A spring scale $A$ and a lever scale $B$ having equal lever arms are fastened to the roof of an elevator, and identical packages are attached to the scales as shown. Knowing that when the elevator moves downward with an acceleration of $1 \mathrm{~m} / \mathrm{s}^2$ the spring scale indicates a load of $60 \mathrm{~N}$, determine $(a)$ the weight of the packages, $(b)$ the load indicated by the spring scale and the mass needed to balance the lever scale when the elevator moves upward with an acceleration of $1 \mathrm{~m} / \mathrm{s}^2$.

Determine the maximum theoretical speed that may be achieved over a distance of 60 m by a car starting from rest, knowing that the coefficient of static friction is 0.80 between the tires and the pavement and that 60 percent of the weight of the car is distributed over its front wheels and 40 percent over its rear wheels. Assume (a) four-wheel drive, (b) front-wheel drive, (c) rear-wheel drive.

Un hombre de $72\,\mathrm{kg}$ está de pie sobre una báscula de resorte en un elevador. Partiendo del reposo, el elevador asciende, alcanzando una velocidad máxima de $1.2\,\mathrm{m/s}$ en $0.80\,\mathrm{s}$. El elevador viaja con esta rapidez constante durante $5.0\,\mathrm{s}$, experimenta una aceleración negativa uniforme durante $1.5\,\mathrm{s}$ y luego se detiene. ¿Qué registra la báscula de resorte durante los primeros $0.80\,\mathrm{s}$ s del ascenso del elevador?

Steep safety ramps are built beside mountain highways to enable vehicles with defective brakes to stop. A $10$-ton truck enters a $15^{\circ}$ ramp at a high speed $v_0=108 \mathrm{ft} / \mathrm{s}$ and travels for $6 \mathrm{~s}$ before its speed is reduced to $36 \mathrm{ft} / \mathrm{s}$. Assuming constant deceleration, determine $(a)$ the magnitude of the braking force, $(b)$ the additional time required for the truck to stop. Neglect air resistance and rolling resistance.

Steep safety ramps are built beside mountain highways to enable vehicles with defective brakes to stop safely. A truck enters a 750-ft ramp at a high speed v0 and travels 540 ft in 6 s at constant deceleration before its speed is reduced to $v_{0} / 2$. Assuming the same constant deceleration, determine (a) the additional time required for the truck to stop (b) the additional distance traveled by the truck.

The two systems shown start from rest. On the left, two $40 \mathrm{~lb}$ weights are connected by an inextensible cord, and on the right, a constant $40 \mathrm{~lb}$ force pulls on the cord. Neglecting all frictional forces, which of the following statements is true? $(a)$ Blocks $A$ and $C$ will have the same acceleration. $(b)$ Block $C$ will have a larger acceleration than block $A$. $(c)$ Block $A$ will have a larger acceleration than block $C$. $(d)$ Block $A$ will not move. $(e)$ None of the above is true.

Baggage on the floor of the baggage car of a high-speed train is not prevented from moving other than by friction. The train is traveling down a 5-percent grade when it decreases its speed at a constant rate from 120 mi/h to 60 mi/h in a time interval of 12 s. Determine the smallest allowable value of the coefficient of static friction between a trunk and the floor of the baggage car if the trunk is not to slide.

A 35 000-Mg ocean liner has an initial velocity of 4 km/h. Neglecting the frictional resistance of the water, determine the time required to bring the liner to rest by using a single tugboat that exerts a constant force of 150 kN.

A baggage conveyor is used to unload luggage from an airplane. The 10-kg duffel bag A is sitting on top of the 20-kg suitcase B. The conveyor is moving the bags down at a constant speed of 0.5 m/s when the belt suddenly stops. Knowing that the coefficient of friction between the belt and B is 0.3 and that bag A does not slip on suitcase B, determine the smallest allowable coefficient of static friction between the bags.

A $54-\text{kg}$ pilot flies a jet trainer in a half vertical loop of 1200-m radius so that the speed of the trainer decreases at a constant rate. Knowing that the pilot's apparent weights at points $A$ and $C$ are $1680 \mathrm{~N}$ and $350 \mathrm{~N}$, respectively, determine the force exerted on her by the seat of the trainer when the trainer is at point $B$.

The velocity of block $A$ is $2 \mathrm{~m} / \mathrm{s}$ to the right at the instant when $r=0.8 \mathrm{~m}$ and $u=30^{\circ}$. Neglecting the mass of the pulley and the effect of friction in the pulley and between block $A$ and the horizontal surface, determine, at this instant, $(a)$ the tension in the cable, (b) the acceleration of block $A,(c)$ the acceleration of block $B$.

A baseball player hits a $5.1 \mathrm{~oz}$ baseball with an initial velocity of $130 \mathrm{~ft} / \mathrm{s}$ at an angle of $40^{\circ}$ with the horizontal as shown. Determine $(a)$ the kinetic energy of the ball immediately after it is hit, $(b)$ the kinetic energy of the ball when it reaches its maximum height, $(c)$ the maximum height above the ground reached by the ball.

A baseball player catching a ball can soften the impact by pulling his hand back. Assuming that a $5-\mathrm{oz}$ ball reaches his glove at $90\ \mathrm{mi} / \mathrm{h}$ and that the player pulls his hand back during the impact at an average speed of $30\ \mathrm{ft} / \mathrm{s}$ over a distance of $6 \mathrm{in}$., bringing the ball to a stop, determine the average impulsive force exerted on the player's hand.

A 2500-lb automobile is moving at a speed of 60 mi/h when the brakes are fully applied, causing all four wheels to skid. Determine the time required to stop the automobile (a) on dry pavement ($\mu_{k}$ = 0.75), (b) on an icy road ($\mu_{k}$ = 0.10).