# EK & Friction in the direction of motion

Discussion in 'MCAT: Medical College Admissions Test' started by Actiq, Nov 17, 2005.

1. ### Actiq New Member

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Can anyone explain how friction can be in the direction of the net force as explained in EK physics Lecture 2.

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3. ### Nikki2002 Moderator Emeritus 7+ Year Member

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found the ek physics book to be confusing as hell....esp. that part about torque with the rhino...ugh i will see that in my nightmares. I liked the physics section from the kaplan comprehensive book much better.

4. ### shantster Eye protection! 10+ Year Member

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I haven't gotten to that point in the EK series, but I remember learning this in physics.

Say you have a pick up truck and a box is in the tailgate. (Let's just pretend this is a Caddilac truck with it's carpeted tailgate so we have lots of friction ) You accelerate in the forward direction and the box does not slide. The net force is in the direction that the truck is moving because the acceleration is in that direction. The friction between the box and the tailgate is also in that direction because otherwise the box would slide towards the back of the tailgate. If you think about it, the box would want to stay in the same place that it was before the truck began moving and so if there was no friction between the two, it would slide off the back of the tailgate. Thus, the friction is occurring opposite to the direction of motion that the box wants to go (backwards) and so it is in the same direction as the net force (forward).

Hope that helps!

5. ### gujuDoc 10+ Year Member

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I used TPR for Physics because I didn't like the way EK explained everything. There were just a couple of pointers i took from the EK physics book, because TPR books didn't cover them.

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6. OP

### Actiq New Member

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Hey!
Thanks, that does help.

7. ### osli Senior Member 5+ Year Member

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Just remember that a static friction force is always opposed to the direction that the "sliding" object would want to move, and that for dynamic friction it always opposes the direction of motion of the sliding object. i.e., it is essentially the same in both cases.

You push on a box... it wants to move in the direction your hand pushes it... the friction force opposes and pushes back towards your hand. If you push hard enough to overcome the static friction force, the box moves, but still with a friction force pushing back towards your hand.

The box in the pickup truck... inertia causes the box to want to move towards the rear relative to the truck... friction opposes and keeps it still. If you accelerate hard enough to overcome the static friction force, the box moves towards the rear, but still with a friction force opposing its motion.

You have a box on a board, and start tilting the board... gravity pulls on the box and it wants to slide down the board... static friction opposes this tendancy and pushes back, keeping the board in place. If you tilt the board enough to overcome the static friction force, the box slides but with dynamic friction opposing its motion.

i.e., static friction keeps things in place when they want to move by opposing the force trying to move them. Dynamic friction pushes back against the direction of motion and can be thought of as what keeps the box from sliding really fast down the board, or accelerating quickly when you push on it.

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8. ### Cooolguy Senior Member 7+ Year Member

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ek physics sucks. period.

9. ### abyssinianson Junior Member 5+ Year Member

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it did

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