Physics Test 3 – Flashcards
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Starting from rest, object 1 is subject to a 10Nforce for 2.0 s. Object 2, with twice the mass, is subject to a 15N force for 3.0 s.
Which object has the greater final speed?
answer
let the mass of object 1 be m1
the force on object 1 is F1 = 10 N and the time is t1 = 2.0 s
we know that
F1 = m1 x a1 = m1 x (v1/t1)
or v1 = (F1 x t1/m1)
where v1 is speed of object 1
similarly,we have
v2 = (F2 x t2/m2)
where F2 = 15 N,t2 = 3.0 s and m2 = 2m1
question
A 0.2 kg plastic cart and a 20 kg lead cart both roll without friction on a horizontal surface. Equal forces are used to push both carts forward for a time of 1 s, starting from rest. After the force is removed at t = 1s, is the momentum of the plastic cart greater than, less than, or equal to the momentum of the lead cart? Explain.
answer
Because equal forces are exerted over equal times, the impulses are equal and the changes in momentum are equal. Because both carts start from rest, their changes in momentum are the same as the final momentum for each, so their momenta are equal.
question
Two pucks, of mass m and 4m, lie on a frictionless table. Equal forces are used to push both pucks forward a distance of 1 m.
a. Which puck takes longer to travel the distance? Why?
b. Which puck has the greater momentum upon completing the distance? Why?
answer
a. 4m puck
b. 4m puck
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Suppose a rubber ball collides head-on with a steel ball of equal mass traveling in the opposite direction with equal speed. Physics question: Which ball, if either, receives the larger impulse?
answer
They both receive the same impulse. Since the masses are equal, the resultant velocities are likewise equal.
The center of mass of the system has v = 0 before, during and after the collision.
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Two students stand at rest, facing each other on frictionless skates. They then start tossing a heavy ball back and forth between them.
answer
Each time a student throws the ball to his friend, he gains momentum in the opposite direction. Each time a student catches the ball, he gains momentum in the direction of the ball, also away from his friend. So, the students will throw the ball until they are too far away from each other.
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If you probably know that it feels better to catch a baseball if you are wearing a padded glove. Explain why this is so, using the ideas of momentum and impulse?
answer
You Know that Impulse is (F.dt). This is also the called the momentary force. Let Impulse is I
We can write I=F.dt, I = F x the change in time
Now when we were the padded gloves. the ball takes a bit more time to give the force on our hand, as the force has to travel through the gloves and then to our hand.
From the above equation we see that force is inversely proportional to time. Now more the time taken, less is the force imparted. The glove actually increases the impact time and hence the force imparted is less.
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A 2kg cart rolling to the right at 3 m/s runs into a 3 kg cart rolling to the left. After the collision, both carts are stationary. What was the original speed of the 3 kg cart?
answer
Law of conservation of momenta states that the sum of the momenta before the collision will equal the sum after collided.
m1v1 + m2v2 = vf(m1+m2)
v2 = (-m1/m2)v1
= -2 m/s
Negative sign means it is rolling to the left. the original speed = 2 m/s
question
To win a prize at the county fair, you're trying to knock down a heavy bowling pin
by hitting it with a thrown object. Should you choose to throw a rubber ball or a beanbag
of equal size and weight? Explain.
answer
Being a non-elastic substance by nature, the beanbag will effect a non-elastic collision with the bowling pin. It will either slow down or completely halt after colliding with the pin. If J = the change in P and the change in P = m(vf - v1). Value of impulse will be the largest after the velocity is zero. Hence max impulse = change in P = -mvi
Rubber ball - being an elastic substance, there will be an elastic collision. Thus it will reverse the direction and move with a velocity equal to or lesser than the initial.
Change in P = m(vf - vi). vf = -vi
so, change in P = -2mvi
*rubber ball is ideal choice*
question
Two ice skaters, Megan and Jason, push off from each other on frictionless ice,
Jason's mass is twice that of Megan.
a. Which skater, if either, experiences the greater impulse during the push? Explain.
b. Which skater, if either, experiences the greater speed after the push? Explain
answer
Megan and Jason are exerting the same amount of force on each other. Because they were at rest initially, their initial velocity is zero, so
0 = m1v1 - m2v2
Eventually you will get 2v2 = v1, so megan has a greater speed.
Because their initial momentum was zero before the push off, conservation of momentum occurs and the final momentum will equal zero as well. So, both experience the same amount of impulse.
question
While standing still on a basketball court, you throw the ball to a teammate. Why
do you not move backward as a result? Is the law of conservation of momentum violated?
answer
Law of conservation of momentum states that the momentum of a system is conserved unless and until the external force acting on it is zero.
We have friction on the ground. Hence, the momentum is not conserved. If he was on ice, a frictionless surface, then maybe.
question
If the earth warms significantly, the polar ice caps will melt. Water will move from the poles, near the earth's rotation axis, and will spread out around the globe. In principle, this will change the length of the day. Why? Will the length of the day increase or decrease?
answer
Angular momentum = L
L = I x omega, or L = mr^2 omega
Law of conservation of angular momentum: The angular momentum of a rotating object subject to no external torque is a constant. So, Lf = Li
This process increases the radius.
The days will get longer.
The key concept: angular momentum - when an object is rotating about an axis, the farther a mass is from that axis, the faster it goes, which means that it has more angular momentum.
question
Rub your hands together vigorously. What happens? Discuss the energy
transfers and transformations that take place
answer
When we rub ones hands against one another, the temperature of the hands increase.
The friction between your hands increases the kinetic energies in molecules of your
hands, and this exhibits itself as an increase in thermal energy of your hands. The temperature of your
hands goes up.
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According to the second law of thermo, it is impossible for a heat engine to convert thermal energy solely into work without exhausting some thermal energy to a cold reservoir. Is it possible to do the opposite--to convert work into thermal energy with 100% efficiency? If not, why not? If so, give an example.
answer
It will not attain 100% efficiency. Best example = refrigerators (carnot cycle)
question
Laws of thermodynamics
answer
The first law, also known as Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system.
The second law of thermodynamics states that the entropy of any isolated system always increases. an isolated system's ability to do work decreases over time
The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.
