Unit 2: Forces Flashcards
Force (F)
- A push or a pull
- Causes objects to change their motion and/or shape
- Is a vector quantity
Newton (N)
- Derived SI unit
- 1 N = 1 kg x m/s²
4 fundamental forces (in order of strength)
- Gravitational force
- Weak nuclear force
- Electromagnetic force
- Strong nuclear force
Derived Forces
- Any non-fundamental force
- Comes from the 4 fundamental forces (ex: friction and tension)
Gravitational force
- Known as “force of gravity” or “weight”
- Attraction only
- Acts between all objects in the universe
- Weak force but long range
Electromagnetic force [A]
- Caused by electric charges
- Most common force (ex: light, electricity, magnetic attraction)
Electromagnetic force [B]
- Strong force and long range
- Can attract or repulse objects (both tend to cancel each other out)
Strong nuclear force
- Holds protons and neutrons together
- Very short-range force
- Much stronger than electromagnetic force
Weak nuclear force
- Occurs between “elementary” particles of an atom (ex: electron)
- Responsible for radioactive decay
- Strong force but very short range
Free-body diagram (FBD): A simple drawing representing
An object and all external forces acting on it
FBD: Object is shown as
A rectangle or large dot
FBD: Forces are drawn as arrows
Originating from object and pointing away from the center
FBD: Each force is labelled with
Symbol F and an appropriate subscript that indicates the force
Applied force, F(a)
- Results when one object is in contact with another object
- Either pushes or pulls on it
Tension, F(t):
Pulling force exerted on an object by rope or string
Indirect Forces
- Objects don’t have to directly push/pull another object to be considered a type of force
- A car engine used to apply force against wheels, which push against ground, causing motion, can be an considered an applied force.
Normal force, F(n)
- Perpendicular force exerted on an object by the surface with which it is in contact.
- Even if object is at rest, normal force is applied
Friction force, F(f)
- Resists motion or attempted motion of object
- Always acts parallel to surface
Contact forces
Require one object to be in contact with another
Non-contact forces
Do not require one object to be in contact with another
Force of gravity, F(g)
- F(g) = mg
- m is the mass of the object
- g is gravitational acceleration (9.8 m/s²)
Net force, F(net) [meaning]
- AKA: Resultant force
- Vector sum of all the forces acting on an object
Net force, F(net) [formulae]
- F(net) = F₁ + F₂ + …
- F₁,₂… are the individual forces acting on an object
Object at-rest
- Means the acceleration and velocity of an object is 0
- The net force applied to the object is 0
Inertia
- Property of matter that causes it to resist changes in motion
- Directly proportional to the mass of the object
First Law of Motion (Law of Inertia)
If the net external force on an object is 0, the object will
1. Remain at rest or
2. Continue to move at a constant velocity
Implications of Newton’s First Law [A]
- Objects at rest remain at rest.
- Objects at motion remain in motion at constant speed and direction.
- Internal forces have no effect on an object’s motion.
Implications of Newton’s First Law [B]
- If velocity is constant, net force acting on it must be 0
- If velocity changes (in direction and/or magnitude), change must be caused by net external force acting on object.
2nd law of motion: If the net external force on an object is not zero
The object will accelerate in the direction of this net force.
2nd law of motion: The magnitude of the acceleration is
- Directly proportional to the magnitude of the net force
- Inversely proportional to the mass of the object.
Formula: F(net) = ma
- F(net) is net force acting on object (N)
- m is the mass of the object (kg)
- a is the acceleration of the object (m/s²)
Motion of an object when F(net) = 0
The object is experiencing uniform motion
Motion of an object when F(net) ≠ 0
The object is experiencing non-uniform motion
Finding the motion of an object in a force question
- Draw a free body diagram
- Find F(net)
- Find “a”
- Find v, d, t
Finding the force on an object in a motion question
- Draw a free body diagram (with variables as the forces)
- Find v, d, t and the mass
- Find “a”
- Find F(net)
Where 3rd law of motion is applied
A contact OR non-contact force on an object by another object
Newton’s third law of motion (statement)
For every action force, there is a reaction force equal in magnitude but opposite in direction
Application of Newton’s third law of motion
The action and reaction forces act on different objects simultaneously, not sequentially, on contact
Formula of action-reaction law
F(action) = -F(reaction)
Free body diagrams of action-reaction forces
Because action and reaction forces are acting on different objects, they will appear on separate FBDs.
Are gravitational and normal forces action-reaction forces?
- Gravitational and normal forces are not action-reaction forces
- They act on the same object
Solving problems with multiple objects accelerating [A]:
- Take all objects as one mass
- Use single mass to determine acceleration of whole system
Solving problems with multiple objects accelerating [B]:
- Take masses as individual objects accelerating at same rate (step 1)
- Draw FBD of each mass
- Analyze each to determine action-reaction
Pulley (for strings)
Changes the direction of tension without changing the magnitude
Force sensors or spring scales
Can measure tension in strings if tied between two strings
Principles of Tension
- Tension magnitude is the same at any point of a string
- Tension pulls away from an object at the ends of a string
- Tension does not affect acceleration of objects tied by a string
Solving problems with tension
- Calculate just the magnitude of tension
- Refer to the FBD for direction
Example problem: Two objects of same mass on opposite sides held by pulley, at rest
- By drawing FBD for either object, same value will be given for tension.
- One object just provides force necessary to hold up other object
Kinematics and Net Force Value
Net force on object must be constant if any kinematics equation used
Field
Type of property of space
Source of Field
The object producing the field
Force Field
Region of space around an object that exerts force on other objects placed within that region
Gravitational Field Quantity Type
Vector quantity (g)
Gravitational Field Function
Exerts an attractive force on objects with a mass
Representation of force field around Earth
Drawing series of lines pointing toward Earth’s center
Change in magnitude of field
Becomes weaker as the distance from Earth’s centre increases
Weightlessness
No force of gravity
Microgravity
Very little force of gravity
Free-fall
Motion of an object solely under influence of gravity
Mass vs weight: type of quantity
- Mass: scalar quantity
- Weight: vector quantity
Mass vs weight: measurement taken
- Mass: matter in an object (kg)
- Weight: force of gravity acting on object (N)
Mass vs weight: method of measurement
- Mass: balance
- Weight: spring scale or force sensor
Mass vs weight: values across time
- Mass: constant - only changes if the quantity of matter changes
- Weight: varies - depends on the magnitude of g at that location
Objects launched at Earth’s orbit: Critical Speed
- Object must travel down and around before landing
- When certain critical speed is reached, object’s path curves downward at same rate as Earth’s curvature, “at orbit”
Objects launched at Earth’s orbit: Slower vs. Faster Speeds
- Slower speeds: objects fall quickly to ground
- Faster speeds: objects travel farther and longer (time-wise)
Orbiting Object
- An object in constant free fall
- Always falling toward Earth but never landing
Force of gravity (equation)
F(g) = mg
Force of gravity (variables)
- F(g) is force of gravity on object (N)
- m is mass of object (kg)
- g is gravitational field strength (N/kg)
Gravitational field strength (meaning)
Force per unit mass acting on an object within a gravitational field
Gravitational Field Strength (type of quantity)
Vector quantity (because it has a direction)
Gravitational field strength (on Earth)
- Decreases as altitude increases
- Varies according to location since Earth is not perfect sphere
Relationship between gravitational field strength and acceleration from gravity
Equal magnitudes
Unit of gravitational field strength
N/kg (same magnitude as m/s² for all values)
Gravitational Field Strength (formulae)
- g = Gmₚ/r²
- mₚ is the mass of the planet/celestial object (kg)
Where universal law of gravitation applies
Between any two objects
Variable G
- The universal gravitational constant
- G = 6.67 x 10-¹¹ N·m²/kg²
Variables of universal law of gravitation
- F(g): force of gravity (N)
- m₁ and m₂: mass of each object (kg)
- r: distance between centers of masses (m)
Universal law of gravitation
F(g) = Gm₁m₂/r²
Requirement for force to be noticed
m₁ and/or m₂ must be large relative to “r”
Air Resistance
- Friction from the air
- Acts opposite to direction of object’s motion if there’s no wind
Cross-sectional area
2D area of a 3D object perpendicular to a surface
Air resistance acting on object: cross sectional area
Larger cross-sectional areas experience more air resistance
Air resistance acting on object: speed
Faster-moving objects experience more air resistance
Terminal speed
- The maximum constant speed of a falling object
- Reached after an object falls for certain amount of time
Cause of Friction
Electrical forces between surfaces where two objects are in contact
Static Friction F(s)
Force that prevents stationary object from starting to move
Starting Friction
- The maximum static friction
- Amount of force that must be overcome to start stationary object moving
- Slightly greater than kinetic friction
Kinetic Friction F(k)
- Force that acts against an object in motion
- Includes sliding, rolling, fluid friction
- Slightly smaller than starting friction
If applied force has same magnitude as kinetic friction (horizontally)
Moving object will maintain uniform velocity
Force of Friction F(f)
- = μF(n)
- F(f), F(n) are friction and normal forces (N)
- μ is coefficient of friction
Coefficient of friction (μ)
- Ratio of the friction force to the normal force
2.. Constant - only depends on nature of two surfaces in contact
Coefficient of kinetic friction μ(k)
Ratio of kinetic friction to normal force F(k)/F(n)
Coefficient of static friction μ(s)
- Ratio of static friction to normal force F(s)/F(n)
- μ(s) ≥ μ(k) because F(s) ≥ F(k)
