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# Momentum question. watch

1. Hello!

Here's a pretty basic momentum question. I understand how to work it out if I have made a correct assumption. Here's the question:

"A driver of mass 100kg is driving a bumper car of mass 100kg and reaches a velocity of +3m/s (6.8mph). He drives straight into another bumper car of mass 100kg which is driving straight at him "head on" at a velocity of -2m/s. The mass of the occupant of this car is 30kg. What happens in this case?"

My assumption is that the minus sign is used for the second car's velocity because velocity is a vector, so it just shows that the cars are travelling in opposite directions. Is this a correct assumption? As I have said, if I'm correct I can work it out, but f not, please can you give me some advice on how to solve this problem?

Thank you TSR.
2. I apologise, but, bump
3. Hmm I think it does sound a bit strange.
This is the way I read it: (which may or may not be right)

The question already tells you which direction the second guy is going,
"is driving straight at him "head on" at a velocity "

So it implies that this second driver is driving the opposite direction to the first driver.

But then it says the second driver's velocity is -2 m/s.

This sort of implies that he is going 2 m /s in the opposite direction to what direction he was already going in.

So then this sort of suggests that he is actually, going the same direction as the first driver.
4. (Original post by v2p)
Hmm I think it does sound a bit strange.
This is the way I read it: (which may or may not be right)

The question already tells you which direction the second guy is going,
"is driving straight at him "head on" at a velocity "

So it implies that this second driver is driving the opposite direction to the first driver.

But then it says the second driver's velocity is -2 m/s.

This sort of implies that he is going 2 m /s in the opposite direction to what direction he was already going in.

So then this sort of suggests that he is actually, going the same direction as the first driver.
That's what I thought at first, but then they wouldn't be driving at each other "head on" as the question leads you to believe. I worked it out using vector arrows anyway, I hope I did it correctly!
5. (Original post by IQ Test)
That's what I thought at first, but then they wouldn't be driving at each other "head on" as the question leads you to believe. I worked it out using vector arrows anyway, I hope I did it correctly!
Yes, the minus sign means motion in the opposite direction to the agreed direction of positive motion, so the two cars are travelling towards each other when they collide.
6. (Original post by IQ Test)
Hello!

Here's a pretty basic momentum question. I understand how to work it out if I have made a correct assumption. Here's the question:

"A driver of mass 100kg is driving a bumper car of mass 100kg and reaches a velocity of +3m/s (6.8mph). He drives straight into another bumper car of mass 100kg which is driving straight at him "head on" at a velocity of -2m/s. The mass of the occupant of this car is 30kg. What happens in this case?"

My assumption is that the minus sign is used for the second car's velocity because velocity is a vector, so it just shows that the cars are travelling in opposite directions. Is this a correct assumption? As I have said, if I'm correct I can work it out, but f not, please can you give me some advice on how to solve this problem?

Thank you TSR.
Yeah, your assumption is correct. I'm afraid v2p is overcomplicating things. The '+' and '-' signs are here to indicate the direction of movement. The question says that the car is going 'head on' to emphasise it.
7. (Original post by jaroc)
Yeah, your assumption is correct.
Really, it's not even an assumption. It's convention - all the information required is given in the question.
8. Ah okay I see that
9. Thank you everyone!

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