Physics and Human Affairs study guide

motion

A change in an object’s position over time

Speed

How fast an object is moving

Acceleration

A change in velocity (speed or direction) over time

Constant speed

An object moves at the same speed

Increasing speed

Object is speeding up

Decreasing speed

Object is slowing down

At rest

Object is not moving

An object is accelerating if it:

-speeds up

-slows down

-changes direction

Dot Diagrams

An object’s position at equal time intervals

Dot Diagram (constant speed)

dots are equally spaced

Dot Diagram (speeding up)

dots get further apart

Dot Diagram (slowing down)

dots get closer together

Dot Diagram (not moving)

dots are in the same place

Motion Diagrams

An object’s position and often include velocity arrows

Motion Diagram (speeding up)

arrows get longer

Motion Diagrams (slowing down)

arrows get shorter

Motion Diagram (constant velocity)

arrows stay the same length

Changing direction

If an object changes direction, it is accelerating, even if its speed stays the same

Speed-time graphs (constant speed)

A horizontal line

Speed-time graphs (speeding up)

An upward-sloping line

Speed-time graphs (slowing down)

A downward-sloping line

Newton’s First Law (Law of Inertia)

An object will remain at rest or continue moving at a constant velocity in a straight line unless acted on by an unbalanced/net force

Forces

A push or pull that occurs during an interaction between objects

• gravity pulls objects downward

• your hand pushes a door

• friction pushes against motion

• a rope pulls an object

When does force end?

A force exists while the interaction is occuring

• you push a box —> force is acting

• you stop pushing —> your pushing force ends

However, other forces such as gravity or friction may still be acting

← 10 N [BOX] 10 N →

the total/net force = 0 N

← 5 N [BOX] 10 N →

the total/net force = (10-5) = 5 N right

0 N

No acceleration

Right

Accelerates right

Left

Accelerates left

Forward

Accelerates forward

Backward

Accelerates backward

Important for net force

A net force does not necessarily mean the object is speeding up.

It means the object is accelerating.

acceleration can mean:

• speeding up

• slowing down

• changing direction

Object speeding up

The net force is in the same direction as the objects motion

Object slowing down

The net force is opposite the object’s motion

Object moving at constant speed

Net force = 0 (no acceleration)

How to tell when a force ends from a speed-time graph

If a force causes an object to accelerate, you can look for when the graph stops changing

Newton’s Second Law

Fnet​=ma

f = net force, measured in Newtons (N)

m = mass, measured in kilograms (kg)

a = acceleration, measured in m/s²

What is mass?

Describes how much matter an object contains. It also relates to an object’s inertia - how resistant it is to change in motion.

Same force, different mass

more mass —> less acceleration

less mass —> more acceleration

Same mass, different force

Object A gets 10 N

Object B gets 20 N

• Object B will have twice the acceleration

Remember:

• more force —> more acceleration

• less force —> less acceleration

Newton’s Second Law (increase force)

acceleration goes up

Newton’s Second Law (decrease force)

acceleration goes down

Newton’s Second Law (increase mass)

acceleration goes down

Newton’s Second Law (decrease mass)

acceleration goes up

Gravity

An attractive force between objects that have mass. The more mass an object has, the stronger the gravitational attraction. The closer two objects are, the stronger the gravitational force between them

Weight

The force of gravity acting on an object

W = mg

W = weight

m = mass

g= gravitational acceleration

Mass

• amount of matter

• measured in kg

• doesn’t depend on location

• same on Earth and Moon

Weight

• force of gravity

• measured in N

• can change depending on gravity

• different on Earth and Moon

Weightlessness

An object is experiencing apparent weightlessness, commonly because it is in free fall and there is no supporting force acting on it

If the mass of one object increases

—> gravitational force increases

If the mass decreases

—> gravitational force decreases

If the distance between two objects increases:

—> gravitational force decreases

more mass = more gravity

If the distance decreases:

—> gravitational force increases

more distance = less gravity

Special Relativity

Basic Idea:

• Deals with how measurements of space and time depend on observer’s reference frame, especially when objects move at speeds close to the speed of light.

What is relative

If different observers can measure it differently depending on their reference frames

Quantity

• length (relative)

• time (relative)

• speed/velocity (relative)

• mass (invariant)

Reference Frame

The perspective from which motion is measured

Time Dilation

Basic idea:

• a clock moving relative to you can appear to run slower than a clock at rest relative to you

• the person traveling with the clock measures their own proper time, while another observer who sees that clock moving measures a longer elapsed time between the same events

Special Relativity: What to memorize

moving very fast —> compared with a stationary observer:

time: —> moving clock is measured to run slower

length: —> length in the direction of motion is measured shorter

speed of light: —> same for all inertial observers