Forces：练习题

A metal block is placed on a horizontal surface. A force of \(15 \text{ N}\) acts on the block to the right, and a force of \(8 \text{ N}\) acts on it to the left.
What is the resultant force acting on the block?

A uniform beam is balanced at its centre. A weight of \(12 \text{ N}\) is placed \(20 \text{ cm}\) from the pivot on one side.
What weight must be placed \(30 \text{ cm}\) from the pivot on the other side to keep the beam in equilibrium?

A uniform beam is pivoted at its centre point P. A weight of \( 4.0\text{ N} \) is placed at a distance of \( 30\text{ cm} \) to the left of the pivot.
At what distance to the right of the pivot must a weight of \( 6.0\text{ N} \) be placed to maintain equilibrium?

Which statement about solid friction is correct?

Which change would increase the stability of a simple object, such as a wooden block?

Describe the effect of solid friction on the motion of two surfaces in contact and state one energy change that occurs as a result of this force.

A uniform beam of weight \(30\text{ N}\) and length \(2.0\text{ m}\) is pivoted at its left end. It is held in a horizontal position by a vertical upward force F applied at its right end. Calculate the magnitude of force F and describe how its value would change if the pivot was moved to the centre of the beam.

A uniform beam is balanced at its centre. A weight of \(12 \text{ N}\) is placed \(25 \text{ cm}\) from the pivot on the left. Calculate the distance from the pivot where a \(20 \text{ N}\) weight must be placed on the right to keep the beam in equilibrium.

Two perpendicular forces act on a small object located at point P. Force \(F_1 = 12\text{ N}\) acts due North, and force \(F_2 = 16\text{ N}\) acts due East.

(a) Calculate the magnitude of the resultant force acting on the object.
(b) Determine the direction of the resultant force as an angle relative to the North.
(c) If the object has a mass of \(4.0\text{ kg}\), calculate the acceleration produced by this resultant force.
(d) State whether mass is a scalar or a vector quantity.

A rubber ball of mass \(0.20 \text{ kg}\) is moving at a velocity of \(15 \text{ m/s}\) towards a wall. It hits the wall and rebounds in the opposite direction with a velocity of \(12 \text{ m/s}\). The ball is in contact with the wall for a time of \(0.040 \text{ s}\).

(a) Calculate the momentum of the ball before it hits the wall.
(b) Calculate the change in momentum (impulse) of the ball during the collision, taking the initial direction of motion as positive.
(c) Determine the average resultant force exerted by the wall on the ball.
(d) Explain why the kinetic energy of the ball is not conserved during this collision.