Impulse

WOD is underlined.

Impulse

• When two objects exert forces on each other (equal and opposite), they cause each other to accelerate.

Impulse

• When two objects exert forces on each other (equal and opposite), they cause each other to accelerate.

• They do so for a certain amount of time.

Impulse

• Impulse = Force * Time

• I = F*t

• Force is what?

Impulse

• Impulse = Force * Time

• I = F*t– Recall Def’n of force F = ma

• I = ma*t

• a is what?

Impulse

• Impulse = Force * Time

• I = F*t– Recall Def’n of force F = ma

• I = ma*t– Recall Def’n of acceleration a = Δv/t

• I = m(Δv/t) * t Note: the times cancel.

• I = m Δv

Impulse

• Impulse = Force * Time

• I = F*t– Recall Def’n of force F = ma

• I = ma*t– Recall Def’n of acceleration a = Δv/t

• I = m(Δv/t) * t

• I = m Δv = Δp Since mass is constant

• WOD Definition: I = F*t = m Δv

Impulse and Momentum

• The impulse on an object is equal to its change in momentum.

• Remember, forces always come in equal and opposite pairs (on the two different objects), so impulses also come in pairs that cancel out.

Impulse and Momentum• So, if they come in pairs that always cancel, ΣI =

0.

• If I = Δp, we can plug it in and get:

ΣΔp = 0

The Law of Conservation of Momentum is just another form of Newton’s third:

“For every action there is an equal and opposite reaction.”

Add to WOD: Impulse is “Change in Momentum”. In my mind, impulse is how much a force changes the velocity.

Impulse and MomentumLets look at units

• Momentum = mv

= kg * m/s

• Impulse = Ft

= N*s

Impulse and MomentumLets look at units

• Momentum = mv

= kg * m/s

• Impulse = Ft

= N*s

Recall: N = kg*m/s2

so I = (kg*m/s2)*s

I = kg * m/s

Lets do a problem

A 2000 N force acts on a rocket of mass 1000 kg, increasing its speed from rest to 200 m/s. How long did the force act?

Lets do a problem

A 2000 N force acts on a rocket of mass 1000 kg, increasing its speed from rest to 200 m/s. How long did the force act?

Use the 2 different ways to calculate impulse to solve.

Impulse tells you how a force changes the velocity of an object.

Impulse = Change in Momentum

F * t = m * v

2000 N * t = 1000kg * ( 200m/s – 0 )

t = 1000*200/2000 kg*m/s/N = 100 sec

Lets do another problem

A 6 N force acts on a 3 kg object for 10 sec. What will be the final velocity of the object if its initial velocity was 10 m/s?

Lets do another problem

A 6 N force acts on a 3 kg object for 10 sec. What will be the final velocity of the object if its initial velocity was 10 m/s?

F * t = m * v

6N * 10sec = 3 kg ( Vf – 10m/s)

60 kg*m/s = 3kg * Vf – 30 kg*m/s

90 kg*m/s = 3kg*Vf

Vf = 30 m/s

Question

• If I drop a dish on a ceramic floor what will happen?

Question

• If I drop a dish on a ceramic floor what will happen?

“Probably break”

• If I drop it onto carpet what will happen?

Question

• If I drop a dish on a ceramic floor what will happen?

“Probably break”

• If I drop it onto carpet what will happen?• “Less likely to break”

Why? Both experience the same impulse?

i.e., the same change in momentum or change in velocity?

Question

• If I drop a dish on a ceramic floor what will happen?

• If I drop it onto carpet what will happen?

Why? Both experience the same impulse?

Impulse is the same, but the time with the carpet is greater (carpet is more flexible), causing it to apply less force.

Hockey

A hockey puck is 160 grams. Initially, its velocity is 3 m/s East.

a)It gets thwacked with a force of + 25 N for .05 sec. What is its final velocity?

Hockey

A hockey puck is 160 grams. Initially, its velocity is 3 m/s East.

b) What if, instead it got thwacked by -12 N for .05 sec. What would be the final velocity?

HockeyA hockey puck is 160 grams. Initially, its

velocity is 3 m/s East.

It got thwacked by -12 N for .05 sec.

C) The hockey stick was .91 kg and had an initial velocity of 1 m/s West. What was the final velocity of the stick?

Hockey SummaryA hockey puck is 160 grams. Initially, its velocity is 3 m/s East.

a)It gets thwacked with a force of + 25 N for .05 sec. What is its final velocity?

b) What if, instead it got thwacked by -12 N for .05 sec. What would be the final velocity?

C) The hockey stick was .91 kg and had an initial velocity of 1 m/s West. What was the final velocity of the stick?

Hockey

A hockey puck is 160 grams. Initially, its velocity is 3 m/s East.

a)It gets thwacked with a force of + 25 N for .05 sec. What is its final velocity?

F * t = m * v Note: m in kg.

Vf = 10.8 m/s

Hockey

A hockey puck is 160 grams. Initially, its velocity is 3 m/s East.

b) What if, instead it got thwacked by -12 N for .05 sec. What would be the final velocity?

F * t = m * v Note: m in kg and neg. direction.

Vf = .75 m/s West (or -.75 m/s)

HockeyA hockey puck is 160 grams. Initially, its velocity is 3

m/s East.

It got thwacked by -12 N for .05 sec.

C)The hockey stick was .91 kg and had an initial velocity of 1 m/s West. What was the final velocity of the stick?

Note: Force on stick is equal and opposite.

Vf = -.34 m/s = .34 m/s West

F * t = m * v Note: 1 m/s west = -1 m/s vector

12N * (.05s) = .91kg ( Vf – (-1m/s))

Remember these Questions?

• If I drop a dish on a ceramic floor what will happen?

• If I drop it onto carpet what will happen?

Why? Both experience the same impulse?

Impulse is the same, but the time with the carpet is greater, causing it to apply less force.

Cushioning

• If an object is moving, a certain amount of impulse is needed to bring it to rest, or bring the momentum down to zero.

Cushioning

• If an object is moving, a certain amount of impulse is needed to bring it to rest, or bring the momentum down to zero.

• I = F*t. Materials that “give” increase time, bringing down F.

Cushioning

• If an object is moving, a certain amount of impulse is needed to bring it to rest, or bring the momentum down to zero.

• I = F*t. Materials that “give” increase time, bringing down F.

• This is why air bags work.

http://youtu.be/yUpiV2I_IRI

Play through 12:30