Momentum and change in momentum

Question

• What would you rather get hit with?

• A bowling Ball or a steel ball bearing?

Which would you rather get hit by?

• 1 bowling ball? • 1 marble ?

On what information does your answer depend?

DEFINITION OF MOMENTUM

Calculated value

• Momentum is a calculated or derived value from information about forces and motion

• Is not an intuitive value

Momentum

• “Pain upon impact”

• “Inertia in motion”

• Quantity of motion found by multiplying an objects mass by its velocity

• It is a vector value (Magnitude and Direction)

Units of momentum

Kg m/s

WHAT DOES HAVE MOMENTUM?

Who has momentum?

Examples of momentum

• Anything with mass that is in motion

• Any object that is not moving has NO momentum

WHAT DOES NOT HAVE MOMENTUM?

Examples of objects without momentum

• Anything that has no velocity

• A bus that is stopped has less momentum than a flea that is moving, but more inertia

What has more momentum?

• The statue of liberty or a toddler walking at 0.5 m/s?

How can one change momentum?

• Examples of objects changing their momentum

DETERMINATION OF AN OBJECTS MOMENTUM VALUE

Equation of momentum

• P = m v

Momentum equation

P = m v

P

M V

Car Crashes

• Why do some people survive and others do not?

Eggs and momentum

CHANGE IN MOMENTUM“Impulse”

More examples of change in momentum

Impulse

• The measure of a change in momentum

• Any object that changes its mass or velocity over time changes its momentum

• Any change in momentum requires a net force

• Any application of a net force, results in a change of momentum of the object

Examples of net force changing momentum

• Hockey stick on a puck• Wall on a car• Bat on a softball

WHAT CAR HAS THE GREATER IMPULSE?

The Red car that was going 45 mph stopped by the Brick Wall

Or

The same red car that was going 45 mph and stopped by application of Brakes

Answer

• They have the same change in momentum

• The car hitting a brick wall will have more force applied over a shorter time

Newton’s second law and ∆p

• How does 2nd law relate to momentum?

• Acceleration = the change in velocity over change in time a = ∆v / ∆t

• Rewrite the 2nd law in terms of the change of momentum (This is how Newton presented the law)

Impulse equation

mΔv = Δp = Fnet ∆t

∆v = vf - vi

Which parts of the equation = the impulse?

• Impulse = F ∆t

• Impulse = m ∆v

• Impulse = pf - pi

Important

• For a given crash, the impulse is constant regardless of how the crash occurs

• Impulse = mΔv

Time and force

• The length of time it takes for the crash to be complete determines the amount of force applied during the crash

• Seat belts

• Foam padding

• Air Bags

• Crumple zones

Time and force

• The longer the time allowed to exert a force, the less force is exerted

Again

• What causes a change in the momentum?

Answer

• An application of net force, producing a change in velocity (slowing down OR speeding up OR changing direction)

Change of momentum and Sports

• Baseball/ softball

•Golf

Catching a ball

Other examples

• In cars, air bags, seat belts, padding on dashboard, steering wheel

• In sports, catching a ball with a glove instead of a hand, bunting/home runs

• In Dance, Gymnastics: Bending at the knees

To catch a tennis ball

• Your hand must apply force to the ball to make it stop moving

APPLICATIONS OF MOMENTUM

What determines if you live or die in an auto accident?

Safety features

Safety features in cars that protect you during a crash

• Seat belt

• Air bags

• Cushioned dashboard, steering weel

• Padded headrest

• Crumple zones

The way to survive an accident

• Drive a car with:– Seat-belts– Air bag– Crumple zones– Soft dashboards/padded steering wheels

What benefit do these features provide?

What value do safety features extend?

• The time of the crash

• Impulse = Force x time

What value is decreased as a result of those safety features?

• The amount of force needed to stop the car (and you)

The purpose of most safety features in a car is to lengthen the time of the

accident

• Air bags

Seat belts

Padding on dashboard, seats and steering wheel

Crumple zones

• It takes time for the parts of the car in the crumple zone to be crushed, less force applied to the passenger section

Any other means to protect the passengers during a collision?

Other examples of change in momentum

• Catching a Foul Ball • Being hit by a foul ball

Again

• The main point of all safety devices in cars is to extend the time of the impact between human and car

Answer

• The amount of force applied is related to the length of time required to bring the driver to a stop

• The longer the time, the less force required