Originally posted by Ramned You are correct. Hands vary in size = not precise as, say, a meter. The length of a hand varies from person to person, so it isn't a useful standard of length.
Which is why the length of a "hand" is standardised, just as a metre is. People don't generally use their own hand to measure it. Conceptually it's not a problem.
Motion continuing...hope they're not too hard for ya yet 😉
Laws of Motion
4. A large crate is placed on the bed of a truck. (A) As the truck accelerates forward, the crate remains at rest relative to it...what force causes the crate to accelerate forward? (B) If the driver slams on the brakes, what could happen to it?
Be sure to answer them, not critize - PM if you see a problem with my ?'s as I can't "always" be perfect 🙂
Originally posted by Schumi A: Friction
B: It may slide forward relative to the truck
B: probably, unless something prevents that or if the acceleration from slamming the breaks is less than the acceleration under a) and if the friction is symlmetrical. And so many other things that can happen
B: depends on the critical friction and the rate of deceleration.
If the deceleration has magnitude a, then the frictional force F required to stop it sliding is ma. But F is less than mu x mg, where mu is the critical friction factor (or something - I've forgotten the term that is actually used).
Originally posted by Ramned Motion continuing...hope they're not too hard for ya yet 😉
Laws of Motion
[b]4. A large crate is placed on the bed of a truck. (A) As the truck accelerates forward, the crate remains at rest relative to it...what force causes the crate to accelerate forward? (B) If the driver slams on the brakes, what could happen to it?
Be sure to answ ...[text shortened]... r them, not critize - PM if you see a problem with my ?'s as I can't "always" be perfect 🙂[/b]
A: friction, gravity and inertia all acting together.
B: The crate will accelerate forward w.r.t. the truck if the forward inertial force on the crate (pseudo force) produced by the deceleration of the truck, in the frame of reference of the truck, exceeds the opposing frictional force acting on the crate.
Originally posted by Ramned You are correct. Hands vary in size = not precise as, say, a meter. The length of a hand varies from person to person, so it isn't a useful standard of length.
QUESTION 2 FOLLOWS.
LOL
You dont actually think people use their own hands to measure horses?!??!
LOL
You think the doctor uses his own feet to measure the height of your kids for check-ups?
Originally posted by Ramned A harder one on motion...this is still basic though.
Two Dimensional Motion
[b]3. A wrench is dropped from the top of a 10-m mast on a sailing ship while the ship is traveling in a straight line at 20 knots. Where will the wrench hit the deck?[/b]
If the wind is not a factor, right by the mast from which it was dropped.
Originally posted by Ramned Motion continuing...hope they're not too hard for ya yet 😉
Laws of Motion
[b]4. A large crate is placed on the bed of a truck. (A) As the truck accelerates forward, the crate remains at rest relative to it...what force causes the crate to accelerate forward? (B) If the driver slams on the brakes, what could happen to it?[/b]
The crate remains at rest relative to the truck because of the force of Friction. If the driver slams on the breaks the forward motion of the crate could over come the force of friction and it could continue moving after the truck stops.
Originally posted by Schumi A: Friction
B: It may slide forward relative to the truck
Correct.
For (B) the reason for this is inertia.
Some other answers I have seen are applicable as well.
To answer people for the "hand" problem: Hands vary is size. Measuring a horse with my hand compared to some 8 feet tall will produce 2 different precise measure. Therefore, it is NOT ALWAYS a good STANDARD of measurement, as compared to measuring a horse using meters.
PM If you wish to correct me; I am solely interested to see who's the Master here!
Next Question to follow!
5. As a simple pendulum swings back and forth, the forces acting on the suspended object are the force of gravity, tension on the suspending cord, and air resistance. (A) Which of these 3 forces exerts no work on the pendulum? (B) Which of these forces does negative work at all times during the pendulum's motion? (C) Describe the work done by the force of gravity while the pendulum is swinging.
Make sure to explain each one - if not I will ask follow-ups because this requires some description.
[b]5. As a simple pendulum swings back and forth, the forces acting on the suspended object are the force of gravity, tension on the suspending cord, and air resistance. (A) Which of these 3 forces exerts no work on the pendulum? (B) Which of these forces does negative work at all times during the pendulum's motion? (C) Describe the work done ...[text shortened]... to explain each one - if not I will ask follow-ups because this requires some description.[/b]
(A) The tension on the suspending cord does no work on the pendulum, because the force it exerts is always tangential to the movement of the pendulum.
(B) The air resistance always does negative work on the pendulum, because air resistance opposes the motion of the pendulum and therefore always acts opposite to the direction of movement of the pendulum.
(C) The work done by gravity on the pendulum changes from negative to positive depending on whether the pendulum is on an upswing (negative work) or on a downswing (positive work).
19 Sep '07 12:14
Anyone a Master of Physics?
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