Ch. 8.2 Acceleration and Force

Motion and Forces

Section 8.2 ObjectivesCalculate the acceleration of an object.

Describe how force affects the motion of an object.

Distinguish between balanced and unbalanced forces.

Explain how friction affects the motion of an object.

AccelerationAcceleration is the

change in velocity divided by the time interval in which the charge occurred.

Any change in velocity is an acceleration.

Three ways to Accelerate:

Object speeding up

Object slowing down

Object changing direction

This cyclist accelerates when he turnsa corner even if his speed doesn’t change.Why?

Calculating Acceleration Acceleration = final velocity – initial

velocity time a = ∆ v / t

Where : a is in meters per second squared (m/s2)

v is in meters per second ( m/s ) t is in seconds ( s)

Acceleration has dimensions of length divided by time squared.

The units of acceleration in SI are meters per second per second. m/s2

Acceleration can be determine from a velocity – time graphAcceleration has

direction and magnitude.

The slope and shape of a graph plotting velocity vs. time describes the object’s motion.

When velocity on graph is

increasing : acceleration is positive

decreasing : acceleration is negative

constant : there is no

acceleration

ForceForce is the cause of

acceleration, or change in an object’s velocity.

Net force is the combination of all the forces acting on an object.

Net force determines whether the velocity of the object will change.

An object accelerates in the direction of the net force.

If net force equals zero, then the object won’t accelerate.

ForceBalanced Forces

are forces acting on an object that combine to produce a net force equal to zero.

Balanced forces completely cancel each other out.

Unbalanced Forces are forces acting on an object that combine to produce a net nonzero force.

FrictionFriction is the force between two objects in

contact that opposes the motion of either object.

Frictional force is different depending on the surfaces in contact.

Smooth – less friction Rough – more friction

FrictionAir resistance is a form

of friction.

Air resistance is caused by the interaction between the surface of a moving object and the air molecules.

Objects with larger surfaces can experience greater air resistance.

GravityGravity is the force

of attraction between two particles of matter due to their mass.

The force of gravity between two objects is proportional to the product of their masses.

Because the Earth has more mass, it has a great gravitational pull than the moon.

Section 8.2 SummaryAcceleration is a change

in the velocity of an object. An object accelerates when it speeds up, slows down, or changed direction. Acceleration is caused by a force.

For straight-line motion, average acceleration is defined as the change in an object’s velocity per unit of time.

The SI unit for acceleration is meters per second squared ( m/s2).

The forces that act on an object combine to act effectively as one force.

Friction is the force between two objects in contact; it opposes the motion of either object.

Gravity is the force of attraction that two particles of matter exert on each other. It is proportional to their mass and inversely proportional to the square of the distance between them.