Forces in Motion

Newton’s Laws of Motion

Terms• Inertia~the tendency of all objects to resist

any change in motion

• Momentum~ a property of a moving object that depends on the object’s mass and velocity

Newton’s First Law of Motion• An object at rest remains at rest and

an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.

• Objects at rest– Not moving– Won’t move unless a push or pull is

exerted on them

• Object in motion– Continues forever at same speed & in

same direction unless an unbalanced force acts on it.

Newton’s First Law of Motion• Friction and Newton’s First Law

– Friction makes it difficult to observe – Friction is an unbalanced force which

changes motion

• Inertia is related to mass– 1st law sometimes called the law of inertia– Why you slide when car makes a turn

• Mass is a measure of inertia– Small mass → less inertia– Easier to change motion of object with a

small mass

Newton’s Second Law of Motion• The acceleration of an object depends

on the mass of the object and the amount of force applied

• Acceleration depends on mass– Acceleration decreases as mass increases

when the same amount of force is applied

• Acceleration depends on force– Acceleration increases as force increases

when the mass is the same

• Expressing it mathematically– F=m·a– A=F/m

Newton’s Third Law of Motion• Whenever one object exerts a force on

a second object, the second object exerts an equal and opposite force on the first.

• Force pairs do not act on the same object

• The effect of a reaction can be difficult to see

• More examples:– Rabbit hopping– Bat hitting ball– Shuttle taking off

Momentum is a Property of Moving Objects

• More momentum ~ harder it is to stop or change its direction

• Momentum is Conserved– Any time 2 or more objects interact, they

may exchange momentum, but the total amount of momentum stays the same

– Momentum before collision = momentum after collision

• Conservation of momentum & Newton’s third law– Because action & reaction forces are equal

and opposite, momentum is conserved