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Physics 11 Assignment #3
Forces
1.
While riding in a helicopter, you drop two ping-pong balls, one filled with air and the other with
water. Both experience air resistance as they fall. Which ball reaches terminal velocity first?
Do both hit the ground at the same time? Explain.
2.
Cheetahs are bigger and faster than small gazelles, but more often than not gazelles escape a
pursuing cheetah by zigzagging. Exactly why does this put the cheetah at a disadvantage?
(Note: use Newton’s laws of motion in your answer.)
3.
According to legend, a horse learned Newton’s laws. When it was told to pull a cart, it refused,
saying that if it pulled the cart forward, according to Newton’s 3 rd law there would be an equal
force backwards. Thus, there would be balanced forces and, according to Newton’s 2nd law, the
car would not accelerate. How would you reason with this rather weird, educated horse?
4.
Which creates a greater force of friction: sliding your physics book across the table on its back
cover or along one of its edges?
5.
Can the coefficient of friction ever be greater than one? Less than one? Explain. What
information would you need to calculate  precisely?
6.
An 873-kg dragster, starting from rest, attains a speed of 26.3 m/s in 0.59 s.
a. Find the average acceleration of the dragster during this time interval.
b. What is the size of the average force on the dragster during this time interval?
c. Assume the driver has a mass of 68 kg. What horizontal force does the seat exert on the
driver?
d. Is the driver’s mass in part c an inertial mass or gravitational mass?
7.
After a day of testing race cars, you decide to take your own 1550-kg car onto the test track.
While moving down the track at 10 m/s, you suddenly accelerate to 30 m/s in 10 s. What is
the average net force that you have applied to the car during the 10-s interval?
8.
When you drop a 400-g apple, Earth exerts a force on it that accelerates it at 9.81 m/s2 toward
Earth’s surface. According to Newton’s 3rd law, the apple must exert an equal and opposite
force on Earth. If the mass of Earth is 5.98 x 1024 kg, what’s the magnitude of Earth’s
acceleration?
9.
A 95.0-kg boxer has her first match in the Canal Zone (g = 9.782 m/s2) and her second match
in Iqualuit, Nunavut (g = 9.832 m/s2).
a.
b.
c.
d.
e.
What is her mass in the Canal Zone?
What is her weight in the Canal Zone?
What is her mass in Iqualuit?
What is her weight in Iqualuit?
Explain the difference between her “mass” and her “weight”.
10.
You place a 7.50-kg television set on a spring scale. If the scale reads 78.4 N, what is the
acceleration of gravity at that location?
11.
If you use a horizontal force of 30.0 N to slide a 12.0-kg wooden crate across a floor at a
constant velocity, what is the coefficient of sliding friction between crate and floor?
Physics 11 Assignment #3
Name: _________________
12.
A student stands on a bathroom scale in an elevator at rest on the 64 th floor of a building. The
scale reads 836 N.
a. As the elevator moves up, the scale reading increases to 935 N, then decreases back to
836 N. Find the acceleration of the elevator.
b. As the elevator approaches the 74th floor, the scale reading drops as low as 782 N. What is
the acceleration of the elevator?
c. Using your results from parts a and b, explain which change in velocity, starting or
stopping, would take the longer time.
d. Explain the changes in the scale you would expect on the ride back down.
13.
A sled of mass 50 kg is pulled along snow-covered, flat ground. The static friction coefficient is
0.30, and the sliding friction coefficient is 0.10.
a. What does the sled weigh?
b. What force will be needed to start the sled moving?
c. What force is needed to keep the sled moving at a constant velocity?
d. Once moving, what total force must be applied to the sled to accelerate it 3.0 m/s 2?
14.
A 2.0-kg mass (m1) and a 3.0-kg mass (m2) are attached to a
massless cord that passes over a frictionless pulley as shown
below. The hanging masses are free to move. Assume the
positive direction of motion to be when the smaller object moves
upward and the larger mass moves downward. This device is
called Atwood’s Machine.
a. Draw the free-body diagram of this situation.
b. What is the acceleration of the system?
c. What is the tension force acting on the cord?
m2
m1
15.
Change m1 to 1.00 kg and m2 to 4.00 kg in the Atwood’s Machine above.
a. What is the acceleration of the system?
b. What is the tension force acting on the cord?
16.
If a vertical spring is stretched 2.0 cm downward by a mass of 0.55 kg, what is the spring
constant?
17.
A 0.35 kg mass attached to a spring of spring constant 130 N/m is free to move on a
frictionless, horizontal surface. If the mass is released from rest at xo = 0.00 m, find the force
on it and its acceleration at
a. x = 0.10 m
b. x = 0.050 m
c. x = 0.00 m
d. x = -0.050 m
18.
A 9.5 kg mass attached to a spring is free to move on a frictionless, horizontal surface. The
equilibrium position is at xo = 0 m. The mass moves indefinitely between x1= 0.25 m and
x4= -0.25 m. The spring force is -30 N at x1.
a. Find the spring constant for this Hooke's Law type situation.
b. Find the Fs at x2 = 0.125 m and x3 = -0.125 m.
c. What is the acceleration at x1 , x2 , x3 and x4.
19.
If a mass is accelerated -55 m/s when a spring force of 110 N is exerted on it,
a. what is the mass in kg?
b. what is the displacement from equilibrium (xo = 0 m) of the mass at this moment if the
spring constant is 2750 N/m?
20.
Which is the stiffer spring, the spring in question #17 or #19 ?
Physics 11 Assignment #3