# Approaching Physics Problems

Q:
Hi, I'm studying for my finals and I'm having trouble understanding some of my classroom material... Could you help me to understand this please? The question is, "A small cart is rolling on an inclined plane with a constant acceleration of 0.5 m/s^2 in the negative x direction. At time t=0, the cart has a velocity of 2 m/s in the positive x direction. If the cart never leaves the ramp, which of the following describes the motion of the cart at a time t>5s?" The answer choices are: A) It's traveling in the +x-direction and is slowing down. B) It's traveling in the -x-direction and is slowing down. C) It's traveling in the +x-direction and is speeding up. D) It's traveling in the -x-direction and is speeding up. -Thank you, Mardi
- Mardi (age 17)
Henderson, NC, US
A:

We don't do school problems, but we can suggest an approach that helps. Try to make a concrete picture of what's happening. You start off with "A small cart is rolling on an inclined plane with a constant acceleration of 0.5 m/s^2 in the negative x direction. At time t=0, the cart has a velocity of 2 m/s in the positive x direction." Why not draw a sketch showing where that cart would be at each second (1 to 5) if it didn't accelerate. Then think about what the acceleration does in the first second. How fast is the cart going on the average, say at 1/2 sec? Which way? Then do the same for the second second, etc. You'll understand what's happening. You can probably get the answer this way. Now you can make a mathematical expression for what the velocity is at different times. You'll see what's meant by positive and negative velocity. Etc. The math here is really easy, you just have to think about what the words mean, and the pictures should help.

Mike W.

(published on 01/02/2016)

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