BIG Physics Homework help... 1. A car is traveling at a steady 80 km/h in a 50 k
ID: 1730781 • Letter: B
Question
BIG Physics Homework help...
1. A car is traveling at a steady 80 km/h in a 50 km/h zone. A police motorcycle takes off at the instant the car passes it, accelerating at a steady 8.0 m/s* a. How much time elapses before the motorcycle is moving as fast as the car? b. How far is the motorcycle from the car when it reaches this speed? Hint: Use kinematic equations for constant acceleration. Call the point where the motorcycle started the origin 2. A man pulling an empty wagon causes it to accelerate at 1.4 m/s'. What will the acceleration be if he pulls with the same force when the wagon contains a child whose mass is three times that of the wagon? Hint: We can use Newton's second law to find the acceleration of the wagon with the child 3. Riders on the Tower of Doom, an amusement park ride, experience 2.0 s of free fall, after which they are slowed to a stop in 0.50 s. What is a 65 kg rider's apparent weight as the ride is coming to rest? By what factor does this exceed her actual weight? Hint: Use the kinematic equations twice. The first time find out the velocity of the rider at the end of the 2.0 s and then use that as the initial velocity during the second part to compute the acceleration 4. In an old-fashioned amusement park ride, passengers stand inside a 3.0-m-tall, 5.0-m- diameter hollow steel cylinder with their backs against the wall. The cylinder begins to rotate about a vertical axis. Then the floor on which the passengers are standing suddenly drops away! If all goes well, the passengers will "stick" to the wall and not slide. Clothing has a static coefficient of friction against steel in the range 0.60 to 1.0 and a kinetic coefficient in the range 0.40 to 0.70. What is the minimum rotational frequency, in rpm, for which the ride is safe? Hint: Consider the passenger to be a particle in circular motion. 5. The cable lifting an elevator is wrapped around a 1.0-m-diameter cylinder that is turned by the elevator's motor. The elevator is moving upward at a speed of1. 6 m/s. It then slows to a stop as the cylinder makes one complete turn at constant angular acceleration How long does it take for the elevator to stop? Hint: This problem requires a knowledge of one-dimensional kinematics and the fact that as the cylinder goes around once, the elevator will advance an amount equal to the diameter of the cylinder. A 25 kg child bounces on a pogo stick. The pogo stick has a spring with spring constant 2.0 x 104 N/m. When the child makes a nice big bounce, she finds that at the bottom of the bounce she is accelerating upward at 9.8 m/s2. How much is the spring compressed? Hint: Assume the spring obeys Hooke's law Dan is gliding on his skateboard at 4.0 m/s. He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.0 m/s. How fast is Dan going as his feet hit the ground? Dan's mass is 50 kg and the skateboard's mass is 5.0 kg. Hint: We will define our system to be Dan plus skateboard 6. 7.Explanation / Answer
(1)
(a) initial velocity of motorcycle(u) = 0
acceleration of motorcyce = 8 m/s2
Final velocity of motorcycle = 80 km/h = 22.22 m/s
Now using the kinematic equation
v = u + at
22.22 = 0 +8t
t = 2.78 s
Hence it would take 2.8 s to reach the speed of car.
(b) Now again using the kinematic equation
v2 = u2 + 2aS
22.222 = 0 +2*8*S
S = 30.858 m
Hence it would cover 30.858 m to reach the above speed.
(2) Let us consider the mass of the wagon = m
Amounr of force applied = F
We know that
F = ma
we know that a = 1.4 m/s2 , therefore
F =m*1.4 -----------(1)
Now the mass of child = 3m
Hence the total mass of the wagon +child = 3m+m = 4m
F = (4m)*a2 -----------(2)
where a2 is the new acceleration
Since the force is same in both cases therefore equating the above equation
m*1.4 = 4m*a2
a2 = 0.35 m/s2
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