# Review Worksheet of Newtonâ€™s Laws

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```					    Newton’s Laws Review                                                    Name
Physics Honors                                                          Date                       Per

Test 3 will include:
• Problems like those below and from your homework
• A conceptual problem
• A problem based on what you did in lab

Friction Sample
1.   A 2.0 kg silverware drawer becomes stuck, so the owner gradually pulls with more and more
force. When the applied force reaches 8.0 N, the drawer suddenly opens, throwing all the utensils
to the floor. Find the coefficient of static friction between the drawer and the cabinet.

Inclined Plane Sample
2.   The block shown in the figure below lies on a smooth plane tilted at an angle θ = 22o to the
horizontal. Rotate your axes! In this problem there is no friction.
a. Determine the acceleration of the block as it slides down the plane.
y               b. If the block starts from rest 9.1 m up the plane from its base, what will be the block’s
speed when it reaches the bottom of the incline?
Old axes                  y’ Rotated axes
x

x’
22o

Mixed Review Problems

3.   A painting of 10kg mass is held up by two strings. One string is at an angle of 30° to the vertical
and has a tension force on it of 60N. What is the angle and the tension force in the other string?

4.   A woman weighs herself in an elevator.
a. Before the elevator starts moving, the scale reads 120lb., what is her mass?
b. If the scale reads 100lb., is she accelerating upward or downward?
c. What is the acceleration of the elevator if the scale reads 100lb.?

5.   If the Force of friction is 100N, and µs = 1.2,
a. What is the normal force of the object?
b. What is its weight (i.e. force of gravity)

6.   For a boy who has mass of 50kg on Earth (g=9.8 m/s2), what are his mass and weight on the moon
(g=1.63 m/s2)?

7.   A helicopter (mass 600kg ) is rising with an acceleration of 2m/s2 while carrying a person of
mass 90 kg hanging by a cable.
a. Find the tension in the cable.
b. Find the upward force of the helicopter propeller.
8.   George pulls on a box with a force of 100N at an angle of 300 to the horizontal on a frictionless
surface.
a. Find FN
b. Find the acceleration of the box.
c. While George pulls, another boy pulls backwards on the box (parallel to the surface) with
a force of 20N. How does the acceleration of the box change? How does FN change?

F=100N
300
20 kg

9.   Jean, who likes physics experiments, dangles her watch from a thin piece of string while a jetliner
she is in takes off from SFO Airport. She notices that the string makes an angle of 25o with
respect to the vertical while the aircraft accelerates for takeoff, which takes about 18 seconds.
Estimate the takeoff speed of the aircraft.

10. Police lieutenants, examining the scene of an accident involving two cars, measure the skid marks
of one of the cars, which nearly came to a stop before colliding, to be 80 m long. The coefficient
of kinetic friction between the rubber and the pavement is about 0.80. Estimate the initial speed of
the car assuming a level road.

11. In the design of a supermarket, there are to be several ramps connecting different parts of the
store. Customers will have to push grocery carts up the ramps, and it is obviously desired that this
not be too difficult. An engineer has done a survey and found that almost no one complains if the
force required is no more than 50 N. Will a slope of θ = 5o be too steep, assuming a 30 kg grocery
cart (full of groceries)? Assume that there IS friction (wheels against ground, wheels on axels,
etc.) and it can be accounted for by a coefficient µk = 0.10.

ANSWERS (note: some answers use 9.8m/s2 and some 10m/s2)
1. µs = 0.41
2. (a) 3.67 m/s2 (b) 8.17 m/s
3. Force of 57N at an angle of 32°
4. a) 54kg b) down c)1.7 m/s2
5. a)83N b) don’t know, if it’s a flat surface then 83N, but if its on an
incline, need the angle of the incline.
6. (a) 50 kg (b) 82 N
7. a) 1080N b) 8280N
8. (a) 146 N (b) 4.33 m/s2 (c) a = 3.33 m/s2 FN same
9. 82 m/s
10. 35 m/s
11. 55 N > 50 N, too steep

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