Forces & MotionForces & Motion

ForcesForces

Forces are acting all around you.Forces are acting all around you. Objects, including you, are being Objects, including you, are being

pushed and pulled in different pushed and pulled in different directions.directions.

Sometimes forces cause motion:Sometimes forces cause motion:– sliding a chair, throwing or kicking a ball, sliding a chair, throwing or kicking a ball,

pushing a grocery cartpushing a grocery cart More importantly, stationary objects More importantly, stationary objects

(objects sitting still) have forces acting (objects sitting still) have forces acting on them tooon them too– a pop can on the counter, a picture frame a pop can on the counter, a picture frame

on the wall, computer on the deskon the wall, computer on the desk

Objects exert forces on each other:Objects exert forces on each other:– You are exerting a force on your chair, You are exerting a force on your chair,

AND you chair is exerting a force equally AND you chair is exerting a force equally on you.on you.

Forces have magnitude and strength.Forces have magnitude and strength. Strength is measured in Newtons Strength is measured in Newtons

(named after Sir Isaac Newton)(named after Sir Isaac Newton)

Forces affect objects in several ways…Forces affect objects in several ways…– Forces acting on stationary objects can Forces acting on stationary objects can

set the object in motion (wind = forces set the object in motion (wind = forces that blows over a trash can).that blows over a trash can).

– Forces can change a moving object’s Forces can change a moving object’s speed and or direction it’s moving (you speed and or direction it’s moving (you get hit in the head with a ball – your head get hit in the head with a ball – your head (outside force) changes the direction and (outside force) changes the direction and speed that the ball was going).speed that the ball was going).

– A force can affect an object without A force can affect an object without making it move (balanced forces)making it move (balanced forces)

– Forces can change a stationary object’s Forces can change a stationary object’s shape (squeezing a balloon)shape (squeezing a balloon)

Gravity is a force.Gravity is a force.

Gravity acts on all objects that have Gravity acts on all objects that have mass.mass.

All objects have mass.All objects have mass. The strength of gravity depends on two The strength of gravity depends on two

things:things:– The mass of the object.The mass of the object.– The distance between two objects.The distance between two objects.

Mass is the measure of the amount of Mass is the measure of the amount of ‘stuff’ inside an object‘stuff’ inside an object

The greater the mass of either object, The greater the mass of either object, the stronger the gravity between them.the stronger the gravity between them.

Friction is a forceFriction is a force

Friction is a force that opposes motion Friction is a force that opposes motion between two surfaces that are in contact between two surfaces that are in contact with each other.with each other.– Friction may prevent motion from starting, it Friction may prevent motion from starting, it

may slow motion down, it may even stop itmay slow motion down, it may even stop it When two solid surfaces slide against When two solid surfaces slide against

each other, it is known as sliding friction.each other, it is known as sliding friction. When two solid surfaces roll over one When two solid surfaces roll over one

another, it is known as rolling friction.another, it is known as rolling friction.

When a solid surface passes through When a solid surface passes through air or water, it is known as fluid air or water, it is known as fluid friction.friction.

Friction depends on two things:Friction depends on two things:– The type of surfaces involved.The type of surfaces involved.– How hard the surfaces are pressing on How hard the surfaces are pressing on

one another.one another.