# Physics you didn’t know that you already knew.

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Physics you didn’t know that you already knew.
Momentum and Impulse Physics you didn’t know that you already knew.

What is momentum? Momentum is a vector quantity. (Direction matters!) It is a measure of inertia in motion, or mass with velocity. The symbol for momentum is p. Take a look at how momentum applies to football. Watch for direction, mass and velocity of the players and see how they interact. Why did the helmet keep moving?

Finding momentum. p = mv Momentum = (mass) (velocity) Momentum is measured in kgm/s Mass is measured in kg Velocity is measured in m/s and direction is +/-, left/right, etc

Try an example… A soccer ball is moving east with a speed of 2 m/s. The mass of the soccer ball is 1 kg. What is the momentum of the soccer ball?

Consider this… A car with a mass of 1000 kg is stopped at a traffic light. A kid rides by on a bike, total mass 100 kg, at a speed of 5 m/s. Which object has the greatest momentum? Why?

Some momentous tidbits about motion and momentum
Constant momentum = constant velocity What if you are accelerating? Acceleration = changing velocity = changing momentum If you are accelerating then there is a net force It takes a force acting over time to cause momentum to change.

Impulse (Remember - It takes a force acting over time to cause momentum to change.) When a force acts for an amount of time we call it impulse. It is the product of the average force and the time interval that the force acts. J = Favg ∆t = ∆p Unit are kgm/s or Ns Impulse changes momentum.

Impulse Changes Momentum
Impulse =∆p Impulse = Favg ∆t ∆p = pf – pi = mvf – mvi

Impulse We can control the damage done by a force by changing the time the force acts on an object. Consider the difference between hitting a wall with your fist or hitting a pillow with your fist. We use padding in sports to protect players. Watch the video to see how that works.

Try an Impulse example…
A soccer ball is moving west with a speed of 2 m/s. The mass of the soccer ball is 1 kg. A player kicks the ball the in opposite direction, causing it to go 4 m/s east. The foot is in contact with the ball for 0.15s. What is the change in momentum of the ball? What impulse is applied to the ball? Find the average force the foot exerts.

Challenge: What’s the joke?
Here’s the punch line… Sorry sir, but he was acting impulsively You come up with the joke! (Seriously, we need your help!)

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