Download Chapter 12 Forces and Motion 12.1 Forces

Chapter 12 Forces and Motion
12.1 Forces
Force – A force is a push or pull that acts on an object. A force can cause a resting object to move, or
it can accelerate a moving object by changing the object’s speed or direction.
Units of Force – A Newton (abbreviated as N) is the force that causes a 1 kg mass to accelerate at a
rate of 1 m/s2. It was named after Sir Isaac Newton (the guy under the apple tree) – who explained how
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force, mass, and acceleration are related. 1 N = 1 kg • m/s
Combining Forces
Arrows can be used to represent force.
You can combine force arrows to show the result of how forces combine.
Forces in the same direction add together.
Forces in opposite directions subtract from one another.
The net force is the overall force acting on an object (after all the forces are combined).
Balanced Forces
When the forces on an object are balanced, the net force is zero – so there in no change in the
object’s motion.
Unbalanced Forces
An unbalanced force is a force that results when the net force acting on an object is not equal to
zero.
When an unbalanced force acts on an object, the object accelerates.
Net force = 7 N
Friction
Friction is a force that opposes the motion of objects that touch as they move past each other.
Without friction, everything would be slippery. (even more slippery than a sheet of ice)
Note that “touching” each other includes solid objects that are directly touching one another as
well as objects moving through a liquid or gas.
There are 4 main types of friction: static friction, sliding friction, rolling friction, and fluid
friction.
Static Friction
Static friction is the friction that acts on objects that are not moving.
Always opposes your push. (acts in the direction opposite to that of the applied force)
Sliding Friction
Sliding friction is a force that opposes the direction of motion of an object as it slides over a
surface.
Because sliding friction is less than static friction, less force is needed to keep an object moving
than to start it moving.
Rolling Friction
Rolling friction is the friction that acts on rolling objects.
Rolling friction is much less than static and sliding friction – which is why wheels are so useful
for moving heavy objects.
It is caused by the slight change of shape of the rolling object and the surface it is rolling on.
Fluid Friction
Fluid friction opposes the motion of an object through a fluid.
The “fluid” can be a fluid (like water) or a mixture of gases (like air).
Fluid friction increases as the speed of the object moving through it increases. (So the faster you
stir, the greater the friction is.)
Air resistance is fluid friction acting on an object moving through the air. (So the faster you
drive, the more air resistance there is.)
Gravity
Gravity is a force of attraction that acts between two objects. (examples: Earth and the sun, the
moon and Earth, you and Earth)
We will just look at the gravity Earth exerts on objects.
Earth’s gravity acts downward toward the center of the earth.
Gravity causes objects to accelerate downward, while air resistance acts in the direction opposite
the motion and reduces acceleration.
The speed of a falling object increases (the object accelerates) until the force of the air resistance
equals the force of gravity. At this point, the object falls at a constant velocity.
This is called the terminal velocity.
The acceleration due to gravity:
G = 9.8 m/s2
Projectile Motion
Throw a ball and it has a curved path.
Air resistance and gravity are the two forces acting on a projectile.
The combination of an initial forward velocity and the downward vertical force of gravity causes
the ball to follow a curved path.