula:
MA=Force out / Force in
Solution:
MA=300 N / 100 N =3
Which of the following is the correct formula for kinetic energy?
a) KE = mgh c) KE = Fd
b) KE = ½mv² d) KE = m/v
b) KE = ½mv²
What is the purpose of this simple machine:
a screw
to hold things together
to fasten things together
What is the formula for work?
What are the units for work?
Work = Force x distance
Units = Joules, J
A student does 400 J of work pushing a cart 8 meters across the classroom floor.
The force was applied in the same direction as the motion.
How much force did the student apply to the cart?
Formula:
W=F×d
Rearrange:
F= W/d = 400 J / 8 m = 50 N
Answer:
The student applied a 50 N force to the cart.
A student uses a crowbar to lift a rock.
The force applied to the crowbar is 60 N, and the force exerted on the rock is 300 N.
What is the mechanical advantage of the crowbar?
MA = Force output / force input
MA = 300 N / 60 N = 5
Which type of energy is stored due to an object’s position above the ground?
a) Elastic potential energy b) Chemical potential energy
c) Gravitational potential energy d) Thermal energy
c) Gravitational potential energy
What is the purpose of this simple machine:
an inclined plane
to make it easier to move heavy objects to a higher elevation by reducing the force needed
What is the formula for Power?
What are the units for Power?
Power = Work / time
Units = Watts
A student does 300 J of work pushing a cart in 6 seconds.
What is the power of the student’s push?
Power = Work / Time
Power = 300 J / 6 seconds = 50 Watts
A pulley system has a mechanical advantage of 4.
If the pulley lifts a load that is 240 N, what is the input force?
Formula: MA = Force out / Force in
Rearrange formula:
Force in = Force out / MA
Force in = 240 N / 4 =60 N
Stretching a rubber band stores what type of energy?
A) Kinetic energy
B) Elastic potential energy
C) Mechanical energy
D) Chemical potential energy
B) Elastic potential energy
What is the purpose of this simple machine:
lever
to make work easier changing the amount of an applied force or changing the direction an applied force
What is the formula for Mechanical Advantage?
What are the units?
Mechanical Advantage = Force out / Force in
There are no units. Force / Force ; units cancel out
A small motor produces 200 watts of power while doing 1,000 joules of work.
How much time does it take the motor to do the work?
Formula:
P=Work/time
Rearrange:
time =Work / Power
t=1,000 J / 200 W = 5 seconds
What is the efficiency of a heater that uses 500 J of electricity but only emits 400 J of heat?
Efficiency=(Work out/ Work in)×100
Efficiency= (400J / 500J) x 100 = 80%
Which type of energy is the sum of potential and kinetic energy in a system?
A) Thermal energy
B) Mechanical energy
C) Chemical energy
D) Nuclear energy
B) Mechanical energy
What is the purpose of this simple machine:
pulley
to change the direction of a force or the amount of a force needed to lift or move a heavy object
What is the formula for Mechanical Efficiency?
What are the units?
Why will Mechanical Efficiency never be greater than 100%?
Mechanical Efficiency = (Work out/ Work in) x 100%
Units = no units, expressed as a percentage
Always less than 100% because some energy leaves the system. Nothing is 100% efficienct
A student uses a force of 150 N to lift a box straight up onto a shelf 2 meters high in 3 seconds.
What is the students power?
Step 1: Find Work
W = F×d = 150 N x 2 m = 300 J
Step 2: Find Power
P=Work / time = 300 J / 3 seconds = 100 Watts
A student uses a lever that has a mechanical efficiency of 85%.
If the work input is 400 J, how much work output does the lever produce?
Formula: Efficiency=(Work out / Work in)×100
Rearrange:
Work out = ( Efficiency / 100 ) x Work in
Work out= (85 /100 )× 400 J = 340 J
According to the Law of Conservation of Energy, energy can be:
A) Created but not destroyed
B) Destroyed but not created
C) Created and destroyed
D) Neither created nor destroyed, only transformed
D) Neither created nor destroyed, only transformed
What is the purpose of this simple machine:
wheel and axel
to make work easier by increasing the force or distance