Physics 102: Chapter 6 - Momentum

1. How do we quantify the effect of a force over time?

2. How do we look at inertia with both mass and velocity?

3. How are these ideas related?

Impulse

The force of the foot on the ball is an impulsive force.

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Graphical Interpretation of Impulse

J = Impulse = area under the force curve =FavgΔt

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Momentum

Momentum is the product of an object’s mass and its velocity:

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p = mv

The Impulse-Momentum Theorem

Impulse causes a change in momentum:

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J =pf - pi = ∆p

Example

A 0.5 kg hockey puck slides to the right at 10 m/s. It is hit with a hockey stick that exerts the force shown. What is its approximate final speed?

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Checking Understanding

Two 1-kg stationary cue balls are struck by cue sticks. The cues exert the forces shown. Which ball has the greater final speed?

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A. Ball 1B. Ball 2C. Both balls have the same final speed

Two 1-kg stationary cue balls are struck by cue sticks. The cues exert the forces shown. Which ball has the greater final speed?

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C. Both balls have the same final speed

Answer

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A 500 kg rocket sled is coasting at 20 m/s. It then turns on its rocket engines for 5.0 s, with a thrust of 1000 N. What is its final speed?

Example

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Starting and Stopping

A car with a mass of 1 metric ton speeds up to highway speed from rest on a strait section of Central Blvd. A little while later, the car comes to a stop as it approaches a red light.

Part 1 - Determine the net impulse and average net force on the car as it goes from rest to highway speed.

Part 2 - Determine the net impulse and average net force on the car as it goes from highway speed to a complete stop.

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Impulses and Car Crashes

1. Consider a car going at highway speeds colliding in a front-end collision with a brick wall. Compare the impulse needed to bring the passenger in the front seat to a stop if they are stopped by each of the following:

• Their seatbelt• The dashboard• An airbag

2. Rank the force being applied for each case.

Shut the DoorYou are sitting on your bed in your dorm room, and suddenly you hear the voice of your ex coming down the hall. You really want to avoid any contact (you broke things off a week ago), and so you want to shut the door. But you don't have time to get up and shut it and act like it wasn't on purpose. You need something fast. Sitting beside you, you happen to have a super ball and a ball of clay that you fidget with when you're studying on your bed. What do you do?

Explain your answer and show why you chose one and not the other..(Demonstration movie =>http://groups.physics.umn.edu/demo/collisionframe.html)

Momentum, Impulse, and the Bouncing BallSuppose you drop a 200 g rubber ball on the floor from a height of 2.00

m and it rebounds to a height of 1.50 m. If the force that the floor exerts on the ball has the shape and duration as shown below, find the max value and avg. value of the force of the floor on the ball?

3 steps:1. Find the velocity of the ball immediately before and after contact with

the floor.2. Use these velocity values to find the

momentum of the ball immediately before and after contact with the floor.

3. Calculate the change in momentum and set that equal to the impulse. Solve for the maximum value of F_floor=>ball.

4.5.

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Bouncing off the Wall

In the overhead view shown below, a 290 g ball with a speed v of 4.6 m/s strikes a wall at an angle of 30° and then rebounds with the same speed and angle. It is in contact with the wall for 11 ms.Overhead View

(a) What is the impulse on the ball from the wall?(b) What is the average force (magnitude & direction) on the wall

from the ball?

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