Bellwork – 4/12/18 What is momentum?

Bill Nye- Momentum Video Answer the Questions

Momentum – “Mass in Motion” CC image courtesy kanesue on wikimedia.org

Momentum Momentum is a measure of how hard it is to stop a moving object. CC image courtesy WillMcC on wikimedia.org

Formula: Momentum depends on mass (how big something is) and velocity ( how fast something is moving in a certain direction). Momentum is a vector quantity Momentum = mass x velocity (p=mv) Free clipart courtesy www.gnurf.net

Dissect the equation: p = mv Symbol = p What are the units for momentum? How would you increase an object’s momentum?

Units of Momentum If an object has a mass of 5kg and is moving at a velocity of 10 m/s, east what is it’s momentum? P=mv P= (5kg)x(10m/s) P= 50 kg.m/s,east

Units of Momentum If an object has a mass of 5kg and is moving at a velocity of 10 m/s, east what is it’s momentum? P=mv P= (5kg)x(10m/s) P= 50 kg.m/s,east

Use the formula, p = mv to answer the following questions Consider a Mack truck and a roller skate moving down the street at the same speed. Which one will have the greater momentum? If the Mack truck is not moving, which one has the greater momentum?

QUESTIONS How would doubling the mass of an object affect its momentum? How would doubling the velocity of an object affect its momentum?

Classwork / Homework Page 199 #’s 1-3

Bellwork 4/13/18 Is momentum a vector or a scalar quantity? What is the formula for momentum?

Understanding Car Crashes Video and Questions

Impulse - Momentum Theorem

How hard is it to stop a moving object? To stop an object, we have to apply a force over a period of time. This is called Impulse Impulse = FΔt Units: N∙s F = force (N) Δt = time elapsed (s)

In a collision, an object experiences a force for a given amount of time that results in its mass undergoing a change in velocity (i.e., that results in a momentum change).

There are four physical quantities mentioned in the above statement - force, time, mass, and velocity change. The force multiplied by the time is known as the impulse and the mass multiplied by the velocity change is known as the change in momentum. The impulse experienced by an object is always equal to the change in its momentum.

In terms of equations, this is expressed as  F Δ t = Δp This is known as the Impulse - Momentum Theorem. F Δ t = mvf – mvi If you want to change an object’s momentum, you need to change its impulse.

There are two ways you can calculate Impulse  F Δ t OR… since Impulse is equal to a change in an object’s momentum.... F Δ t = mvf - mvi

Real World Applications Highway safety engineers use the impulse-momentum theorem to determine the stopping distances and safe following distances for cars and trucks. Safety engineers also use this theorem to design safety equipment that reduces the force exerted on the human body during collsions.

If you drop an egg from a building- how could you decrease its momentum to keep it from breaking? FΔt = Δmv (Assuming the mass and the velocity remains the same)

ANSWER You will have to decrease the force acting on the egg or increase the time that the egg falls to the ground).

How can baseball players use changes in momentum to hit the ball further?

ANSWER If the batter keeps the bat in contact with the ball for a longer period of time, with a constant force, a greater change in momentum will result.

Practice Problem A 1400 kg car moving westward with a velocity of 15m/s collides with a utility pole and is brought to rest in 0.30s. Find the force exerted on the car during the collision. Given: m = 1400 kg vi= 15m/s west , so -15m/s t = 0.30s vf = 0 m/s Unknown: F F (t) = mvf – mvi F (0.30s) = ( 1400kg x 0) – (1400 kg x -15m/s) F (0.30s) = 21000 kg.m/s 0.30s F = 7.0 x 104 N ; east

Classwork PAGE 201 #’S 1-3

BELLWORK 4/16/18 Use the Impulse-Momentum Theorem to explain the mechanics behind not bursting a water balloon when you catch it from a distance.

EXPLANATION

Stopping Distance Stopping times and distance depend on the impulse-momentum theorem Example: A truck with a mass of 1200 kg and a car with a mass of 600 kg are both traveling at 30 km/hr. If the brakes in both vehicles apply the same force, how would the stopping times compare for them? Stopping time is twice as much for the truck compared to the car because the momentum is twice as much and if the force applied is the same, then time must also be twice as much

Stopping Distance Example: A truck with a mass of 1200 kg and a car with a mass of 600 kg are both traveling at 30 km/hr. If the brakes in both vehicles apply the same force, how would the stopping distance compare for them? The truck’s time period is twice that of the car’s, but the acceleration is half that of the car (f=ma). Going back to our ugly ladies, x=vit +1/2at2, the truck’s stopping distance is four times that of the car

Example Problem A 2250 kg car traveling to the west slows down uniformly from 20 m/s to 5 m/s. How long does it take to decelerate if the force on the car is 8450 N to the east? How far does the car travel during the deceleration? m = 2250 kg vi = -20 m/s vf = -5 m/s F = 8450 N t = ?  x = ?

To find distance, we can use the impulse-momentum theorem: x = ½ (vi + vf) t x = ½ (-20 m/s + -5 m/s) 4 s x = -50 m x = 50 m to the west