Constant velocity forces and kinetic energy ratio problems

[wmsaqqa]

1)- An object is moving with constant velocity. Wich of the following statements is true?

A) There are no forces acting on the object.
B) There is no frictional force acting on the object.
C) The net force on the object is zero N.
D) A constant force is being applied in the direction of motion.

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2)- The ratio of the mass of object A to the mass of object B is 2:1 and the ratio of their speeds is 1:2 in the same oreder. What is the ratio of their kinetic energies?

A) 1:3
B) 2:1
C) 1:2
D) 3:1


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* Please help me by giving me the answers for these 2 questions and explain the answer, thanks.

 

[LawrenceC]

I won't give you the answer but I'll help you think it through.

Hints:

For Q1, think of Newton's second law.

For Q2, think of the formula for kinetic energy and compare the two situations.

 

[wmsaqqa]

For q1, i get the answer C, because the object is moving with constant velocity.. so the net force on the object is zero .. right? I am not sure

 

[cupid.callin]

For q1, i get the answer C, because the object is moving with constant velocity.. so the net force on the object is zero .. right? I am not sure

Yes you're right ...

 

[LawrenceC]

For q1, i get the answer C, because the object is moving with constant velocity.. so the net force on the object is zero .. right? I am not sure

Ditto previous post. You are correct.

 

[LawrenceC]

Hint for Q2:

Write the ratio of kinetic energies.

 

[wmsaqqa]

i didn't understand this question anymore, its complicated!
can i have some help please?

 

[BloodyFrozen]

As LawrenceC already said, write the ratio of the kinetic energies.

$$KE = \frac{1}{2}mv^{2}$$

 

[wmsaqqa]

1/2 x 2:1 x (1:2)2

and then? how can i multiply 1/2 and 2:1 .. ?

 

[BloodyFrozen]

$$KE=\frac{1}{2}mv^{2}$$

Object A mass : Object B mass → 2:1
Object A velocity : Object B velocity → 1:2

For example, let's say $m_A = 6$, then this forces $m_B = 3$.
Again, we can choose $v_A = 3$, then this forces $v_A = 6$.

Now apply the formula for kinetic energy and compare their values.

$$KE_A : KE_B$$

What do you get?

 

[Morgoth]

if you have two quantites A,B what is their ratio?

A:B= A/B
So eg for masses 1:2 means M_A/M_B =1/2 or M_B=2M_A

so writting KE of A and KE of B KE_A and KE_B their ratio would be
KE_A/KE_B (or KE_B/KE_A above is the one you want to compare with the one below).

writing their = will give you the answer.

 

[wmsaqqa]

i've got the answer C.. thank you guys for help