6. Intro to Forces (Dynamics)

Forces & Kinematics

Multiple Choice
An 800-kg car is traveling along a horizontal road directly towards a cliff. The driver notices and brakes, resulting in a 5,000-N net force slowing the car down. If the car's initial speed was 20 m/s and the car stops just before going over the cliff, how far away was the car from the cliff when the driver hit the brakes?
Multiple Choice
The following forces are acting on a $250 g$ box on a horizontal surface. A downward force of gravity with magnitude $2.45 N$ . An upward normal force with magnitude $2.45 N$ . A rightward pull from a rope of $5.0 N$ . A leftward kinetic friction force with magnitude $7.5 N$ . At this instant, what is the magnitude of the acceleration of the box?
Multiple Choice
Tabletop shuffleboard is a popular pub game where players slide pucks across a tabletop trying to get them as close to the end of the table as possible, without falling off. Shuffleboard tables are typically covered in shuffleboard wax, which gives a very low coefficient of kinetic friction between the table and puck. Suppose that one particular table is 6.0 long m and has a coefficient of kinetic friction of 0.080. What speed should a player give to a puck on one end of the table so the puck just makes it to the far end?
Open Question
(II) A child on a sled reaches the bottom of a hill with a velocity of 10.0 m/s and travels 25.0 m along a horizontal straightaway to a stop. If the child and sled together have a mass of 60.0 kg, what is the average retarding force on the sled on the horizontal straightaway?
Open Question
When jumping straight up from a crouched position, an average person can reach a maximum height of about 60 cm. During the jump, the person's body from the knees up typically rises a distance of around 50 cm. To keep the calculations simple and yet get a reasonable result, assume that the entire body rises this much during the jump. (a) With what initial speed does the person leave the ground to reach a height of 60 cm?
Open Question
FIGURE EX6.10 shows the force acting on a 2.0 kg object as it moves along the x-axis. The object is at rest at the origin at t = 0s. What are its acceleration and velocity at t = 6 s?
Open Question
The forces in FIGURE EX6.9 act on a 2.0 kg object. What are the values of ax and ay, the x- and y-components of the object's acceleration?
Open Question
Compressed air is used to fire a 50 g ball vertically upward from a 1.0-m-tall tube. The air exerts an upward force of 2.0 N on the ball as long as it is in the tube. How high does the ball go above the top of the tube? Neglect air resistance.
Open Question
A 20,000 kg rocket has a rocket motor that generates 3.0 x 10^5 N of thrust. Assume no air resistance. (a) What is the rocket's initial upward acceleration?
Open Question
A horizontal rope is tied to a 50 kg box on frictionless ice. What is the tension in the rope if: (b) The box moves at a steady 5.0 m/s?
Open Question
a. A rocket of mass m is launched straight up with thrust Fₜₕᵣᵤₛₜ. Find an expression for the rocket's speed at height h if air resistance is neglected.
Open Question
Your forehead can withstand a force of about 6.0 kN before fracturing, while your cheekbone can withstand only about 1.3kN. Suppose a 140 g baseball traveling at 30 m/s strikes your head and stops in 1.5 ms.a.What is the magnitude of the force that stops the baseball?
Open Question
A 500 g model rocket is on a cart that is rolling to the right at a speed of . The rocket engine, when it is fired, exerts an 8.0 N vertical thrust on the rocket. Your goal is to have the rocket pass through a small horizontal hoop that is 20 m above the ground. At what horizontal distance left of the hoop should you launch?
Open Question
As a science fair project, you want to launch an 800 g model rocket straight up and hit a horizontally moving target as it passes 30 m above the launch point. The rocket engine provides a constant thrust of 15.0 N. The target is approaching at a speed of 15 m/s. At what horizontal distance between the target and the rocket should you launch?
Open Question
Two rock climbers, Paul and Jeanne, use safety ropes of similar length. Jeanne's rope is more elastic, called a dynamic rope by climbers. Paul has a static rope, not recommended for safety reasons.
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(a) Jeanne (Fig. 4–72) falls freely about 2.0 m and then the rope stops her over a distance of 1.0 m. Estimate how large a force (assume constant) she will feel from the rope. (Express the result in multiples of her weight.)