Spring 2005

MWF 11:30

**Instructor**: Dr. Bruce Law, CW
327, Tel: 532-1618.

**R3. The
force of friction must be equal in magnitude but opposite in direction to the
force that you are pushing with. This is because the net force is zero.**

**R5.
You are pushing with a force which is equal in magnitude but opposite in direction
to the force of friction so that the net force is zero and you can maintain a
constant velocity.**

**R12.
Mass is more fundamental because it is an intrinsic property of an object.
Weight varies with location depending upon the acceleration due to gravity eg. for a mass m = 10kg on Earth it`s
weight is W = mg = 10 x 10 = 100N. However, in space where g = 0m/s ^{2}
this objects weight is W = mg = 10 x 0 = 0N whereas its
mass still remains 10kg.**

**R16. W
= mg = 1 x 10 = 10N**

**R23.
The acceleration a = F(net)/m therefore if F(net) is
tripled then a is also tripled.**

**R24. Acceleration
a = F(net)/m, therefore, for a constant F(net) if m is
tripled then the acceleration must decrease by a factor of 3.**

**R25. a
= F(net)/m. If both F(net)
and m are tripled then a remains unchanged.**

**R26.
The acceleration and net force point in the same direction.**

**E6. No, it is
not possible to go around a curve in the absence of a force. When you go around
a curve, your direction changes and hence your velocity changes, therefore you
are accelerating. According to ****Newton****'s 2nd Law of motion a net force must be present to
cause this acceleration.**

**E9. The bear
has a weight W = mg = 400 x 10 = 4000N, down. As the bear is sliding at
constant velocity it`s net force must be zero i.e.
there must be a frictional force of 4000N, up.**

**E15. W(moon) = mg(moon) = 10 x 10/6 = 16.7N (here I divided by 6
gravity on the moon is only 1/6 that of Earth). W(Earth)
= m g(Earth) = 10 x 10 = 100N. The mass m = 10kg on both the moon and Earth. **

**E33. You have
to run the engine in order to balance the frictional force which opposes the
motion.**

**E35. The apple
is held at rest, therefore, the net force is 0N. When
you drop the apple the net force on the apple is just due to its weight W = 1N.
(Note that in the problem they call it a 1-N apple therefore they are referring
to its weight (rather than it`s mass which would be
in kilograms (kg))).**

**P2.
Acceleration, a = F/m = 200N/40kg = 5m/s/s in the direction of the force. **

**P3. Upward
force = 300N. Weight of pail W = mg = 20 x 10 = 200N.**

**Therefore the
net force on the pail is F(net) = 300 – 200 =
100 N, up.**

**The
acceleration of the pail is a = F(net)/m = 100N,
up/20kg = 5m/s ^{2}, up.**