# Physics brain teasers

1. Jul 8, 2004

### dajugganaut

hi! i have two brain teasers (more like logic questions) which I received in class today...if anyone is able to tell me the actual answers that would be excellent!!! thanks!!

1. Falling Rocks

A boulder is many more itmes heavier than a pebble- that is, the gravitational force that acts on a boulder is many more times that which acts on a pebble. Yet if you drop a boulder and a pebble at the same time, the will fall together with equal accelerations (neglecting air resistance). The principal reason the haveier boulder does not acclerate more than the pebble has to do with:

a)energy
b)weight
c)inertia
d)surface area
e) none of the above

2. Horse and Buggy (Which of the following is correct?)

a)If action always equals reaction, a horse cannot pull a buggy becasue the action of the horse on the buggy is exactly cancelled by the reaction of the buggy on the horse. The buggy pulls backward on the horse just as hard as the horse pulls forward on the buggy, so they cannot move.

b) The horse pulls forward slightly harder on the buggy than the buggy pulls backward on the horse, so they move forward.

c) The horse pulls before the buggy has time to react, so they move forward.

d) The horse can pull the buggy frward only if the horse weighs more than the buggy.

e) The force on the buggy is as strong as the frce on the horse, but the forces are acting on two different objects, each with different masses and frictional conditions.

2. Jul 8, 2004

### Gokul43201

Staff Emeritus
1. This is a ridiculously worded question. I can convincingly argue that the answer could be any of a,b,c or e.

2. This if definitely (e)

3. Jul 8, 2004

### dajugganaut

could you explain gokul43201???

4. Jul 8, 2004

### Gokul43201

Staff Emeritus
Which one ?

5. Jul 8, 2004

### dajugganaut

the second one perhaps, and maybe a brief reasoning for why a,b,c,e for number1?

6. Jul 8, 2004

### Gokul43201

Staff Emeritus
1. The gravitational force on the object is proportional to its mass, as described in the first line of the question. So, F = km, where k is some proportionality constant that is independent of the object. The acceleration is related to the force by the relation, F = ma. So, a = F/m = km/m = k. So, the acceleration is a constant independent of the object. So...
(a)energy : this is a bit of a stetch, so I'll skip it,
(b)weight : The weight IS the gravitational force, and is given by W = mg. This implies that the free acceleration of an object is always = g, no matter how heavy it is.
(c)inertia : This is just another word for mass, and is probably the answer your teacher wants. A greater inertia means that it takes a greater force to cause the same acceleration.
(e)none of the above : Because the correct reason is what I gave in the discussion at the top.

7. Jul 8, 2004

### Gokul43201

Staff Emeritus
2. For an object to move from rest, there must be some unbalanced force on it. The horse moves forward because friction is pushing it. The buggy moves because the horse is pulling it. The buggy pulls the horse back with the same force that the horse pulls the buggy, but this is not the same as the force of friction that pushes the horse forward. The net forward force on the horse = F(friction) - F(pull from buggy). This net forward force results in motion.

8. Jul 8, 2004

### dajugganaut

thanks for helping me!