# High School Physics : Momentum

## Example Questions

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### Example Question #1 : Understanding Elastic And Inelastic Collisions

car travelling at  collides with another  car that is at rest. The two bumpers lock and the cars move forward together. What is their final velocity?

Explanation:

This is an example of an inelastic collision, as the two cars stick together after colliding. We can assume momentum is conserved.

To make the equation easier, let's call the first car "1" and the second car "2."

Using conservation of momentum and the equation for momentum, , we can set up the following equation.

Since the cars stick together, they will have the same final velocity. We know the second car starts at rest, and the velocity of the first car is given. Plug in these values and solve for the final velocity.

### Example Question #2 : Understanding Elastic And Inelastic Collisions

ball moving at  strikes a  ball at rest. After the collision the  ball is moving with a velocity of . What is the velocity of the second ball?

Explanation:

This is an example of an elastic collision. We start with two masses and end with two masses with no loss of energy.

We can use the law of conservation of momentum to equate the initial and final terms.

Plug in the given values and solve for .

### Example Question #2 : Understanding Elastic And Inelastic Collisions

ball strikes a  ball at rest. After the collision the  ball is moving with a velocity of  and the second ball is moving with a velocity of . What is the initial velocity of the first ball?

Explanation:

This is an example of an elastic collision. We start with two masses and end with two masses with no loss of energy.

We can use the law of conservation of momentum to equate the initial and final terms.

Plug in the given values and solve for .

### Example Question #1 : Momentum

ball moving at  strikes a second ball at rest. After the collision the  ball is moving with a velocity of  and the second ball is moving with a velocity of . What is the mass of the second ball?

Explanation:

This is an example of an elastic collision. We start with two masses and end with two masses with no loss of energy.

We can use the law of conservation of momentum to equate the initial and final terms.

Plug in the given values and solve for .

### Example Question #1 : Understanding Elastic And Inelastic Collisions

Which of these would NOT be an example of an inelastic collision?

Two cars crash into each other, and stop with a loud bang

Neutrons fuse with hydrogen atoms in a nuclear reactor core such that kinetic energy is conserved

A man is clapping his hands such that they move with equal, but opposite velocities

Two hydrogen atoms fuse together to form a helium atom and gamma radiation

A match scrapes a matchbook and bursts into flame

Neutrons fuse with hydrogen atoms in a nuclear reactor core such that kinetic energy is conserved

Explanation:

The difference between an elastic and an inelastic collision is the loss or conservation of kinetic energy. In an inelastic collision kinetic energy is not conserved, and will change forms into sound, heat, radiation, or some other form. In an elastic collision kinetic energy is conserved and does not change forms.

Remember, total energy and total momentum are conserved regardless of the type of collision; however, while energy cannot be created nor destroyed, it can change forms.

In the answer options, only one choice preserves the total kinetic energy. The resulting bang from the car crash, the flame from the match, the sound of hands clapping, and the gamma radiation during hydrogen fusion are all examples of the conversion of kinetic energy to other forms, making each of these an inelastic collision. Only the neutron fusion described maintains the conservation of kinetic energy, making this an elastic collision.

### Example Question #6 : Understanding Elastic And Inelastic Collisions

Hockey puck A (2kg) travels with a velocity of  to the right when it collides with hockey puck B (1.6kg), which was originally at rest. After the collision, puck A is stationary. Assume no external forces are in play and that the momentum of the pucks are conserved. What is the final velocity of puck B after the collision?

Explanation:

Elastic collisions occur when two objects collide and kinetic energy isn't lost. The objects rebound from each other and kinetic energy and momentum are conserved. Inelastic collisions are said to occur when the two objects remain together after the collision so we are dealing with an elastic collision.

Above, the subscripts 1 and 2 denote puck A and B respectively, and the initial momentum of puck B is zero, so that term is not included in the equation above.

Plug in initial and final velocities and mass:

### Example Question #7 : Understanding Elastic And Inelastic Collisions

A space vehicle, in a circular orbit around Earth, collides with a small asteroid that ends up in the vehicle’s storage bay. For this collision

Both momentum and kinetic energy are conserved

Only kinetic energy is conserved

Only momentum is conserved

Neither momentum nor kinetic energy is conserved

Only momentum is conserved

Explanation:

This is an inelastic collision as the two objects stick together and move together with the same velocity. Inelastic collisions conserve momentum, but they do not conserve kinetic energy.

### Example Question #8 : Understanding Elastic And Inelastic Collisions

A bullet with mass  hits a ballistic pendulum with length  and mass  and lodges in it. When the bullet hits the pendulum it swings up from the equilibrium position and reaches an angle  at its maximum. Determine the bullet’s velocity.

Explanation:

We will need to start at the end of the situation and work backward in order to determine the velocity of the bullet.  At the very end, the pendulum with the bullet reaches its maximum height and therefore comes to a stop.  It has gravitational potential energy.  At the bottom of the pendulum right after the bullet collides with it, it has kinetic energy due to the velocity of the bullet.  With the law of conservation of energy, we can set the kinetic energy of the pendulum right after the collision equal to the gravitational potential energy of the pendulum at the highest point.

To determine the height of the pendulum we will need to use trig and triangles to find the height.  We know that the pendulum makes a 30-degree angle with the equilibrium position at its maximum height.  The length of the pendulum is provided which is the hypotenuse of this triangle.  We need to find the adjacent side of this triangle.  We can use cosine to determine this.

We can now subtract this value from the length of the pendulum to determine how high off the ground the pendulum is at its highest point.

We can now set the kinetic energy of the pendulum right after the collision equal to the gravitational potential energy of the pendulum at the highest point.

The mass is the same throughout so it falls out of the equation.

The pendulum with the bullet was moving  after the collision.  We can now use momentum to determine the speed of the bullet before the collision.  Conservation of momentum states that the momentum before the collision must equal the momentum after the collision.

They both move together after the collision

Since the pendulum was not moving at the beginning

We can now plug in these values and solve for the missing piece.

### Example Question #1 : Momentum

A man with a mass of is painting a house. He stands on a tall ladder of height . He leans over and falls straight down off the ladder. If he is in the air for  seconds, what will be his momentum right before he hits the ground?

Explanation:

The problem tells us he falls vertically off the ladder (straight down), so we don't need to worry about motion in the horizontal direction.

The equation for momentum is:

We can assume he falls from rest, which allows us to find the initial momentum.

.

From here, we can use the formula for impulse:

We know his initial momentum is zero, so we can remove this variable from the equation.

The problem tells us that his change in time is  seconds, so we can insert this in place of the time.

The only force acting upon man is the force due to gravity, which will always be given by the equation .

### Example Question #2 : Momentum

ball is thrown west at  and collides with a  ball while in the air. If the balls stick together in the crash and fall straight down to the ground, what was the velocity of the second ball?

Explanation:

We know that if the balls fell straight down after the crash, then the total momentum in the horizontal direction is zero. The only motion is due to gravity, rather than any remaining horizontal momentum. Based on conservation of momentum, the initial and final momentum values must be equal. If the final horizontal momentum is zero, then the initial horizontal momentum must also be zero.

In our situation, the final momentum is going to be zero.

Use the given values for the mass of each ball and initial velocity of the first ball to find the initial velocity of the second.

The negative sign tells us the second ball is traveling in the opposite direction as the first, meaning it must be moving east.

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