Download Physics 1A Week 2 Questions Fall 1998

Physics 1A
Week 2 Questions
Fall 1998
1. A flea can jump 3 cm with a takeoff angle of 70o. What is its initial velocity? The flea achieves this
speed in a takeoff distance of 0.5 mm (that is about the size of a flea), what is its average
acceleration during takeoff?
2. Suppose you have a horse pulling a buggy along a road. Which statement is correct:
(a) If action always equals reaction a horse cannot pull a buggy because the action of the horse on
the buggy is exactly cancelled by the reaction of the buggy on the horse. The buggy pulls backwards
on the horse just as hard as the horse pulls forward on the buggy, so they cant move.
(b) The horse pulls forward slightly harder on the buggy than the buggy pulls 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 forward only if the horse weighs more than the buggy,
(e) The force on the buggy is as strong as the force on the horse, but the horse can plant its feet on
the ground to pull and the buggy rolls on its wheels.
3. Light carries momentum, so when a beam impinges on a surface it will exert a force on that surface.
If the light is reflected rather than absorbed the force will be (a) less (b) greater (c) equal (d) not
enough information given.
4. A garbage truck crashes head-on into a Volkswagen, and the two come to rest in a cloud of flies.
Which experiences the greater impact force? (a) the truck (b) the Volkswagen (c) both the same
(d) not enough information to tell
5. It is fairly easy to catch a 20 N (4 lb) steel weight dropped from a height of 20 cm or so. However, if
you attempt to catch the weight while resting your hand flat on a table you are likely to be injured.
Why is this? (Don't try this at home)
Physics 1A
Week 2 Solutions
Fall 1998
1. A flea can jump 3 cm with a takeoff angle of 70o. What is its initial velocity? The flea achieves this
speed in a takeoff distance of 0.5 mm (that is about the size of a flea), what is its average
acceleration during takeoff?
Range =
vo2 sin 2
, substitute range=0.03m and  = 70o solve for vo = 0.68 m/s
g
At takeoff flea starts with vo=0 and ends with v=0.68 m/s in distance of 0.5 x 10-3m, use v2=vo2+2as,
solve for a = 470 m/s2
2. Suppose you have a horse pulling a buggy along a road. Which statement is correct:
(a) If action always equals reaction a horse cannot pull a buggy because the action of the horse on
the buggy is exactly cancelled by the reaction of the buggy on the horse. The buggy pulls backwards
on the horse just as hard as the horse pulls forward on the buggy, so they cant move.
(b) The horse pulls forward slightly harder on the buggy than the buggy pulls 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 forward only if the horse weighs more than the buggy,
(e) The force on the buggy is as strong as the force on the horse, but the horse can plant its feet on
the ground to pull and the buggy rolls on its wheels.
It is true that the force backwards on the horse is the same as the force forwards on the buggy. We
need to look at the resultant force on the system to see how it will accelerate. There will be a net
forward force on the "horse and buggy" system, provided by the difference between the forward
force of friction from the horse's feet on the ground, and the smaller backwards rolling friction
force.
3. Light carries momentum, so when a beam impinges on a surface it will exert a force on that surface. If
the light is reflected rather than absorbed the force will be (a) less (b) greater (c) equal (d) not enough
information given. The change in momentum is twice as large when the light is reflected, so the
force is twice as large.
4. A garbage truck crashes head-on into a Volkswagen, and the two come to rest in a cloud of flies.
Which experiences the greater impact force? (a) the truck (b) the Volkswagen (c) both the same
(d) not enough information to tell
Ans. is (c), Both experience the same force, but since the VW has less mass it has a larger
acceleration.
5. It is fairly easy to catch a 20 N (4 lb) steel weight dropped from a height of 20 cm or so. However, if
you attempt to catch the weight while resting your hand flat on a table you are likely to be injured.
Why is this? (Don't try this at home)
The change in momentum of the weight will be the same in each case, but the time it takes to stop
will be larger if you do not allow your hand to move backwards during the catch. Change of
momentum = impulse = force x time interval. So if the time interval is smaller the force is larger
and it hurts more.