Momentum

← Friction Questions All Lessons Momentum Problems →

Momentum — the property of a moving object that makes it difficult to stop. Fill in the Blank Notes

Warm-Up

Compare the force the big box exerts on the little box with the force the little box exerts on the big box.

Momentum Song — The Hush Sound — Video SMTP

How Tough Is It to Stop?

"Which is tougher to stop, a bowling ball or a baseball?"

↓

It Depends ....

If the baseball had a much larger velocity, it would be harder to stop.

(technically, product of mass and velocity)

Defining Momentum

Momentum = Mass x Velocity

p = mv

(kg·m/s) (kg)(m/s)

a) vector quantity

b) p has same direction as velocity

c) units - kg·m/s

Momentum and Collisions

Momentum helps us understand collisions

[Flash]

Kinetic Energy is conserved in perfectly elastic collisions

Perfectly Elastic Collision?

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The ball's energy is lost in the collision with the floor. It does not return its original position.

Same Momentum, Different Mass

"Could a moving baseball have the same momentum as a moving bowling ball?"

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"Yes. p = mv. The baseball's velocity would have to be many times faster."

Force and Momentum

1) Relationship between force and momentum

When a force acts on a body it
changes its' momentum.

Mathematical Relationship, F, p

p = mv

so ...

Δp = mΔv

F_net = ma

so ...

F_net = mΔv / t

(Cross Multiply)

F_nett = mΔv

Since Δp = mΔv, then ...

F_nett = Δp

Force is the time rate
of change in momentum

F_net = Δp / t

Δp = Ft

Force F, 'Follow through', t

Soccer ball kick.wmv

Dr. Dave's Hall of Fame

What Determines the Change in Momentum?

The change in momentum that an unbalanced force produces on an object depends on ....

Noble and Greenough School H.S.

... how long the force acts on a body.

Follow Through Matters

"Could a female golfer EVER change the momentum of a golf ball more than a male golfer?"

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Yes.

She can compensate for a smaller force
by employing a good 'follow through'

2) Impulse

J = F_nett

(N·s) or (kg·m/s)

Combining all the equations

* J = F_nett = mΔv = Δp *

Write on your test immediately
before answering questions

Given this equation, what is another unit for momentum besides kg·m/s?

* J = F_nett = mΔv = Δp *

↓

N·s

Explore: Impulse in a Crash

Same change in momentum, different stopping times.

Choose a speed and protection, then press Crash! The shaded area on the graph is the impulse.

Try This

Crash with no seat belt, then with a seat belt and airbag. Is the impulse the same? Is the force the same?

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The impulse is the same (same mass and speed, so same Δp), but the longer stopping time makes the force much smaller.

More: Collision Safety lessons

Evolution of hand protection in boxing: a fist wrapped in oxhide strips, an early laced leather glove, and a modern padded red boxing glove

NYSSLS Reading Passage · Dynamics

The Physics of the Fist: Force, Impulse, and the Evolution of the Boxing Glove

Open Reading Passage →

Check Your Understanding

5 quick questions — pick an answer to see if you’re right.

1. Which statement correctly describes momentum?

2. Besides kg·m/s, which unit can also be used for momentum?

3. What is the momentum of a 0.15 kg baseball moving at 40 m/s?

4. Can a moving baseball have the same momentum as a moving bowling ball?

5. How can a golfer who swings with less force still give a golf ball a larger change in momentum?

Momentum Problems →

Also see: Collision Safety — impulse, airbags, crumple zones, and an egg-drop design challenge