Inertia and Momentum
Review: What is inertia?
How do we measure inertia?
How does the inertia of a cement mixer compare to a roller skate?
1 What is Momentum?
Moving objects have a quantity that objects at rest don’t have.
In the past, this was called impedo .
2 What is Momentum?
Objects at rest had no impedo .
A rock rolling down a hill had impedo .
More faster it moved, the more impedo it had.
The change in impedo depended on force and how long the force acted.
3 What is Momentum?
Momentum = Inertia in motion
We calculate momentum as:
A moving object can have a large momentum if either its mass or its velocity are large. 4 Defining Momentum Activity
1. What is the product of the mass and the speed called?
Momentum
5 Defining Momentum Activity
2. The greater an object’s momentum, the more force is needed to bring it to a stop – true or false?
True
6 Defining Momentum Activity
3. Which object had the greater momentum, the ball bearing or the marble? What is your evidence?
Which object had more mass? Which object had more speed? (distance/time)
7 Defining Momentum Activity
4. Use the results of this experiment to define momentum two ways: 1) With a mathematical expression momentum = mass x speed
2) a description of what momentum does
8 What is Momentum?
We decided that a cement truck will always have more inertia than a roller skate.
Will a cement truck will always have more momentum than a roller skate?
9 CHECK YOUR NEIGHBOR
Which has more momentum? A wagon at rest or a wagon rolling down a hill?
A small car or a semi truck moving at 60 mph?
A 1 kg rock moving at 10 m/s or a 10 kg rock moving at 1 m/s?
10 Momentum in Action Supertankers cut off their power when they are 25 or so kilometers from port. Because of their huge momentum (due mostly to their huge mass), about 25 kilometers of water resistance are needed to bring them to a halt.
11 Momentum & Impulse
What gets the rock moving down the hill?
A force is needed to get the rock moving, but it is also important how long the rock is pushed. So, for changing an object’s momentum, both force and the time interval are important
12 Momentum & Impulse
Impulse = Change in Momentum
To calculate impulse, Impulse = Force x Time Interval
13 CHECK YOURSELF
1. Does a moving object have impulse?
2. Does a moving object have momentum?
14 CHECK YOURSELF
1. Does a moving object have impulse?
No – just like an object doesn’t “have” a force, an object doesn’t have an impulse. An impulse requires an interaction with another object.
15 CHECK YOURSELF
2. Does a moving object have momentum?
Yes!
16 Impulse – Momentum Relationship
Newton’s 2 nd Law acceleration = Force/mass We also said that acceleration = (velocity/time)
Force/mass = (velocity/time) multiply both sides by (mass x time)
(Force)(time) = (mass)(velocity) Impulse = Momentum
17 Increasing Momentum
To increase the momentum of an object, you need to: 1) increase the mass or 2) increase the velocity or 3) increase the force or 4) increase the time interval or 5) or any combination!
(mass)(velocity) = (Force)(time)
18 Increasing Momentum
To increase the momentum of an object, you need to: increasing the time interval where the force is applied causes the canon ball to farther
19 Increasing Momentum
More examples: Pulling a slingshot or arrow in a bow all the way back
Driving a golf ball – follow through with swing!
20 Decreasing Momentum
If your car is going to crash, would you rather crash into a concrete wall or a pile of hay?
Why?
21 Decreasing Momentum
What does having your hand go backward when catching a ball do?
How about falling on a surface with give (gym mats) vs. a rigid surface (contrete)?
22 Decreasing Momentum
To decrease the momentum of an object, assuming you can’t decrease the mass and velocity, you will either: 1) decrease the time interval and increase the force or 2) decrease the force and increase the time (mass)(velocity) = (Force)(time)
23 How are you feeling about this concept?
Reflection
How well do you understand Momentum? Impulse?
What can you do to improve your understanding?
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