The Nature of ForcesThe Nature of ForcesThe Nature of ForcesThe Nature of Forces

Force• a push or pull that

gives energy to an object

• sometimes causes a change in motion

To describe a force you need to know two things:

The direction The strength

Used to draw Force Diagrams

Units of Force

English – pounds (lb)

(That’s right. Your weight is a force, not a measure of mass.)

Metric – newtons (N)

One pound is about 4.5 N.

Forces can give energy to objects, sometimes causing them to:

• start movingstop movingchange direction

The combination of all forces acting on an

object

Net Force

Equal and opposite (balanced) forces produce

NO motion.

NONet

Force

Unequal (unbalanced) forces produce motion in the direction of the larger

force.

Forces in the same direction add together.

What forces can you name?

What forces are shown here?

What forces are shown here?

Friction

•Force opposing motion•ALWAYS works in opposite direction of motion

– Causes objects to slow down and stop

Types of Friction•Types:1.Static: acts on objects NOT moving

• Holds objects in place 2.sliding friction: objects slide to produce friction

• Brakes, basketball shoes3. rolling friction: objects roll;

• bicycle or car tires4. fluid friction: fluids (air, water,etc.) moving over a

surface1.Aerodynamic: shapes that make lower air friction

How much friction?

• Depends on materials and force between objects– Rough surfaces = more or less friction?– Smooth surfaces = more or less friction?– Heavy objects = more or less friction?– Lighter objects = more or less friction?

Gravity:Tim and Moby explain• Force that pulls objects

towards each other• depends on the mass of

and the distance between them– Accelerates objects

towards Earth

• Law of universal gravitation: all objects have a gravitational pull on all other objects

Which object has the most gravity? Why?

Which object has the least gravitational pull on it? Why?

Weight vs. Mass

Weight vs. Mass• Mass: amount of matter in an object• Weight: measure of force of gravity on an

object– It’s a FORCE!– Weight = mass X gravity– The mass does not change but the weight is

determined by the force of gravity!– Man on the moon