Momentum and ImpulseChapter 7-1 and 7-2

Momentum• We already know that it is harder to stop a large truck than a small car when they are both moving at the same velocity.

• Because of this fact, we can say that the truck has more momentum.

Momentum• Momentum is inertia in motion.

• More specifically Momentum = Mass x Velocity

• p=mv is how we write out our linear momentum formula

Try it!

• What would have to exist for the small sumo wrestler to have more momentum than the larger sumo wrestler?

Try it!• What is the momentum of a 15 kg toy truck moving at 30 m/s?

• What is the velocity of a golf ball that has a momentum of 200 kg(m/s) if the golf ball has a mass of 1.3 kg?

Impulse Changes Momentum

• If the momentum of an object changes, either the mass or the velocity or both MUST change.

• If the the mass remains unchanged, as is usually the case, then the velocity changes and acceleration occurs.

Impulse Changes Momentum

• We also know that force produces acceleration.

• Therefore, the greater the force acting on an object, the greater its change in velocity, and the greater its change in momentum is going to be.

Bill Nye the Science Guy Shows us Momentum

• Great Moments in Momentum!

Impulse Changes Momentum

• We have two formulas that we can use to express momentum

• The first is called “the impulse”

• The second is called “the change in momentum”

• Notice how they will always equal each other.

Try this!

• A 3kg bowling ball is rolled down a lane at 15m/s. The force of the ball is 25 N. What is the time it takes the ball to roll down the lane?

Increasing Momentum• The forces involved in impulses usually vary from instant to instant.

• When a baseball player hits the ball with a bat we are looking at the average force of the impact because there is no force on the ball until the bat come in contact with the ball.

Increasing Momentum• After the ball is hit, force increases rapidly. This is what increases our momentum on the ball.

• Remember when we talk about average force of impact we are talking about the Newtons from the bat hitting the ball. When we talk about impulse we are talking about impact force x time, which is measured in Newton-seconds.

Decreasing Momentum

• If your car was out of control and the only way to stop it would you choose to run your car into a pole or into some bushes and small trees?

• Why?

Decreasing Momentum

• The reason you should choose the bottom car crash over the top one is because in the bottom one the car impacts for a longer period of time because the tree will bend back with the force of the car but the pole does not.

Decreasing Momentum

• If an impact takes longer to happen and the same amount of momentum is used then the force of the impact is decreased.

• If an impact is very short and the same amount of momentum is used then the force of the impact is increased.

Try it!• When a dish falls, will the impulse be less if it lands on a carpet than if it lands on a hard floor?

• If the boxer to the right is able to make the impact time fice times longer by “riding” with the punch, how much will the force of the impact be reduced?

Try it!• 1. Determine the momentum of a ...

a. 60-kg halfback moving eastward at 9 m/s.

b. 1000-kg car moving northward at 20 m/s.

c. 40-kg freshman moving southward at 2 m/s.

 

• 2. A car possesses 20 000 units of momentum. What would be the car's new momentum if ...

a. its velocity was doubled.b. its velocity was tripled.

c. its mass was doubled (by adding more passengers and a greater load)

d. both its velocity was doubled and its mass was doubled.

Try It!

• Now consider a collision of a tennis ball with a wall. Depending on the physical properties of the ball and wall, the speed at which the ball rebounds from the wall upon colliding with it will vary. The diagrams below depict the changes in velocity of the same ball. For each representation (vector diagram, velocity-time graph, and ticker tape pattern), indicate which case (A or B) has the greatest change in velocity, greatest acceleration, greatest momentum change, and greatest impulse. Support each answer.