When an objectWhen an object’’s distance from s distance from another object is changing.another object is changing.

You must have a You must have a reference pointreference point in in order to determine if something is order to determine if something is in motion.in motion.

Does the sun Does the sun movemove

or the or the Earth?Earth?

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The The pushpush or or pullpull on on an object.an object.

Force between surfaces Force between surfaces that that resists the movementresists the movement of one surface past of one surface past another surface.another surface.

Friction also exists Friction also exists between an object between an object rolling across another rolling across another surface.surface.

ForceForce or or attractionattraction between two between two objects.objects.

Gravity and Motion

•The gravitational constant is 9.8 m/s².

•Objects fall to Earth at the same acceleration regardless of mass.

Measure of the force of Measure of the force of attraction between an attraction between an objectobject and and EarthEarth..

A A NewtonNewton is the metric is the metric unit used to measure unit used to measure weight.weight.

Many forces act together Many forces act together on an object.on an object.

However, it is not However, it is not always easy to always easy to determine what is determine what is exerting a force or exerting a force or what is receiving a what is receiving a force.force.

When forces are applied in the When forces are applied in the samesame direction, the forces are direction, the forces are addedadded

togethertogether..

Example: paddling a boat in the Example: paddling a boat in the same direction helps the boat to same direction helps the boat to go faster.go faster.

When forces are applied in When forces are applied in oppositeopposite directions, directions, subtract the smaller force subtract the smaller force to determine the direction.to determine the direction.

Example: Example: tug-of-tug-of-warwar

The resulting force is The resulting force is called the called the net forcenet force..

When a net force acts on When a net force acts on an object, the forces are an object, the forces are unbalancedunbalanced..

This causes an object to This causes an object to start start movingmoving, stop , stop movingmoving, , or change or change directiondirection..

Equal forces on an object Equal forces on an object in opposite directions are in opposite directions are called called balancedbalanced forcesforces..

This does not change This does not change an objectan object’’s motion.s motion.