PHYSICS 231 Lecture 14: revision. Remco Zegers Walk-in hour: Monday 9:15-10:15 am Helproom BPS 1248. Chapter 4: Newton’s Laws.
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Walk-in hour: Monday 9:15-10:15 am
Helproom BPS 1248
Newton’s second law: F=ma
Gravitational Force: F=mg F=Gm1m2/r2
Equilibrium: Fx=0 Fy=0 (object not moving)
normal force: force perpendicular to surface the object
is resting on, and balancing the component of
the gravitational force perpendicular to the
friction: F=sn (s: coef. of static friction)
F=kn (k: coef. of kinetic friction) s>k
Hooke’s law: Fs=-kx(k: spring constant)
To lift a patient, 4 nurses grip the sheet on which the patient
is lying and lift upward. If each nurse exerts an upward
force of 240 N and the patient has an upward acceleration
of 0.504 m/s2 what is the weight of the patient?
A 5.0 kg bucket of water is raised from a well by a rope.
If the upward acceleration of the bucket is 3.0 m/s2, find
the force exerted by the rope on the bucket.
A force F (10N) is exerted on
the red block (1 kg). The coef.
of kinetic friction between the
red block and the blue one is 0.2.
If the blue block (10kg)rests on a
frictionless surface, what will
be its acceleration?
A rocket is fired from a launching pad. The velocity AND
acceleration of the rocket increase with time even though
the thrust of the engine is constant. Why?
a)The gravitational force becomes smaller because the mass
of the rocket becomes smaller (losing fuel)
b) The gravitational constant g becomes smaller if the rocket
travels away from the earth
c) While traveling away from earth, the gravitational pull
from other planets/sun becomes stronger supporting the
d) Because the density of air becomes small, the friction
position in grav. field.
k is the spring constant
A pendulum is pushed with initial
velocity 0.1 m/s from a height of
1 cm. How far does it compress the
spring? (assume m does not rise
significantly after hitting the spring)
A ‘smart’ student decides to save energy by connecting his
exercise treadmill to his laptop battery. If it takes
70 J to move the belt on the treadmill by 1 meter and
50% of the generated energy is stored in the battery, how
‘far’ must the student run to use his 100 W laptop for free
for 2 hours?
A block of 1 kg is pushed up a 45o
slope with an initial velocity of 10 m/s.
How high does the block go if:
a) there is no friction
b) if the coefficient of kinetic friction
A crate of 50kg is starting to slide from a slope. When it
reaches the bottom, it is caught by a spring with a spring
constant of 1000 N/m. a) If the crate was originally at a height
of 10 m and friction can be ignored, how much is the spring
b) if the frictional force is 100N and
the length of the slope is 15m, what is
the maximal compression?
A 70-kg diver steps of a 10-m tower and drops, from rest
straight down into the water. If he comes to rest 5.0 m
below the water surface, determine the average resistive
force exerted by the water.