Identify
This term is defined by "a moving object's resistance to a change in its motion" (or "moving inertia").
Momentum
A large train is sitting motionless in a station.
How much momentum does the train have?
The train has no momentum.
(even without knowing the mass, if an object has a velocity of zero, then its momentum will be zero)
An object that has a mass of 6 kg and a velocity of 5 m/s.
Calculate its momentum.
30 kg*m/s
p=mv
p=6*5
p=30
What two variables must be found in order to calculate the momentum of any object?
The object's mass and its velocity
This term is defined by "an event in which two objects collide and bounce off of each other".
Elastic Collision
A small car and a larger truck (with more mass) are traveling with the same velocity.
Which vehicle has more momentum? Explain your reasoning.
The larger truck
The truck's momentum would be found by calculating a greater mass multiplied by the same velocity.
A rabbit has a momentum of 8 kg*m/s and a mass of 2 kg.
Calculate its velocity.
4 m/s
p=mv
8=2*v
8/2=v
4=v
Many athletes in contact sports (where players collide) would benefit from increasing their momentum. What are two ways in which they can achieve this?
Possible responses:
1.Increasing their mass (muscle and/or fat) by eating more and exercising.
2. Increasing their velocity by training muscles used in sprinting, or using tactics that allows them to build speed before contact.
This term is defined by "an event in which two objects collide and stick to each other".
Inelastic Collision
Two marbles are rolling towards each other with the same speed. The blue marble is rolling right, while the red marble rolls left. After the marbles collide, both marbles roll to the left.
Which marble had the most momentum? Explain your reasoning.
The red marble.
Since both marbles roll to the left after the collision, the total momentum must point left. In order for the total to be pointing left, the red marble (which was originally traveling to the left) had to have more momentum than the blue marble (which was traveling to the right.
A bowling ball has a velocity of 12 m/s and a momentum of 12 kg*m/s.
Calculate its mass.
1 kg
p=mv
12=m*12
12/12=m
1=m
Explain, using concepts from this unit, how bending your knees when landing on your feet after a jump can decrease the magnitude of force your body receives.
Bending your knees increases the time of the collision between you and the ground. If time is increased, then the force required to change your momentum is decreased.
This law states that "the total momentum among objects in a system must be maintained." (The momentum before a collision is equal to the momentum after a collision)
Conservation of Momentum
Two cars of equal mass are traveling in the same direction, side-by-side, and with the same velocity. When approaching a stop light, the red car starts breaking sooner and gradually comes to stop, while the blue car waits until the last moment to come to an abrupt stop.
Which car's brakes must apply a greater magnitude of force? Explain your reasoning.
The blue car's brakes.
Both cars experience the same change in momentum, which is equal to force multiplied by time. Sine the blue car's change in momentum happened in a smaller amount of time, the force required to stop it was greater than the force required to stop the red car.
A distracted driver, in a car with mass 500 kg and velocity of 20 m/s, collides into the back a motionless car. The motionless car has a mass of 350 kg.
Calculate the total momentum after the collison.
10,000 kg*m/s
total p = (mv)+(mv)
total p = (500*20)+(350*0)
total p = 10,000+0
total p = 10,000
Explain how air bags, seat belts, and crash barriers use concepts from this momentum unit to reduce injuries of people involved in vehicle collisions.
Possible responses:
- these safety devices prevent elastic collisions with rigid objects, which reduces the change in momentum for the people involved, thus reducing the force required to change the momentum.
- these safety devices increase the amount of time that the objects collide for, thus decreasing the amount of force required to change the momentum for the people involved.
This term defines "a certain amount of force exerted on an object each second for a certain amount of time".
Impulse
A soccer ball is kicked towards a goal two times. The first kick sends the ball into the hard frame of the goal, and the ball bounces off in the opposite direction. A second kick, just as hard as the first, sends the ball into the net which brings the ball to rest.
Does the ball bouncing off the frame, or stopping in the net, cause a greater change in momentum for the ball? Explain your reasoning.
Bouncing off the frame.
When bouncing off the frame, the balls momentum goes from a positive to a negative value. When stopping in the net, the ball only goes from the same positive value to zero.
Two cars traveling towards each other collide and stick together. One car has a mass of 20 kg and a velocity of 5 m/s, while the other car has a mass of 30 kg and a velocity of -4 m/s.
Calculate the cars' velocity after the collision.
-0.4 m/s
p(1)=20*5=100 ; p(2)=30*-4=-120 ; total p= -20
total m=20+30=50
total p =total m*v
-20=50*v
-20/50=v
-0.4=v
Many sports require a player to hit an object, changing the object's direction and speed. To get the object to move the fastest, it is important to "follow through" - moving your body as if you didn't hit the object at all.
How does "following through" result in a large velocity for the object you hit? Use concepts from this unit in your explanation.
Following through with a certain force increases the time that the force is applied, resulting in a greater change in momentum for the object. Since the objects mass doesn't change, the greater change in momentum is directly related to its velocity.