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1516 – Physics

Term 2 Week 10

Week 10 (Feb. 28 – Mar. 03)

AMS : 40021 Sample Questions Exam

1. Give the definition of "moment of a force".

2. What is the magnitude of the moment of a force around a pivot?

3. A door of weight 150 N has a height of 2.0 m and a width of 1.0 m. The center of gravity of the door is at its

geometric center, and the door rests on two pivots vertically above each other on one edge. What is the moment

of its weight around the lower hinge?

4. Find the magnitude and direction of the moment of F (2.0 N) around O..

5. The two forces in the figure act perpendicular to the ruler at d and d’ from the pivot. What is their resultant

moment (magnitude and direction)?

Numerical application: F = 3.0 N, F’ = 3.0 N, d = 2.5 m, and d’ = 2.0 m.

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6. F1 and F2 are two opposite forces separated by a distance d, and acting normally on an object. F1 and F2 form

a COUPLE; what is the moment of this couple?

7. Two masses M1 and M2 are suspended from the extremities of a horizontally meter stick of negligible mass.

Find the point of application of the single force needed to balance the stick.

Numerical application: M1 = 4.0 kg, and M2 = 1.0 kg.

8. Given the meter-stick of negligible mass shown below, find the resultant of the two downward pulls on the

stick.

Numerical application: M1 = 4.0 kg, and M2 = 1.0 kg.

9. The center of gravity can be thought of as the point of application of a certain force in the body. What force

is that?

10. A girl of mass m1 = 45 kg sits on the seesaw beam a distance d = 3.5 m from the pivot. Where should her

brother of mass m2 = 40 kg sit in order to prevent the seesaw from turning?

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11. Force F exerted at the top of the cube of weight W just prevents the cube of side s from tipping over the

edge of the desk, to which it is pivoted at O. What magnitude must F have?

Numerical application: W = 15 N.

12. Consider a system of two frictionless trolleys at rest on a flat surface. T has a mass m and U has an

unknown mass. The two trolleys are separated by a compressed spring S tied up by a string. When the string is

burned the spring pushes the carts apart. The two trolleys hit the bumpers at the same instant. If the distance X

is three times the distance Y, what is then the mass of U in terms of m?

Figure not to scale

13. A body of mass 5.0 kg was subjected to a net constant force of (16 N)j for a time interval t.

During that time the velocity of the body changed from (12 m s-1

)j to (2.0 m s-1

)j. Find t.

14.

a) Given the displacement versus time graph of a body, how to determine graphically the velocity at any instant

t?

b) If the graph is a straight line, what does its slope represent?

c) What does a zero slope represent?

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Homework : PLH31 – WRCC

Sample Questions Exam

1. Determine the magnitude and direction of the moment vector of the force using the figure below.

2. Determine the magnitude and direction of the moment vector of the force using the figure below.

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3. Determine the magnitude and direction of the moment vector of the force using the figure below.

4. Determine the magnitude and direction of the moment vector of the force using the figure below.

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5. Determine the magnitude and direction of the resultant moment in this case:

6. Determine the magnitude and direction of the resultant moment in this case:

7. A meter ruler AB of weight 0.75 N is supported on a knife edge placed at 25 cm from A. The meter ruler

balances horizontally when a mass of 50 g is suspended at a distance of 15 cm from A. Find the position of the

center of gravity of the meter ruler.

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8. A girl of mass 50 kg sits on the right-hand side of a seesaw at a distance of 3.4 m from its pivot. Another girl

of weight 450 N sits on the left-hand side. At what distance from the pivot must she sit to balance the seesaw?

Reference: Physics L Part 1, Chapter 4.11, Exercise 4.6, Page 244

Solution:

m1 = 50 kg, d1 = 3.4 m, m2g = 450 N, g = 10 N/kg, d2?

Sum of the clockwise moments around O = Sum of the anticlockwise moments around O.

1 11 1 2 2 2

2

50 10 3 43 7 3 8 m

450

m gdm gd m gd d

m g

.. .

9. What is terminal velocity

10. A spring has an original length of 10 cm. When a load of 500 g is applied to the spring, it produces an

extension of 20 mm without exceeding the elastic limit of the spring. Calculate its length when a load of 250 g

is applied.