Transcript Acceleration and Force

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
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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
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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
– 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.