Notes – Chapter 4 – Forces in One Dimension

Force

• Force - Any push or pull acting on an object

• F = vector notation for the magnitude and direction

• F = only size of forceMeasured in :

Newtons (N)

Force - Systems

• System – Objects or objects of interest

• External world – Everything around the system with which the system can interact.– hand, Earth, string,

table, etc.

Forces

• Contact forces – When an object from the external world touches a system, exerting a force

• Field forces – Other ways in which the motion of objects can change. ie. gravity

Forces

• Contact forces – When an object from the external world touches a system, exerting a force

• Field forces – Other ways in which the motion of objects can change. ie. gravity

Fundamental Forces• 1) Gravity – Causes an

attraction between the mass of objects and produces weight

• 2) Electromagnetic - Causes an attraction between positive and negative charges

• 3) Nuclear – Strongest fundamental force. It holds protons and neutrons together in the nucleus of atoms

Forces - Agent

• Agent – The specific and identifiable cause of every contact and field force

• Eg. When you push your textbook, your hand (the agent) exerts a force on the textbook (the system)

• Contact forces – When an object from the external world touches a system, exerting a force

• Field forces – Other ways in which the motion of objects can change. ie. gravity

• Free-body diagram

Net force

• Net force – The vector sum of all the forces on an object– Can be adding or

subtracting

Net force

Laws of Motion

• Isaac Newton – 1686 – English scientist discovered the three laws of motion

• Motion – The change in position when compared to a reference point– Reference point – A place

or object used to determine if something is in motion

• 2nd Law: States that the acceleration of an object is proportional to the net force and inversely proportional to the mass of the object being accelerated

• a = Fnet_

m

2nd Law of Motion

• 2nd Law: States that the acceleration of an object is proportional to the net force and inversely proportional to the mass of the object being accelerated

• a = Fnet_

m

2nd Law of Motion

• 2nd Law: States that the acceleration of an object is proportional to the net force and inversely proportional to the mass of the object being accelerated

• a = Fnet_

m

2nd Law of Motion

• 2nd Law: States that the acceleration of an object is proportional to the net force and inversely proportional to the mass of the object being accelerated

• a = Fnet_

m

1 N = 1 kg x 1 m/s2

= 1 kg- m/s2

2nd Law of Motion

2nd Law of Motion

• Acceleration of an object depends on the amount of mass and the size of the force.

• Acceleration = change in speed or velocity over time. It could be speeding up, slowing down, or changing directions

2nd Law of Motion

• Acceleration of an object depends on the amount of mass and the size of the force.

• Acceleration – change in speed or velocity over time. It could be speeding up, slowing down, or changing directions

• The acceleration of an object is directly proportional to the force and inversely proportional to the mass

• a = F / m acceleration = Force / massLarge force = large accelerationLarge mass = small acceleration

2nd Law of Motion

• The acceleration of an object is directly proportional to the force and inversely proportional to the mass

• a = F / m acceleration = Force / massLarge force = large accelerationLarge mass = small acceleration

2nd Law of Motion

Large force + small mass = high acceleration

small force + large mass = low acceleration

Larger force = higher acceleration

• Falling objects – All objects fall at the same rate

• Large mass small mass

2nd Law of Motion

Big Forcelittle acceleration

Little force

big acceleration

• Falling objects – All objects fall at the same rate

• Large mass small mass

2nd Law of Motion

Big Forcelittle acceleration

Little force

big acceleration

1st Law of Motion

• 1st Law : An object that is at rest will remain at rest, and an object that is moving will continue to move in a straight line with a constant speed, if and only if the net force acting on that object is zero.2nd Law : if Fnet = 0, a = 0

1st Law of Motion

• Law of Inertia.• Inertia – Causes

resistance to change in motion= Force? No : force= interactions of 2 objects–Friction – A force that slows down motion.–1st Law examples: It takes force to move an object. A hockey puck slides in a straight line across ice. Gravity keeps things stationary on Earth (stationary remains at rest)

1st Law of Motion• Law of Inertia.• Inertia – Causes resistance to

change in motion= Force? No – force= interactions of 2 objects–Friction – A force that slows down motion.–1st Law examples: It takes force to move an object. A hockey puck slides in a straight line across ice. Gravity keeps things stationary on Earth (stationary - remains at rest)

1st Law of Motion• Law of Inertia.• Inertia – Causes resistance to change in motion

– Friction – A force that slows down motion.

– 1st Law examples: It takes force to move an object. A hockey puck slides in a straight line across ice. Gravity keeps things stationary on earth(stationary -

remains at rest)

– Equilibrium :• net Force = 0

1st Law of Motion

• Law of Inertia.• Inertia – Causes

resistance to change in motion– Friction – A force that

slows down motion.– 1st Law examples: It takes

force to move an object. A hockey puck slides in a straight line across ice. Gravity keeps things stationary on earth

Weight

• Weight = Gravitational force due to earth’s mass

• Fg = mg• g = gravitational field• Earth’s surface =

– 9.8 N/kg

• Apparent Weight = Gravity plus the support force acting on an object.– Elevator up = feel

heavier– Elevator down = feel

lighter

Weight

• Apparent Weight = Gravity plus the support force acting on an object.– Elevator up = feel heavier– Elevator down = feel

lighter

• Weightlessness – No contact forces acting to support the object. Apparent weight = zero

Weight

• Apparent Weight = Gravity plus the support force acting on an object.– Elevator up = feel heavier– Elevator down = feel

lighter

• Weightlessness – No contact forces acting to support the object. Apparent weight = zero

Drag force

• Drag force – Force exerted by a fluid on an object opposing motion through the fluid.– Depends on: motion of

the object, properties of the object, properties of the fluid

Terminal velocity

• Terminal velocity – Constant velocity reached when drag force equals the force of gravity.– Light objects with large

surface area = higher drag force

– Heavy compact objects = lower drag force

• Air resistance– Air friction slows down falling objects

• In a vacuum (no air) a feather would fall at the same rate as a bowling ball

Terminal velocity

• Terminal velocity – Constant velocity reached when drag force equals the force of gravity.– Light objects with large

surface area = higher drag force

– Heavy compact objects = lower drag force

• Vterminal

• Tennis ball = 9 m/s• Basketball = 20 m/s• Baseball = 42 m/s

Object shape influences drag force Fusiform – Egg shape – reduces drag

Sky diver – Spread eagle = 60 m/s+ parachute = 5 m/s

Terminal velocity

• Terminal velocity – Constant velocity reached when drag force equals the force of gravity.– Light objects with large

surface area = higher drag force

– Heavy compact objects = lower drag force

• Vterminal

• Tennis ball = 9 m/s• Basketball = 20 m/s• Baseball = 42 m/s

Object shape influences drag force Fusiform – Egg shape – reduces drag

Sky diver – Spread eagle = 60 m/s+ parachute = 5 m/s

Terminal velocity

• Terminal velocity – Constant velocity reached when drag force equals the force of gravity.– Light objects with large

surface area = higher drag force

– Heavy compact objects = lower drag force

• Vterminal

• Tennis ball = 9 m/s• Basketball = 20 m/s• Baseball = 42 m/s

Object shape influences drag force Fusiform – Egg shape – reduces drag

Sky diver – Spread eagle = 60 m/s+ parachute = 5 m/s

Terminal velocity

• Terminal velocity – Constant velocity reached when drag force equals the force of gravity.– Light objects with large

surface area = higher drag force

– Heavy compact objects = lower drag force

• Vterminal

• Tennis ball = 9 m/s• Basketball = 20 m/s• Baseball = 42 m/sObject shape influences drag force Fusiform – Egg shape – reduces dragSky diver – Spread eagle = 60 m/s

+ parachute = 5 m/s

• Interaction Pairs – a set of two forces that are in opposite directions, have equal magnitude, and act on different objects

• F A on B = - F B on A

• The force of A on B is equal in magnitude and opposite in direction of the force of B on A

3rd Law of Motion

• Tension – The force that a string or rope exert. Tension equals the weight hanging force, or the pulling force from either side.

3rd Law of Motion

FT

Fg

FA FB

FAFB

• Normal force – The perpendicular contact force that a surface exerts on another surface

• FN = mg Fnet = 0

3rd Law of Motion

3rd Law of Motion

• If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction.

• For every action (force) there is an equal and opposite reaction (force)

3rd Law of Motion

• If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction.

• For every action (force) there is an equal and opposite reaction (force)

3rd Law of Motion

• For every action (force) there is an equal and opposite reaction (force)

• Ex: Recoiling of a fired gun, a balloon travels in the opposite direction of air flow

3rd Law of Motion

• Momentum - An object’s mass multiplied by its velocity

• Conservation of Momentum – When two or more objects collide, the total momentum of the object is the same after as before

• M1V1 = M2V2

1 kg x 10 m/s = 5 kg x ___ m/s

3rd Law of Motion

• Conservation of Momentum – When two or more objects collide, the total momentum of the object is the same after as before

• Angular momentum – Velocity of rotation increases as the distance from the center becomes smaller

3rd Law of Motion

• Conservation of Momentum – When two or more objects collide, the total momentum of the object is the same after as before

• Angular momentum – Velocity of rotation increases as the distance from the center becomes smaller