Newton's Third Law

Newton’s Third Law

Projectile Motion  Horizontal

and Vertical Motions affect the projectile motion of an object.  Horizontal motion  motion parallel to the ground.  Ex.

A ball is thrown straight forward; the ball would continue at a constant velocity parallel to the ground.

 Vertical  Force

Velocity

of gravity pulls objects vertically (downward)  Ex. Ball being thrown will curve downward due to vertical velocity (gravity)

Centripetal Force  Remember

the requirements for something to accelerate?  (change

in speed, change in direction)

 Acceleration

toward the center of a curved or circular path is called Centripetal Acceleration.

 Ex.

Figure 18 – Ball moving through a curved pipe  Objects move in a circle, because centripetal force is accelerating them toward the center.  Ex. Moon in orbit; Earth’s gravity exerts a centripetal force on the moon.

Newton’s Third Law  “For

every action force, there is an equal and opposite reaction force.”  Ex. If you pushed on a wall, wearing rolling skates, would you move? If so, which direction?  Answer  Yes, Backwards! The wall exerts as much force as you exert on it.

 Ex.

Jumping up and down on a trampoline; you exert a downward force, while the trampoline exerts an equal force upward.  Rocket propulsion is another good example. A

Rocket exerts a force on gases, while the gases exert a force on a rocket.

Momentum  Momentum

= mass x velocity

or (p = mv)  Units for momentum  kg m/s **Momentum has direction because velocity has direction**

 Which

would be easier to stop? A slowmoving bicycle or a speeding bicycle?  Answer  A slow moving bicycle; it has a lower velocity.  Which would be easier to stop? A moving “Hot Wheels” car or a Monster Truck?  Answer  A “Hot Wheels” Car; Hot Wheels has less mass.

 More

mass = more momentum  More velocity = more momentum

Force and Changing Momentum  Remember  (Final

velocity – initial velocity)/time

 Remember  Force

the formula for acceleration? Newton’s Second Law?

= mass x acceleration

 These

two formulas are combined when referring to momentum. F

= (mvf – mvi)/t

 When

you catch a ball, your hand exerts a force on the ball that stops it. The force depends on the mass and speed of the ball and how long it takes to come to a stop.

Law of Conservation of Momentum  Momentum

can be passed from one object to another.  If a group of objects exerts forces only on each other, their total momentum doesn’t change.