Chapter 3

Forces

Newton’s Second Law

• Forces and motion are connected– An object will have greater

acceleration if a greater force is applied to it.

– The mass of an object and the force applied to it affect acceleration.

Newton’s Second Law

• Acceleration equals net force divided by mass.

• Friction- force that opposes motion between two surfaces that are touching each other.

F=ma

Newton’s Second Law

– microwelds, areas where surface bumpers stick together, are the source of friction.

– Static friction=two surfaces not moving past each other

– Sliding friction= opposes the motion of 2 surfaces sliding past each other

– Rolling friction=between the rolling object and the surface it rolls on (tire on pavement)

Newton’s Second Law

• Air Resistance-opposes force of gravity

– amount depends on object’s shape, size and speed

– terminal velocity- forces on a falling object are balanced and the object falls with constant speed

upward force exerted on an object falling through air-explains why a

brick falls faster than a feather

Gravity

• Law of Gravitation-any two masses exert an attractive force on each other.

– One of the 4 basic forces that also include the electromagnetic force, the strong nuclear force, and the weak nuclear force.

– Long range force that gives the universe its structure

Amount of gravitation force between objects depends on their masses and the distance between them.

Gravity

• Due to inertia, all objects fall with the same acceleration regardless of mass.

An object that is in free fall seems to be weightless

9.8 m/s²

Gravity

• Weight– decreases as an object moves away from

Earth.– weight results from a force; mass is a measure

of how much matter an object contains

weight=measure of the force of

gravity on an object

N

mass=measure of the amount of matter in an

object

g and kg

Gravity

• Objects in the space shuttle float because they have no force supporting them.

Gravity

• Projectiles have horizontal and vertical velocities due to gravity, and follow a curved path.

Always curved

parallel to Earth’s surface

Gravity

• Acceleration toward the center of a curved path is called centripetal acceleration; it is caused by centripetal force, an unbalanced force. (car rounding a curve)– satellites are anything that moves around

another body in a generally circular path.

Circle

moon=natural satellite

space station=artificial satellite

The Third Law of Motion

• To every action force there is an equal and opposite reaction force.– action-reaction forces act on different

objects and differ from balanced forces– example= rocket propulsion

action force up=reaction force down

action force right=reaction force left

The Third Law of Motion

• Momentum-related to how much force is needed to change an object’s motion.

momentum=mass times velocityp=mv

p=momentum

kg X m/s

The Third Law of Motion

• Law of conservation of momentum-momentum can be transferred between objects; momentum is not lost or gained in the transfer

When two objects collide…