12-May-15 Physics 1 (Garcia) SJSU Chapter 6 Momentum Object A hits Object B. What happens?

Apr 21, 2023 Physics 1 (Garcia) SJSU

Chapter 6 Momentum

Object A hits Object B. What happens?


ForksIn 7th Century royal courts of the Middle East

began to use forks at the table for dining but was not until the 16th Century that forks were common in Europe.

Early table forks were modeled after kitchen forks but in the late 17th Century forks with four curved tines appear.

The additional tines made diners less likely to drop food, and the curved tines served as a scoop so people did not have to constantly switch to a spoon while eating.

By the early 19th Century, multi-tined forks were also used in England and slowly began to spread to America.

http://www.calacademy.org/research/anthropology/utensil/details1.htm


Force, Momentum, Energy

With Newton’s Laws, we can understand motion just using forces. Can also eat food just using knives.

Easier to understand motion by introducing concepts of momentum and energy. Think of them as the fork and spoon of mechanics.


Momentum

Momentum of an object is,

(Momentum) =

(Mass) X (Velocity)

Examples of objects with large momentums are supertanker (large mass) and bullet (large velocity).


Check YourselfA 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h..Which vehicle, the car or the truck, has greater momentum? What would the car’s speed have to be for the momentums to match? Aren’t you forgetting something? How does that matter?


Momentum and Force

To stop an object with a large momentum requires either:

• Large force (stopping the object quickly).

• Small force applied for a long time.

Notice that changing object’s momentum depends on force and time interval.


Impulse

Define impulse acting on an object as,

(Impulse) =

(Force on object) X (Time interval)

Objects have momentum.

Impulse acts on an object.


Impulse & Momentum

Impulse is related to momentum by,

(Change in momentum) = (Impulse)

or

(Mass) X (Change in velocity) =

(Force) X (Time interval)

This relation comes from Newton’s 2nd law.


Demo: Egg Throw

Throw a raw egg as fast as possible at a plastic sheet that’s held loosely.

X X

X

X

X (Hold here)


Check YourselfThrow egg at sheet or wall with

same speed. Which case has:Greater change of velocity? Greater change of momentum? Largest impulse on the egg? Largest time of impact? Largest force on the egg?

LONG TIME

short time

small force

LARGE FORCE


Demo: Vampire Stake

Safest when slow moving stake is placed on a soft, fleshy spot on the chest.

(force) x (TIME)

X XOuch!

(FORCE) x (time)

Not safe if stake strikes hard skull


Check Yourself

A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h..The force of impact is greatest on which vehicle, the car or the truck? The impulse is greatest on which vehicle, the car or the truck? Change of momentum greatest? Change of velocity greatest? Driver injury greatest?


Automobile SafetyMaximizing the time of impact on the driver minimizes

the force of impact. This principle used in design of:

Seatbelts Air Bags CrumpleZones


CollisionsWhen two objects collide, impulse is equal and

opposite for the two objects.

Before collision

Impact

After collision

IMPULSEIMPULSE

Each object has equal and opposite change in momentum.


Conservation of Momentum

Since change of momentum in a collision is equal and opposite, the momentum gained by one object is the amount lost by the other.

MomentumObject A

Before Collision

MomentumObject B

Before Collision+

A B

MomentumObject A

After Collision

MomentumObject B

After Collision= +

A B

Actual amount of momentum exchanged depends on the details of the collision, such as whether or not collision is elastic.


Demo: Elastic CollisionsObjects of equal mass exchange momentum on elastic

collisions.


Demo: Newton’s Balls

Steel balls collide elastically, exchanging momentum on collision.


Demo: Don’t ScratchTo sink a billiard ball that is very close to the

pocket without having the cue ball go in as well (“scratching”) strike the cue ball hard so it makes a crisp, elastic collision.

As with Newton’s balls, cue ball will stop after giving all its momentum to the other ball in the collision.


Demo: Blaster Balls

When masses unequal, momentum change can be large.

Pingpongball

Golfball

Speed ofping-pong ball

is 3x larger(Slingshot effect)


Demo: Inelastic Collisions

Objects stick together after colliding.

A

A

A

B

B

B

Note: Use this concept in lab experiment entitled “Projectiles”


Check Yourself

Large (4 kg) fish swims at 3 m/s towards a small (2 kg) fish and swallows it for lunch.

Total momentum before lunch?

Total momentum after lunch?

Velocity of the large fish (with small fish inside)?


Recoil

Momentum conservation also explains recoil

(MASS) x (velocity)(mass) x (VELOCITY)

Recoil effect is like an inelastic collision in reverse.


Complicated Collisions

Collisions at an angle (not head-on) are more complicated. Learn by playing pool.

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12-May-15 Physics 1 (Garcia) SJSU Chapter 6 Momentum Object A hits Object B. What happens?

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