Forces and Newton’s Laws of Motion

Sections 4.1 – 4.3

4.1 The Concepts of Force and Mass

Force Push or Pull– Contact Forces pulling on rope, pushing on ball– Non-contact Forces gravity pulling object

Mass How much matter

Sir Isaac Newton studied force and mass– Made Newton’s Laws of Motion

4.2 Newton’s First Law of Motion

Ice Hockey– When nothing happening puck sits on ice– After hit, puck moves in straight line until hits

something

Newton’s First Law of Motion

An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force.

Net Force

Net Force Vector sum of all forces

Net Force required to change velocity

Inertia and Mass

Takes more force to change velocity of some objects

Inertia tendency to stay at rest or in constant motion

Mass measurement of inertia– Kilogram (kg)

Physics of Seat Belts

An Inertial Reference Frame

Reference frame in which Newton’s law of inertia is valid.

Noninertial frame reference frames that are accelerating.

4.3 Newton’s Second Law of Motion

What happens when a net force is applied to a hockey puck?– Accelerates (changes velocity)

More net force More acceleration

Newton’s Second Law Components

Force

Mass

Newton’s Second Law of Motion

When a net external force F acts on an object of mass, m, the acceleration, a, that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the net force.

m

Fa

OR

Newton’s Second Law of Motion

F = ma

Force

External force– Come from the environment

Internal force – One part of object pushing (or

pulling) another part of object

Unit of Force

Newton (N)

Force = mass x accelerationN = kg x m /s2

N = kg m /s2

Calculations with Newton’s 2nd Law

F needed

Free-body diagram– Draw only forces acting on the object– Represent the forces are vector arrows

Example

Four people are having a tug-o-war game. Ashley pulls left with 200 N, Bert pulls left with 278 N, Charlie pulls right with 304 N, and Dannie pulls right with 189 N. What is the net force, acceleration of the 5 kg rope, and who wins the game?

Free-body diagramA = -200 N

B = -278 N

C = +304 N

D = +189 N

Example

What is the net force, acceleration of the 5 kg rope, and who wins the game?

F = +15N a= 3 m/s2

Charlie and Dannie win!

A = -200 N

B = -278 N

C = +304 N

D = +189 N

Practice Work

You should be able to try

115 #CQ 1- 5, P 1- 7

Total of 13 problems