CER Forces Lesson

​Acceleration vs. Mass

By Robert Johnson

Claim

The bigger they are the harder they fall.

What is the evidence?
What is the scientific reasoning?
But, first...

Background information

Key terms

• inversely proportional

• mass (of system)

• kilogram (kg)

• acceleration

• m/s/s (m/s²)

• Force

• ​Newton (N)

• Net Force

• Inertia

• Opposing Forces

• gravity (g)

• Weight

• ​Friction

• coefficient of friction

• Tension Force

• Normal Force

• Force Diagram

• momentum

Inversely Propotional

As x increases, y decreases.
As x decreases, y increases
Their product is constant

Multiple Choice

Inversely proportional means...

• Two quantities are directly proportional if their sum is constant.
• Two quantities are inversely proportional if their difference is constant.
• Two quantities are inversely proportional if their ratio is constant.
• Two quantities are inversely proportional if their product is constant.

Multiple Choice

The equation for inversely proportional is y = ?

• y = k/x
• y = k+x
• y = k*x
• y = k-x

Multiple Choice

As x increases, y decreases and vice versa is ...

• an inverse relationship
• directly proportional

• inversely proportional

• a positive correlation

Mass

The quantity of matter or how much stuff something is made up of.

• Measured in kilograms preferably.

• Proportional to Net Force, inversely proportional to acceleration.

• the mass of a system is defined as the total quantity of matter it contains, which is calculated by summing the individual masses of all objects within the system

Multiple Choice

The amount of matter in an object is 

• pressure
• weight
• mass
• force

Multiple Choice

The SI unit for mass is...

• ounce
• pound
• gram
• kilogram

Multiple Choice

What is meant by the mass of a system?

• The total amount of matter in a system defined by the experimenter.

• The weight of a system in motion.
• The volume occupied by a system.
• The energy contained within a system.

Multiple Choice

Mass is directly proportional to______ and inversely proportional to ______, according the Newton's 2nd Law

• force; distance
• mass; velocity
• force; acceleration
• energy; time

Inertia

Law of Inertia (Newton's 1st Law of Motion)

• Inn-ur-sha

• Increases with mass

• resistance to a change in motion.

• An object in motion will remain in motion unless acted on by a net force.

Multiple Choice

The Law of Inertia states that

• An object will remain at rest or in uniform motion unless acted upon by an external force.
• An object will change its state only when it is in motion.
• An object will accelerate indefinitely without any force.
• An object will stop moving if no force is applied to it.

Multiple Choice

A resistance to a change in motion is ______

• inertia
• acceleration
• friction
• momentum

Multiple Choice

Inertia increases with ...

• temperature
• velocity
• mass
• friction

Acceleration

​Speeding up, Slowing Down, or changing directions

• Measured in m/s/s or m/s²

• Directly proportional to force and indirectly proportional to mass

• Must be > 0 to produce a net force.

Multiple Choice

acceleration is

• speeding up

• slowing down

• changing directions

• all of the above

Multiple Choice

The units for acceleration are ...

• kilometers per hour (km/h)
• meters per minute (m/min)
• meters per second squared (m/s²)
• feet per second squared (ft/s²)

Multiple Choice

_____ must be greater than zero to produce a net force.

• acceleration

• Gravitational pull
• Frictional force
• Inertial mass

Force and Net Force

Push or Pull (Force) that causes an acceleration (Net Force)

• Measured in newtons (N)

• F_net =m_system*a (Newton's 2nd Law)

• Not all pushes or pulls cause a change in motion.

Multiple Choice

Forces are measured in _____

• pascals
• joules
• watts
• newtons

Multiple Choice

Compare and contrast force with net force.

• Force is a push or pull; net force produces a change in motion.

• Force is a vector quantity; net force is a scalar quantity affecting motion.
• Force is a total of all interactions; net force is an individual force acting on an object.
• Force is the sum of all interactions; net force is a single force acting on an object.

Multiple Choice

How do you calculate net force?

• Net force is found by dividing total mass by acceleration.
• Net force is multiplying mass times acceleration

• Net force is determined by averaging all forces applied.
• Net force is the product of mass and velocity of an object.

Opposing Forces

Law of Action and Reaction

• For every action there is an equal and opposite reaction.

• Newton's 3rd Law

Multiple Choice

An opposing force is defined as ...

• a force that enhances the effect of another force
• a force that remains stationary and does not interact
• a force that is always directed upwards regardless of other forces
• a force that acts in the opposite direction to another force

Multiple Choice

Newtons's 3rd Law of Motion is also known as...

• Push and Pull
• Action and Reaction
• Mass and Acceleration
• Force and Motion

Multiple Choice

Newton's Third Law of Motion States...

• For every force, there is a corresponding force in the same direction.
• Every action creates a reaction that is twice as strong.
• For every motion, there is a counter motion that cancels it out.
• For every action, there is an equal and opposite reaction.

Newtonian Gravity

Gravity is a downward acting force.

Gravity = g = 9.8 m/s^{2
or 9.8 N/kg, because net force/mass = acceleration
Often written as F}_g or the force due to gravity.

Multiple Choice

Gravity is a________ acting force.

• medium-range
• downward

• upward

• short-range

Multiple Choice

Fg represent

• The normal force

• The force due to gravity

• tension force

• friction force

Weight

Downward acting force due to mass and gravity

Replacing a with g, Newton's 2nd Law can be rewritten as:
Fnet =m_system*g or F =mg
g = 9.8 m/s²

Multiple Choice

What is the weight of a 2.00-kilogram object on the surface of Earth?

• 4.91 N

• 2.00 N

• 9.81 N

• 19.6 N

Multiple Choice

A 60.-kilogram astronaut weighs 96 Newtons on the surface of the Moon. The acceleration due to gravity on the Moon is

• 0.0 m/s²

• 1.6 m/s²

• 4.9 m/s2

• 9.8 m/s²

Multiple Choice

An object has a weight (Fg) of 200 N. If the object is on the surface of the Earth, What is this object's mass?

• 200 kg

• 100 kg

• 20.4 kg

• 20.4 N

Multiple Choice

A 50-kilogram student, standing on the Earth, attracts the Earth with a force closest to

• 0 N

• 5 N

• 50 N

• 500 N

Friction

Opposes motion, slows things down, and results in waste heat.
μ = coefficient of friction.
Multiplied by the calculated force to determine the net force.

Multiple Choice

Friction ...

• Opposes motion

• slows objects down

• creates waste heat

• all of the above

Multiple Choice

A 2.0-N force acts horizontally on a 10-N block that is initially at rest on a horizontal surface. The coefficient of static friction between the block and the surface is 0.50.What is the magnitude of the frictional force that acts on the block?

• 5 N

• 8 N

• 2 N

• 0 N

Multiple Choice

There are 25N of friction that keep a 10kg brick moving at a constant rate. What is the coefficient of friction?

• 0.25

• 0.25N

• 4

• 4N

Multiple Choice

How does friction affect speed?

• more friction decreases speed
• more friction increases speed
• less friction decreases speed

Multiple Choice

 What is the force of friction between a block of ice that weighs 950 N and the ground if μ = .12? 

• 65 N
• 226 N
• 175 N
• 114 N

Other types of Forces

Normal Forces, Tension Forces, etc.

• Normal Force is an upward force that opposes gravity.

• Tension force is an upward force that opposes gravity.

• The difference is that a tension force involves hanging, like an elevator. Normal forces occur when objects are resting, like on the floor.

Force Diagrams

aka Free-body Diagrams

• Extremely useful in solving problems in physics.

• Keeps track of all the forces, which is necessary to determine the net force.

• uses arrows to show the magnitude and direction of forces.

• The object can be represented with a dot or a box.

• You don't have to be artistic to draw one of these.

Multiple Choice

The down-arrow in this diagram is most-likely:

• gravity

• normal

• a push or pull

• friction

Multiple Choice

If this object were moving right, the left-arrow in this diagram is most-likely:

• gravity

• normal

• a push or pull

• friction

Multiple Choice

Which diagram best shows person pushing a couch (accelerating) across the floor?

Multiple Choice

Name the missing force....

• weight force
• normal force
• frictional force
• tension force

Multiple Choice

Name the missing force...

• tension force
• frictional force
• normal force
• spring force

Multiple Choice

Name the missing force...

• frictional force
• weight force
• thrust
• buoyant force

Momentum

ρ = mv

• Momentum is a way to quantify motion, and is calculated by multiplying mass times velocity.

• Unlike net force is does not require acceleration.

• Objects with large momentums are hard to stop and vice versa.

• Unit is kg m/s

Multiple Choice

The momentum of an object depends upon the object's ___________&_____________.  

• size and shape
• mass and speed
• mass and veloctiy
• mass and energy

Multiple Choice

The equation to calculate momentum is

• p=mv
• p=1/2mv²
• p=mgh
• p=mg

Multiple Choice

Determine the momentum of 1000-kg car moving southward at 20 m/s......... 

• 20000 N
• 20000 Joules
• 20000 Kg * m/s south
• 10000 kg * m/s south

Multiple Choice

What is the unit for momentum?

• m/sec
• kg/sec
• kg/m
• kg m/sec

Multiple Choice

A 0.250 kg cart moving at 0.400 m/s has how much momentum?

• 0.1 kg-m/s
• 1 kg-m/s
• 10 kg-m/s
• 100 kg-m/s

Multiple Choice

Determine the momentum of 1000-kg car moving northward at 20 m/s......... 

• 20000 N
• 20000 Joules
• 20000 Kg * m/s
• 20000 kg * m/s north

Multiple Choice

The definition of momentum is

• Mass in Motion
• The size of an object
• The velocity of an object
• A comparison between two objects

Evidence

Experimental Design

Results

Linearization

​y=mx+ b or acceleration = m*1/mass

Reasoning

Newton's 2nd Law

The results show a y =1/x relationship, so squaring the mass values will linearize the graph. Then we can use y=mx + b
a = F_net/m_{system
acceleration is directly proportional to Net force, and inversely proportional to mass}

What is the slope of an acceleration vs mass graph?

​​y=mx+ b or acceleration = m*1/mass

• Rise/run with units = m/s² /(1/kg) rewritten as kg* m/s² (Reciprocal Rule of fractions)

• kg* m/s² = 1 N according to Newton's second law abbreviated F =ma

• Also expressed as acceleration = Force/mass or a=Fnet/m.

• So, slope =m = Force on an acceleration vs. mass graph.