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physics
mechanics
Questions and Answers of
Mechanics
A person driving her car at 35km / h approaches an intersection just as the traffic light turns yellow. She knows that the yellow light lasts only 2.0 s before turning to red, and she is 28 m away
A car is behind a truck going 18 m / s on the highway. The car's driver looks for an opportunity to pass, guessing that his car can accelerate 0.60 m/s2 at and that he has to cover the 20-m length of
Agent Bond is standing on a bridge, 15m above the road below, and his pursuers are getting too close for comfort. He spots a flatbed truck approaching at 25m/s, which he measures by knowing that the
A conveyor belt is used to send burgers through a grilling machine. If the grilling machine is 1.2 m long and the burgers require 2.8 min to cook, how fast must the conveyor belt travel? If the
A horse trots away from its trainer in a straight line, moving 38 m away in 9.0 s. It then turns abruptly and gallops halfway back in 1.8 s. Calculate(a) Its average speed(b) Its average velocity for
Two children are playing on two trampolines. The first child bounces up one-and-a-half times higher than the second child. The initial speed up of the second child is 4.0 m/s.(a) Find the maximum
If there were no air resistance, how long would it take a free-falling skydiver to fall from a plane at 3200 m to an altitude of 450 m, where she will open her parachute? What would her speed be at
You stand at the top of a cliff while your friend stands on the ground below you. You drop a ball from rest and see that she catches it 1.4 s later. Your friend then throws the ball up to you, such
On an audio compact disc (CD), digital bits of information are encoded sequentially along a spiral path. Each bit occupies about 0.28 μm. A CD player's readout laser scans along the spiral's
A person jogs eight complete laps around a 400-m track in a total time of 14.5 min. Calculate(a) The average speed(b) The average velocity, in m/s.
You are on the street trying to hit a friend in his dorm window with a water balloon. He has a similar idea and is aiming at you with his water balloon. You aim straight at each other and throw at
A projectile has the least speed at what point in its path?
A car is driven 225 km west and then 98 km southwest (45°). What is the displacement of the car from the point of origin (magnitude and direction)? Draw a diagram.
For the vectors given in Fig. 3-35, determine(a) A - B + C,(b) A + B - C,(c) C - A - B,
A tiger leaps horizontally from a 7.5-m-high rock with a speed of 3.0 m / s. How far from the base of the rock will she land?
Estimate by what factor a person can jump farther on the Moon as compared to the Earth if the takeoff speed and angle are the same. The acceleration due to gravity on the Moon is one-sixth what it is
A delivery truck travels 21 blocks north, 16 blocks east, and 26 blocks south. What is its final displacement from the origin? Assume the blocks are equal length.
A ball is thrown horizontally from the roof of a building 7.5 m tall and lands 9.5 m from the base. What was the ball's initial speed?
A ball thrown horizontally at 12.2 m/s from the roof of a building lands 21.0 m from the base of the building. How high is the building?
A diver running 2.5 m / s dives out horizontally from the edge of a vertical cliff and 3.0 s later reaches the water below. How high was the cliff and how far from its base did the diver hit the
A fire hose held near the ground shoots water at a speed of 6.5 m / s. At what angle(s) should the nozzle point in order that the water land 2.5 m away (Fig. 3-36)? Why are there two different
A grasshopper hops along a level road. On each hop, the grasshopper launches itself at angle θ0 = 45ο and achieves a range R = 0.80 m. What is the average horizontal speed of the grasshopper as it
Extreme-sports enthusiasts have been known to jump off the top of El Capitan, a sheer granite cliff of height 910 m in Yosemite National Park. Assume a jumper runs horizontally off the top of El
A projectile is fired with an initial speed of 36.6 m / s at an angle of 42.2° above the horizontal on a long flat firing range. Determine(a) The maximum height reached by the projectile,(b) The
An athlete performing a long jump leaves the ground at a 27.0° angle and lands 7.80 m away.(a) What was the takeoff speed?(b) If this speed were increased by just 5.0%, how much longer would the
If Vx = 9.80 units and Vy = -6.40 units, determine the magnitude and direction of V.
Suppose the rescue plane of Problem 31 releases the supplies a horizontal distance of 425 m in advance of the mountain climbers. What vertical velocity (up or down) should the supplies be given so
Suppose the kick in Example 3-6 is attempted 36.0m from the goalposts, whose crossbar is 3.05 m above the ground. If the football is directed perfectly between the goalposts, will it pass over the
A stunt driver wants to make his car jump over 8 cars parked side by side below a horizontal ramp (Fig. 3-41).(a) With what minimum speed must he drive off the horizontal ramp? The vertical height of
Huck Finn walks at a speed of 0.70 m / s across his raft (that is, he walks perpendicular to the raft's motion relative to the shore). The heavy raft is traveling down the Mississippi River at a
Graphically determine the resultant of the following three vector displacements:(1) 24 m, 36° north of east;(2) 18 m, 37° east of north; and(3) 26 m, 33° west of south.
Two planes approach each other head-on. Each has a speed of 780 km / h, and they spot each other when they are initially 10.0 km apart. How much time do the pilots have to take evasive action?
A passenger on a boat moving at 1.70 m / s on a still lake walks up a flight of stairs at a speed of 0.60 m / s Fig. 3-43. The stairs are angled at 45° pointing in the direction of motion as
A person in the passenger basket of a hot-air balloon throws a ball horizontally outward from the basket with speed 10.0 m / s (Fig. 3-44). What initial velocity (magnitude and direction) does the
An airplane is heading due south at a speed of 688 km / h. If a wind begins blowing from the southwest at a speed of90.0 km / h (average), calculate (a) The velocity (magnitude and direction) of the
In what direction should the pilot aim the plane in Problem 44 so that it will fly due south? Problem 44 An airplane is heading due south at a speed of 688 km / h. If a wind begins blowing from the
A swimmer is capable of swimming 0.60 m / s in still water. (a) If she aims her body directly across a 45-m-wide river whose current is 0.50 m / s, how far downstream (from a point opposite her
A boat, whose speed in still water is 2.50 m / s, must cross a 285-m-wide river and arrive at a point 118m upstream from where it starts (Fig. 3-45). To do so, the pilot must head the boat at a
A child, who is 45 m from the bank of a river, is being carried helplessly downstream by the river's swift current of 1.0 m / s. As the child passes a lifeguard on the river's bank, the lifeguard
V̅ is a vector 24.8 units in magnitude and points at an angle of 23.4° above the negative x axis.(a) Sketch this vector.(b) Calculate Vx and Vy.(c) Use Vx and to obtain (again) the magnitude and
Two cars approach a street corner at right angles to each other (Fig. 3-47). Car 1 travels at a speed relative to Earth ν1E = 35 km / h, and car 2 at ν2E = 55 km / h. What
On mountainous downhill roads, escape routes are sometimes placed to the side of the road for trucks whose brakes might fail. Assuming a constant upward slope of 26°, calculate the horizontal and
A light plane is headed due south with a speed relative to still air of 185 km / h. After 1.00 h, the pilot notices that they have covered only 135 km and their direction is not south but 15.0° east
Romeo is throwing pebbles gently up to Juliet's window, and he wants the pebbles to hit the window with only a horizontal component of velocity. He is standing at the edge of a rose garden 8.0 m
Apollo astronauts took a "nine iron" to the Moon and hit a golf ball about 180 m. Assuming that the swing, launch angle, and so on, were the same as on Earth where the same astronaut could hit it
(a) A long jumper leaves the ground at 45° above the horizontal and lands 8.0 m away. What is her "takeoff" speed v0?(b) Now she is out on a hike and comes to the left bank of a river. There is no
A projectile is shot from the edge of a cliff 115 m above ground level with an initial speed of 65.0 m / s at an angle of 35.0° with the horizontal, as shown in Fig. 3-51.(a) Determine the time
Vector V1 is 6.6 units long and points along the negative x axis. Vector V2 is 8.5 units long and points at to the positive x axis.(a) What are the x and y components of each vector?(b) Determine the
A hunter aims directly at a target (on the same level) 38.0 m away.(a) If the arrow leaves the bow at a speed of 23.1 m / s, by how much will it miss the target?(b) At what angle should the bow be
Figure 3-33 shows two vectors, A and B, whose magnitudes are A = 6.8 units and B = 5.5 units. Determine C if(a) C = A + B,(b) C = A - B,(c) C = B - A. Give the magnitude and direction for each.
An airplane is traveling 835 km / b in a direction 41.5° west of north (Fig. 3-34).(a) Find the components of the velocity vector in the northerly and westerly directions.(b) How far north and how
Why is the stopping distance of a truck much shorter than for a train going the same speed?
If an object is moving, is it possible for the net force acting on it to be zero? Explain.
(a) Why do you push down harder on the pedals of a bicycle when first starting out than when moving at constant speed? (b) Why do you need to pedal at all when cycling at constant speed?
1. A truck is traveling horizontally to the right (Fig. 4-38).When the truck starts to slow down; the crate on the (frictionless) truck bed starts to slide. In what direction could the net force be
1. You are pushing a heavy box across a rough floor. When you are initially pushing the box and it is accelerating, (a) You exert a force on the box, but the box does not exert a force on you. (b)
1. The normal force on an extreme skier descending a very steep slope (Fig. 4-42) can be zero if(a) His speed is great enough. (b) He leaves the slope (no longer touches the snow). (c) The slope is
You are trying to push your stalled car. Although you apply a horizontal force of 400 N to the car, it doesn't budge, and neither do you. Which force(s) must also have a magnitude of 400 N? (a) The
Matt, in the foreground of Fig. 4-39, is able to move the large truck because(a) He is stronger than the truck. (b) He is heavier in some respects than the truck. (c) He exerts a greater force on the
1. What causes the boat in Fig. 4-41 to move forward?(a) The force the man exerts on the paddle. (b) The force the paddle exerts on the water. (c) The force the water exerts on the paddle. (d) The
1. When a skier skis down a hill, the normal force exerted on the skier by the hill is (a) Equal to the weight of the skier. (b) Greater than the weight of the skier. (c) Less than the weight of the
How much tension must a cable withstand if it is used to accelerate a 1200-kg car vertically upward at 0.70 m/s2?
A 14.0-kg bucket is lowered vertically by a rope in which there is 163 N of tension at a given instant. What is the acceleration of the bucket? Is it up or down?
Can cars "stop on a dime"? Calculate the acceleration of a 1400-kg car if it can stop from 35 km/h on a dime (diameter = 1.7cm). How many g's is this? What is the force felt by the 68-kg occupant of
A person jumps from the roof of a house 2.8 m high. When he strikes the ground below, he bends his knees so that his torso decelerates over an approximate distance of 0.70 m. If the mass of his torso
What is the weight of a 68-kg astronaut?(a) On Earth,(b) On the Moon (g = 1.7 m/s2),(c) On Mars (g = 3.7 m/s2),(d) In outer space traveling with constant velocity?
A window washer pulls herself upward using the bucket-pulley apparatus shown in Fig. 4-48.(a) How hard must she pull downward to raise herself slowly at constant speed?(b) If she increases this force
The two forces FÌ…1 and FÌ…2 shown in Fig. 4-52a and b (looking down) act on an 18.5-kg object on a frictionless tabletop. If FÌ…1 = 10.2 N FÌ…2 and FÌ…2 = 16.0N, find the net force on
(a) If mA = 13.0kg and mB = 5.0 kg in Fig. 4-53, determine the acceleration of each block.(b) If initially mA is at rest 1.250 m from the edge of the table, how long does it take to reach the edge of
Three blocks on a frictionless horizontal surface are in contact with each other as shown in Fig. 4-54. A force F is applied to block A (mass mA).(a) Draw a free-body diagram for each block.
Suppose the pulley in Fig. 4-55 is suspended by a cord C. Determine the tension in this cord after the masses are released and before one hits the ground. Ignore the mass of the pulley and cords.
If the coefficient of kinetic friction between a 22-kg crate and the floor is 0.30, what horizontal force is required to move the crate at a steady speed across the floor? What horizontal force is
A force of 35.0 N is required to start a 6.0-kg box moving across a horizontal concrete floor.(a) What is the coefficient of static friction between the box and the floor?(b) If the 35.0-N force
The coefficient of static friction between hard rubber and normal street pavement is about 0.90. On how steep a hill (maximum angle) can you leave a car parked?
A 2.0-kg silverware drawer does not slide readily. The owner gradually pulls with more and more force, and when the applied force reaches 9.0 N, the drawer suddenly opens, throwing all the utensils
A box is given a push so that it slides across the floor. How far will it go, given that the coefficient of kinetic friction is 0.15 and the push imparts an initial speed of 3.5 m/s?
A 1280-kg car pulls a 350-kg trailer. The car exerts a horizontal force of 3.6 × 103 N against the ground in order to accelerate. What force does the car exert on the trailer?Assume an effective
In Fig. 4-56 the coefficient of static friction between mass mA and the table is 0.40, whereas the coefficient of kinetic friction is 0.20.(a) What minimum value of mA will keep the system from
A small box is held in place against a rough vertical wall by someone pushing on it with a force directed upward at 28° above the horizontal. The coefficients of static and kinetic friction between
Two crates, of mass 65 kg and 125 kg, are in contact and at rest on a horizontal surface (Fig. 4-57). A 650-N force is exerted on the 65-kg crate. If the coefficient of kinetic friction is 0.18,
Superman must stop 120-km/h a train in 150 m to keep it from hitting a stalled car on the tracks. If the train's mass is 3.6 × 105 kg, how much force must he exert? Compare to the weight of the
A wet bar of soap slides down a ramp 9.0 m long inclined at 8.0°. How long does it take to reach the bottom? Assume µk = 0.060.
A skateboarder, with an initial speed of 2.0 m/s rolls virtually friction free down a straight incline of length 18 m in 3.3 s. At what angle is the incline oriented above the horizontal?
Uphill escape ramps are sometimes provided to the side of steep downhill highways for trucks with overheated brakes. For a simple 11° upward ramp, what minimum length would be needed for a runaway
A block is given an initial speed 4.5 m/s of up the 22.0° plane shown in Fig. 4-59.(a) How far up the plane will it go?(b) How much time elapses before it returns to its starting point? Ignore
The crate shown in Fig. 4-60 lies on a plane tilted at an angle θ = 25.0° to the horizontal, with µk = 0.19.(a) Determine the acceleration of the crate as it slides down the
A person has a reasonable chance of surviving an automobile crash if the deceleration is no more than 30 g's. Calculate the force on a 65-kg person accelerating at this rate. What distance is
A crate is given an initial speed of 3.0m/s up the 25.0° plane shown in Fig. 4-60.(a) How far up the plane will it go?(b) How much time elapses before it returns to its starting point? Assume
A car can decelerate at -3.80 m/s2 without skidding when coming to rest on a level road. What would its deceleration be if the road is inclined at 9.3° and the car moves uphill? Assume the same
A skier moves down a 12° slope at constant speed. What can you say about the coefficient of friction, µk? Assume the speed is low enough that air resistance can be ignored.
A child slides down a slide with a 34° incline, and at the bottom her speed is precisely half what it would have been if the slide had been frictionless. Calculate the coefficient of kinetic
What average force is required to stop a 950-kg car in 8.0 s if the car is traveling at 95 km/h?
A 7180-kg helicopter accelerates upward at 0.80m/s2 while lifting a 1080-kg frame at a construction site, Fig. 4-68.(a)What is the lift force exerted by the air on the helicopter rotors?(b) What is
Estimate the average force exerted by a shot-putter on a 7.0-kg shot if the shot is moved through a distance of 2.8 m and is released with a speed of 13 m/s.
Three mountain climbers who are roped together in a line are ascending an ice field inclined at 31.0° to the horizontal (Fig. 4-69). The last climber slips, pulling the second climber off his
A motorcyclist is coasting with the engine off at a steady speed of 20.0 m/s but enters a sandy stretch where the coefficient of kinetic friction is 0.70.Will the cyclist emerge from the sandy
The 70.0-kg climber in Fig. 4-72 is supported in the "chimney" by the friction forces exerted on his shoes and back. The static coefficients of friction between his shoes and the wall, and between
A 28.0-kg block is connected to an empty 2.00-kg bucket by a cord running over a frictionless pulley (Fig. 4-73). The coefficient of static friction between the table and the block is 0.45 and the
A 72-kg water skier is being accelerated by a ski boat on a flat ("glassy") lake. The coefficient of kinetic friction between the skier's skis and the water surface is µk = 0.25 (Fig. 4-74).(a)
Two blocks made of different materials, connected by a thin cord, slide down a plane ramp inclined at an angle to the horizontal, Fig. 4-76 (block B is above block A). The masses of the blocks are mA
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