Physics Unit 2: Forces Flashcards Preview

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Flashcards in Physics Unit 2: Forces Deck (21):

Define a force

Force: PULL or a PUSH upon an OBJECT resulting from the object's interaction with another object.
• Whenever there is an INTERACTION between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.
• Vector quantity (force has direction)
• Forces can cancel each other out (if equal)
• UNBALANCE force changes the motion


Types of Forces
-non-contact/action-at-a-distance forces

Contact Forces: must be in contact with another object
• If in the air, there is no contact = no contact forces (below)

Applied Force (Fa): a force that results when one object makes contact with another and pushes or pulls on it
• Ex: pushing a wagon has an Fa forward →force exerted on the wagon by a person
• Force is “applied” to an object by a person or another object

Tension Force (FT): a pulling force from a rope or string on an object that always points toward the rope or string
• Pulling wagon has a FT forward
• *direction of tension: always pulls the object toward the rope/string
**When an object is held by two ropes, then the tension is divided between the two ropes
o Ft = Fg then divide by two for tension in each rope

Normal Force (FN): a perpendicular/support force exerted by a surface on an object in contact with the surface; the normal force always points away from the surface (perpendicular)
• FN is a push from the surface onto the object
• “Support Force” exerted upon an object that is in contact with another stable object.
• Ex: if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.

Friction (Ff): a force that acts opposite to the motion or attempted motion exerted; a force exerted by a surface on an object. →opposes the sliding of two surfaces across one another
• Parallel to the surface
• Force exerted by a surface as an object moves across it or makes an effort to move across it.
• Sliding and Static friction force
o A static friction force exists between the surfaces of the floor and the box to prevent the box from being set into motion.
o The static friction force balances the force that you exert on the box such that the stationary box remains at rest. When exerting 5 Newton of applied force on the box, the static friction force has a magnitude of 5 Newton
• Ex: if the wagon is moving to the right, then the friction on the wagon acts toward the left (opposite to motion).
• Ex: If the wagon is at rest even if the children are pushing and pulling on it, then the friction is left, opposite to the tension and applied force keeping the wagon at rest
• Ex: when car applies breaks, the wheels lock and force of friction causes car to decrease velocity to zero.

Air resistance Force:
• Type of frictional force as an object moves through air

Spring Force: the force exerted by a compressed or stretched spring upon any object that is attached to it.
• Always acted upon by a force that restores the object to its rest or equilibrium position

Non-contact forces/action-at-a-distance forces:

Force of Gravity (Fg-the gravitational force): force of attraction between any two objects due to their mass
• Always points down toward Earth’s centre (doesn’t matter if surface is sloped)
• Weight (Fg): force that acts on its mass as a result of gravity = mass*acceleration due to gravity (Newton’s)
o Fg = m*g

Electric force:
• Protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation.

Magnetic force:
• Two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters.

*external forces: caused by one object pushing or pulling on another
**Internal force: an object exerts a force on itself


Net force

Net Force (Fnet): the sum of all forces acting on an object (resultant force)
• Use of Free Body diagrams
• When the sum of all forces = 0 →object has constant velocity or is stationary
• When the sum of all forces ≠0 →object accelerates in the direction of Fnet

**When an object is not sinking or flying, then the normal force and force of gravity cancel


Newton's first law

Galileo’s Law of Inertia:
• Once an object starts moving, it will continue moving at a constant velocity if there is no friction present

Inertia: the property of matter that causes it to resist changes in motion; inertia is directly proportional to the mass of the object
• Object with more mass has more inertia (harder to change motion)
• Object with less mass has less inertia
• Example of changes in motion: accelerating

Newton’s first law of motion (aka Law of Inertia): an object will remain at rest or continue to move at constant velocity when the net force on the object is zero
• Therefore: if the velocity of an object is constant, then the net force is zero
o When the acceleration is zero, the net force must also be zero
• A non-zero net force will change the velocity of an object
o Ex: velocity can change in magnitude, direction, or both

• A net force is not required to maintain the velocity of an object

• External forces are required to change the motion of an object. Internal forces have no effect on the motion of an object.

**FORCE is not needed to keep an object in motion
Ex: slide book across table eventually will stop due to the presence of friction. If no friction then the book would continue the motion forever

**Ice has no friction

Ex: Wagon pulled across a rough surface by a child at constant velocity
•Ground exerts a normal force up and the force of gravity pulls down on the wagon →forces cancel to give net force = 0
•The force of friction acts backward on the wagon and the child applies tension forward on the wagon →forces cancel to give net force = 0


Newton's second law

Newton’s Second Law: an object will ACCELERATE in the DIRECTION of the NET FORCE; the magnitude of the acceleration is directly proportional to the magnitude of the net force and inversely proportional to the object’s mass

• When the net external force on an object is NOT zero
• Mass = kg
• Large net force = large acceleration
• Small net force = small acceleration
• Large mass = small acceleration
• Small mass = large acceleration

Acceleration = Net force/mass
a = Fnet/m
Fnet = a*m
*→ Fg = mass*acceleration due to gravity (Newton’s)

Big 5 equations (motion in one direction)


SI units

mass: kg
Velocity: m/s (NOT km/h)


Calculations using Newton's laws

Calculations involving Newton’s laws
Find acceleration:
• a = Fnet/m
• a = change V/change t (one dimension)
• Big 5 equations (one dimension)

Find Fnet:
• Fnet = am
• Fnet = sum of all forces (label + and -)

If given mass hanging,
• Can calculate Fg = m*9.81
If also forces cancel out,
• Fg = Ft

*rearranging Fnet formula for acceleration and mass


Pully questions

Pulley questions:
• Forces on cart:
o Fn = Fg therefore cancel
o If no friction, then Fnet = Fg (on the hanging weight)
o If friction, then Fnet = Fg – Ff
o *Fg = gravity applied on hanging weight, which becames the force that causes the cart to accelerate (horizontal force)
• Forces on hanging weight
o Fg
• If cart is accelerating, then starting from rest (vi = 0)
*tension is created by Fg (hanging weight) therefore no Ft
Pulley translates from vertical force to horizontal force →frictionless pulley

a) Calculate acceleration
a = Fnet/m
a = (mass of hanging weight*9.81) + (friction if applicable)/total mass (because force is accelerating both objects)


Gravitational Force

Force due to gravity


Electrostatic Force

Force due to electric charges


Magnetic Force

Force due to magnets or electric currents


Normal Force

Force between two surfaces that acts at right angles to the surface


Tension Force

force that the end of a string applies inward to an object that is holding


Compression Force

Force that a rod applies outward to an object when the rod is being compressed


Upthrust Force

upward force that acts on an object that is immersed in a fluid


Lift force

Force caused by a fluid flowing over an object in an asymmetrical way


Newton's Third Law

Newton’s Third Law of Motion: For every action force, there is a simultaneous reaction force that is equal in magnitude but opposite in direction.
For every action, there is an equal and opposite reaction
• For every interaction, there is a pair of forces acting on the two interacting objects.

• When drawing FBD, the action and reaction forces will appear on separate FBDs. Since they are on separate FBDs, they are not added together

Ex: Swimmer moves through the water
• The swimmer’s arms and legs exert an ACTION force backwards on the water, causing the water to accelerate backwards. (object water)
• The water also exerts an equal but opposite force forward on the swimmer, propelling the swimmer forward through the water. (object swimmer)

**Identify the two interacting objects and make two statements describing who is pushing on whom and in what direction.


Internal and external forces

*differentiate between external force (causing motion of entire object) and internal force (object exert force on another object)

To calculate the internal contact force: i.e. tension
-choose FBD with fewer forces (one being applied force)
Fnet = contact – friction
ma = contact – friction
*a = total of all objects



• If objects are connected by a rope, then treat as entire system (accelerating all masses; ignore internal forces like tension). After find acceleration, then can look at the objects individually (look at internal forces)
o Ffrictiontotal!

• Always find acceleration! Acceleration of entire is the same as the acceleration of objects that are connected

• If objects have different masses and are applied with the same force in opposite directions (i.e. newton’s third law), then the objects will have different acceleration
o Greater mass = lower acceleration


Static and Kinetic

Static Friction (Fs): the force of friction that prevents two surfaces in contact from sliding relative to one another. Maximum force of static friction is a force of friction exerted on a stationary object by a surface that prevents the object from starting to move
• Objects are at rest
• Applied force must be greater than max static friction to cause the object to move
• A static friction force exists between the surfaces of the floor and the box to prevent the box from being set into motion.
• Has a range (minimum to max)
o Ffrict-static ≤ μfrict-static• Fnorm
o Ex: max static friction is 25
• If applied 5 N, then Fs = 5 N (no motion)
• If applied 25 N, then Fs = 25 N (no motion)
• If applied 26 N, then Fs = 25 N (motion)

Kinetic Friction (Fk): a force exerted on a moving object by a surface that acts OPPOSITE to the direction of motion of the object
• Constant velocity: applied force = kinetic friction
• Refers to many types of friction:
o Sliding, rolling, drag/fluid

**Static friction is greater than kinetic friction (force required to start an object moving is greater than the kinetic resistance acting on the object when it is moving)

What affects force of friction:
• Depend on MASS of object, the type of material the OBJECT is made of, and the type of SURFACE the object is in contact with (MATERIAL)

If two or more objects are attached, you must overcome the COMBINED maximum force of static friction to cause the objects to move.


Coefficient of friction

Coefficient of Friction: the ratio of the force of friction to the normal force (no units)
• Coefficient of static friction
• Coefficient of kinetic friction
• Between 0 and 1 (0-frictionless surface; 1-high friction surface where frictional force is equal to the normal force)
*how rough the surface is times weight (Fn = Fg)