Acceleration and Force Section 7.2

Objectives Calculate 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

Acceleration Definition: The time rate of change of velocity, or “how fast the velocity is changing” Commonly experienced when… You’re in a car You’re falling Include direction with your answer.

What you “think” you know: Objects with no acceleration are at rest. Not always true. How about a ball rolling on a flat surface? Objects at rest have no acceleration Not always true. How about a ball thrown upwards so it reaches its top height? Only objects that speed up are accelerating Not true. Objects that slow down are also experiencing a change in their velocity

Acceleration equation acceleration = (vfinal – vinitial) / time a = ∆v / t “∆v” has units of m/s and “t” has units of s. Units of “a” = m/s2

Sample problem A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot. Given: time, initial velocity, final velocity Unknown: acceleration and direction a = ∆v / t ∆v = 14.7 m/s – 0 m/s = 14.7 m/s a = 14.7 m/s / 1.5 s a = 9.8 m/s2, downward

Force Definition: A push or a pull What does it do? It causes accelerations to happen Balanced forces do not cause acceleration Example: Stapler sitting on a desk Unbalanced forces do not cancel out completely Tug of war, ball rolling downhill The force left over is called the ‘net force’ and causes an acceleration.

Friction Definition: The force between two objects in contact that opposes the motion of either object. It is a force (push or pull) Opposes motion (acts in the opposite direction of the motion) Frictional force depends on the two surfaces in contact Coefficient of friction, μ

Types of Friction Air resistance Surface to surface contact Force increases with velocity Surface to surface contact Force depends on composition of materials Ice-on-ice: very low friction Rubber-on-concrete: very high friction

For the lab tomorrow: Frictional force = μ * force of gravity μ is called the “coefficient of friction” and depends on the surfaces in contact Force of gravity is also called ‘Weight’ Weight (or force of gravity) = mg “g” is called “acceleration due to gravity” and has the value of 9.8 m/s2.