Consider the common experience of pushing open a door. Shown is a top view of a door hinged on the left. Four pushing forces are shown, all of equal strength. Which of these will be most effective at opening the door?

Question 9.1

• A ladder leans against a wall.
• What is the sign of the torque of the normal force of the wall on the ladder, N_W?
• Positive
• Negative
• The torque is zero
• It depends on where we choose the pivot point to be

Question 9.2

• A ladder leans against a wall.
• Let’s choose the pivot point to be at the bottom of the ladder.
• What is the sign of the torque of the normal force of the wall on the ladder, N_W?
• Positive
• Negative
• The torque is zero

Question 9.3

• A ladder leans against a wall.
• Let’s choose the pivot point to be at the bottom of the ladder.
• What is the sign of the torque of the static friction force of the floor on the ladder, f_{s F}?
• Positive
• Negative
• The torque is zero

Question 9.4

• A uniform ladder leans against a wall.
• Let’s choose the pivot point to be at the bottom of the ladder.
• What is the sign of the torque of the weight of the ladder, w?
• Positive
• Negative
• The torque is zero

Question 9.5

A man is standing on a diving board. The board is fixed in place at two points, 1 and 2. The board has negligible mass.

Which of the following is true about F1 and F2, the forces exerted by the posts at points 1 and 2 on the board?

A) F1 is pushing upward, and F2 is pulling downward.

B) Both F1 and F2 are pushing upward.

C) F1 is pulling downward, and F2 is pushing upward.

D) F1 or F2 may be more than one combination of pushing upward or pulling downward, depending on the mass of the man and the length of the board.

​

Question 9.6

Question 9.7

A

B

C

D. All three of the trucks will tip over.

E. None of the three will tip over.

The centres of gravity of the three trucks parked on a hill are shown by the Xs. Which truck(s) will tip over?

X

X

X

• A construction worker of mass m_w sits 2.0 m from the end of a steel beam of mass m_b, as shown.
• The tension in the Cable is T
• The wall exerts a normal force, n on the beam, and an upward force, F₁.
• Define +x = to the right, +y = up, and the axis of rotation is the point where the beam touches the wall.
• What is the force, F₁?

1. (m_b + m_w)g
2. (m_b + m_w)g − T cos(30°)
3. (m_b + m_w)g − T sin(30°)
4. T sin(30°)
5. T cos(30°)

Question 9.8

• A construction worker of mass m_w sits 2.0 m from the end of a steel beam of mass m_b, as shown.
• The tension in the Cable is T
• The wall exerts a normal force, n on the beam, and an upward force, F₁.
• Define +x = to the right, +y = up, and the axis of rotation is the point where the beam touches the wall.
• What is the normal force, n?

1. (m_b + m_w)g
2. (m_b + m_w)g − T cos(30°)
3. (m_b + m_w)g − T sin(30°)
4. T sin(30°)
5. T cos(30°)

Question 9.9