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Forces
What Is a Force?
• Force- a push or a pull that acts on an object
o Can cause a resting object to move, or it can
accelerate a moving object
• Forces are measured in Newtons (N)
• One newton is the forces that causes 1 kg mass to
accelerate at a rate of 1 meter per second each
second
o 1 N = 1 kg*m/s2
Representing Force
• Forces can’t be seen, but their effects can. So
we have come up with a way to represent
forces
Forces can be combined
• 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 the forces are combined
Balanced and Unbalanced Forces
• When forces on an object are balanced, the
net force in zero and there is no change in the
object’s motion
• When an unbalanced force acts on an object,
the object accelerates
Friction
• A force that opposes the motion of objects
that touch as they move past each other
• 4 main types of friction:
1.) static friction
2.) sliding friction
3.) Rolling friction
4.) fluid friction
Static Friction
• Acts on object that are not moving
- Always acts in the direction opposite to that of
the applied force
- You experience static friction every time you
take a step
Sliding Friction
• Force that opposes the direction of motion of
an object as it slides over a surface
- Less than static friction. Therefore, less force is
needed to keep an object moving than to start it
moving
Rolling Friction
• Friction force that acts on rolling objects
- 100 to 1000 times less than the force of static
or sliding friction
- Helps explain why professional movers use
wheeled dollies to move heavy objects
Fluid Friction
• Opposes the motion of an object through a fluid
- Examples can be seen on a submarine moving
through water or an airplane flying through air
- You can feel fluid friction when stirring thick cake
batter
- Increases as the speed of the object moving
through the fluid increases
- Fluid friction acting on an object moving through
the air is known as air resistance
Gravity
• Force that acts between any 2 masses
-attractive force—pulls objects together
-does not require objects to be in contact
-can act over long distances
-Earth’s gravity acts downward toward the
center of Earth
Falling Objects
• Gravity causes objects to accelerate
downward, whereas air resistance acts in the
direction opposite to the motion and reduces
acceleration
Falling objects
• As objects fall to the ground, they accelerate
and gain speed
• Terminal velocity is the constant velocity of a
falling object when the force air resistance
equals the force of gravity
-the doesn’t mean the objects stop in mid air…it
just means it doesn’t accelerate anymore
• Mass does NOT matter, all objects fall at the
same rate
Projectile Motion
• The motion of a falling object (projectile) after
it is given an initial forward velocity.
- Air resistance and gravity are the only forces
acting on a projectile