1. Acceleration and Force
Section 7.2

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

3. 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.

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. 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.