Will someone please help me with these Physics questions.

Question

Which of the following best describes the momentum of two bodies after a two-body collision if the kinetic energy of the system is conserved?

a. must be less

b. it will be double

c. must also be conserved

d. might also be conserved

An 80 kg hockey player is skating at 5 m/s. He collides with the wall and comes to a stop in .2 s. What was the force of his impact on the wall.

a. 600 N

b. 1000 N

c. 2000 N

d. 400 N

A 1,000 kg car is traveling at 20 m/s. The driver applies the brakes over a 10 s time period. What is the magnitude of the braking force?

a. 1000 N

b. 4000 N

c. 3000 N

d. 2000 N

What is the force required to produce a 1.4 Nm torque when applied to a door at a 60.0º angle and 0.40 m from the hinge? 

a. 8.2 N

b. 5.7 N

c. 2.0 N

d. 4.0 N

A moving billiard ball collides with an identical stationary billiard ball in an elastic collision. After the collision, the second ball has the same speed that the first ball had originally. Which of the following is true of the first ball after the collision?

a. It comes to rest.

b. It maintains half of its initial velocity.

c. It moves in the opposite direction of its original motion.

d. It maintains its initial velocity.

Answer 1

1) and 2) use impulse equation FΔt = Δmv (Solve for F)

3) torque is defined as Force (lever arm) sinθ (solve for F)

4) The E_K is conserved, therefor the 1st ball must not be moving if the second ball has the same kinetic energy as the 1st ball before the collision.

Answer 2

Answers to your 5 questions

1. c
2. c
3. b
4. d
5. a

Explanation.

1. The law of conservation of linear momentum is valid for elastic and inelastic collisions. I don't like the word "also" in the answer. The law of conservation of mechanical energy and the law of conservation of linear momentum are independent laws and not connected to each other.
2. To get the answer to the problem use the Newton's Second law in a form Δ(mv) = FΔt.
3. For this problem use Newton's Second law in a form F = ma. To find acceleration use the formula a = v / t.
4. From the formula for torque τ = F·d·sin Θ, find the force F.
5. In case of elastic collision if two balls with the same masses collide, then after the collision they just exchange their speeds. This result could be obtained, if you apply to the collision the law of conservation of linear momentum and the law of conservation of mechanical energy.·

Answer 3

Question:

Which of the following best describes the momentum of two bodies after a two-body collision if the kinetic energy of the system is conserved?

Answer:

The momentum is always conserved, whether kinetic energy is conserved or not.

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Question:

An 80 kg hockey player is skating at 5 m/s. He collides with the wall and comes to a stop in .2 s. What was the force of his impact on the wall.

Answer:

Let

m = 80 kg me the mass of the player,

v0 = 5 m/s be the player's speed at impact,

v1 = 0 m/s be the player's final speed,

t0 = 0 s be the time of impact,

t1 = 0.2 s be the time he stops.

The reason he stops is the force the wall impacts on him.

By Newton's Second Law the force is

F = ma

where a is his acceleration

a = (v1 - v0)/(t1 - t0) = (0 - 5)/(0.2 - 0) = -25 m/s²

So the wall acts with force

F = 80 × (-25) = -2000 N

The force is negative because the wall's force is in the opposite direction of his

initial speed.

The question is not about the force acting on the player, but about the force

with which the player acts on the wall.

By Newton's Third Law, that force is of equal magnitude, but opposite; that is 2000N.

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Question:

A 1,000 kg car is traveling at 20 m/s. The driver applies the brakes over a 10 s time period. What is the magnitude of the braking force?

Answer:

It is solved just like the previous question.

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Question:

What is the force required to produce a 1.4 Nm torque when applied to a door at a 60.0º angle and 0.40 m from the hinge?

Answer:

Let

τ = 1.4 Nm be the desired torque,

α = 60° be the angle the force forms with door,

r = 0.4 m be the radius of application.

By definition of torque,

τ = rFsin(α)

where F is the force. So

F = τ/rsin(α) = 1.4/(0.4×(√3/2)) = 4.04 N

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Question:

A moving billiard ball collides with an identical stationary billiard ball in an elastic collision. After the collision, the second ball has the same speed that the first ball had originally. Which of the following is true of the first ball after the collision?

Answer:

Let,

m be the mass of both balls,

v0 be the initial velocity of the first ball,

v1 be the final velocity of the first ball,

w0 be the initial velocity of the second ball,

w1 be the final velocity of the second ball.

We are give w0 = 0 and w1 = v0,

and we are being asked about v1.

The collision is governed by conservation of momentum: mv0 + mw0 = mv1 + mw1

Plugging the two given identities into it we get

mv0 = mv1 + mv0

From that v1 = 0.

Notice that we did not use the information that the collision is elastic.

The mere information that the second ball ends up with the original velocity of

the first is sufficient to imply that the first ball must end up with the initial velocity

of the second ball.

And that implies that kinetic energy is conserved.

This tells us that in an inelastic collision it is impossible for the second ball

to end up with the initial velocity of the first.