Force and Motion Notes
3/8/2016
Forces in Action
What is motion?
Motion is movement. Example: moving in a car.
Velocity = is a speed in a given direction.
Example: 50 mph North to the school
Acceleration = a change in velocity.
Example: a car at a stop sigh or a car turning.
Changes in motion are due to forces
Force = a push or a pull.
Example: accelerator or brake
Inertia = the tendency to resist changes in motion.
Example: motion and seatbelts
Penny activity – Why doesn’t the penny move?
Mass and Inertia
“The bigger they come the harder they fall”
Mass = the amount of matter that an object has. It is also a measure of an object’s inertia
– how it resists changes
Weight = force of gravity on your mass
Newton’s 1st Law of Motion
An object at rest remains at rest, an object in motion remains in motion unless
acted upon by an outside force.
Relate back to penny.
Interactions that push or pull
Gravity = a force pulling objects toward the center of the Earth (9.8 m/s2). All objects
accelerate to the Earth at the same rate unless air resistance slows them down.
Friction = an interaction that resists motion.
Example:
walking on ice
vs
walking on sticky surface
No friction,
Need more
lots of friction,
need less
Activity: Is friction a force?
Balanced and Unbalanced Forces
Balanced force = forces acting on the object cancel each other out.
Example: seat, book. No accelerating.
Unbalanced force = a greater force is acting on an object in some direction. This
produces motion.
Action and Reaction Forces
Action force: is when you push or pull objects.
Reaction force: the force that pushes back on you.
Newton,s 3rd Law:
For every action there is an equal, opposite reaction.
“I pull on it, it pull on me” “scooter”
How forces act on objects
Acceleration, mass and force
Newton’s 2nd Law:
F=MxA
Force = mass times acceleration
Pressure
Pressure = a force per unit area.
F
P=
A
Pressure = force divided by area
Example: snowshoes / high heels / bed of nails
Pressure in Fluids:
Fluid = is anything that flows (gases and liquids). Movement in fluids is due to pressure
changes. High pressure moves toward low pressure.
DEMO; imploding can
Buoyancy – the upward force exerted by H2O
This is why objects feel lighter in water. How much lighter?
Archimede’s principle: the weight of the fluid that is displaced is = to the buoyant force
of the object.
(demo: overflow can, rock, graduated cylinder)
Tell Archimedes story
Why else do objects float other than buoyancy?
Density = mass/volume D = m/v
a. Mass = the amount of material a substance has. (TBB – gram)
b. Volume = the amount of space an object occupies
1. regular volume = L x W x H (cm3)
2. irregular volume = water displacement (ml)
Density is like a “fingerprint” for a substance. Every known substance will have a
specific density.
H2O = 1 g/ml or 1 g/ cm3
Aluminum = 2.7 g/ml
(sink)
Wood = 0.5 g/ml
(float)
Ice = 0.92 g/ml
(float)
3
Lead = 11.3 g/cm
(sink)
Example:
Object with a density > than 1 will sink in water.
Object with a density < than 1 will float in water.
Object with a density = to 1 will stay in middle of water.
Activities