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# Newton's First Law

## An object at rest will remain at rest and an object in motion will remain in motion unless acted on by an unbalanced force.

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Ouch!

### Ouch!

Credit: Nick Perrone
Source: http://www.flickr.com/photos/nickperrone/8129436049

Two professional football players collide. Ouch! Collisions occur all the time in football. That’s the nature of the game.

The Back Story

• Football is all about one player trying to run down the field with the ball and other players trying to stop him. When two football players collide, you might think that the player with more mass would always stop the other player. But you would be wrong. It all depends on the player’s momentum.
• Momentum is governed by Newton’s third law of motion.
• Credit: Talbot Troy
Source: https://www.flickr.com/photos/rustydarbonne/2870201437

Force of collision depends on both speed and mass of the football players [Figure2]

• Momentum depends not only on the player’s mass but also on how fast he is moving.

Can You Apply It?

1. State Newton’s third law of motion.
2. Define momentum in terms of football.
3. What is the law of conservation of momentum? How does it apply to football players who collide on the field?
4. Football player A has a mass of 110 kg, and he is running down the field with a velocity of 2 m/s. Football player B has a mass of 120 kg and is stationary. What is the momentum of each player?
5. If football player A collides with football player B, what is their total momentum before and after the collision?
6. Compare and contrast elastic and inelastic collisions. Which type of collision occurs in football?

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