Motion & Forces

Motion & Forces

Speed & VelocitySpeed & Velocity

• Speed - the rate of change in position over time (distance/time)

• Instantaneous speed

• Average speed

• How is velocity different from speed? Velocity describes both

speed and direction

Change in Velocity

Change in Velocity

What makes an object change velocity?

ForceForce• Push or Pull

• Forces can cause a change in velocity

• Examples:

GravityMagnetismFriction

Balanced ForcesBalanced Forces

Forces that– act on the same object

– are opposite in direction

– are equal in size

Balanced ForcesBalanced Forces Book on table

1. Weight of the book is downward force caused by gravity

2. Table pushes back with an equal and opposite force

Net force is zero

Unbalanced Forces

Unbalanced Forces

Forces that are NOT opposite and equal

What do they cause?

CHANGE IN VELOCITY

ACCELERATION

Forces that are NOT opposite and equal

What do they cause?

CHANGE IN VELOCITY

ACCELERATION

FrictionFrictionForce that opposes motion between 2 surfaces

Depends on the:• types of surfaces•force between the surfaces

Force that opposes motion between 2 surfaces

Depends on the:• types of surfaces•force between the surfaces

Types of FrictionTypes of Friction• Static Friction: (Couch Potato)

– Force between objects at rest

• Sliding Friction: (Ice skating)– Solid surfaces slide over each other

• Rolling Friction: (Rollerblading)– An object rolls over a surface.

• Fluid friction: (Submarine)– An object moves through a fluid

FrictionFrictionFriction is greater...

–between rough surfaces

–when there’s greater force between surfaces (such as more weight)

Friction is greater...

–between rough surfaces

–when there’s greater force between surfaces (such as more weight)

LubricantLubricant

Changes sliding friction into fluid friction

FrictionFriction

Is friction always bad?

GravityGravity

The attractive force between all objects in the universe

Acceleration of Gravity

Acceleration of Gravity

Near the surface of the earth the acceleration due to gravity is 9.8 m/s/s

Newton’s 1st Law

Newton’s 1st Law

An object at rest remains at rest and an object in motion will continue moving at a constant velocity unless acted on by unbalanced forces.

Newton’s Laws of Motion

Newton’s Laws of Motion

Newton’s 1st Law

Newton’s 1st Law

INERTIA

• tendency of an object to resist any change in its motion

• increases as mass increases

Newton‘s 2nd Law

Newton‘s 2nd Law

If more force is applied, an object will be accelerated _____more

Newton’s 2nd Law

Newton’s 2nd Law

• More force means more acceleration

• More mass means less accelerationForce = mass X acceleration

F = ma

Newton’s 3rd Law

Newton’s 3rd Law

For every action, there is an equal and opposite reaction.

Forces occur only in pairs– Act on different objects– Equal and opposite

Newton’s 3rd Law

Newton’s 3rd Law

The hammer exerts a force on the nail to the right.

The nail exerts an equal but opposite force on the hammer to the left.

Momentum

Momentum

Terminal VelocityTerminal Velocity

The final speed of a falling object.

Terminal VelocityTerminal Velocity

Air resistance is offsetting the the weight of the object

ForcesForces

EndEnd

ForceForce

Aristotle's idea:

For an object to move at a constant speed, a constant force must be applied.

ForceForce

Newton’s idea:

An object moving at a constant speed will continue at that speed without additional force being applied.

Balanced ForcesBalanced Forces