# 5 Physics Question A toy plane with a mass of 0.97 kg is tied to a string and made to travel at a speed, physics homework help

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*timer*Asked: Jun 27th, 2017

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### Question Description

1.A toy plane with a mass of 0.97 kg is tied to a string and made to travel at a speed of 21 m/s in a horizontal circle with a 18-m radius. The person holding the string pulls the plane in, increasing the tension in the string, increasing the speed of the plane and decreasing the radius of the plane's orbit. What is the net work done on the plane if the tension in the string increases by a factor of four and the radius decreases to 11 m.

J

2.A pendulum bob with a mass of 0.35 kg is attached to a 1.5 m long string as shown. As the pendulum bob swings from point A, where the angle *θ* = 35°, to point B at the bottom of its arc, determine the change in the gravitational potential energy of the pendulum bob-earth system.

J

3.In a downhill ski race, surprisingly, little advantage is gained by getting a running start. (This is because the initial kinetic energy is small compared with the gain in gravitational potential energy even on small hills.) To demonstrate this, find the final speed in m/s and the time taken in seconds for a skier who skies 62.0 m along a 25° slope neglecting friction for the following two cases. (Enter the final speeds to at least one decimal place.)

(a)

starting from rest

final speed m/stime taken s

(b)

starting with an initial speed of 3.00 m/s

final speed m/stime taken s

(c)

Does the answer surprise you? Discuss why it is still advantageous to get a running start in very competitive events.

4.As shown in the figure below, a skateboarder starts at point A on the ramp and rises to point B, a maximum height of *h* = 2.45 m

m/s

5.A 400 kg dragster accelerates from rest to a final speed of 120 m/s in 400 m (about a quarter of a mile) and encounters an average frictional force of 1200 N. What is its average power output in watts and horsepower if this takes 7.30 s?

## Tutor Answer

Hi, for question 2 I used my own figure that I believe is similar to what you have. I couldn't solve number 4 without the figure. Find attached the answers.

PHYSICS FIVE QUESTIONS

1. A toy plane with a mass of 0.97 kg is tied to a string and made to travel at a speed

of 21 m/s in a horizontal circle with a 18-m radius. The person holding the string

pulls the plane in, increasing the tension in the string, increasing the speed of the

plane and decreasing the radius of the plane's orbit. What is the net work done on

the plane if the tension in the string increases by a factor of four and the radius

decreases to 11 m?

𝑚 = 0.97 𝑘𝑔

𝑣=

21𝑚

𝑠

𝑟 = 18𝑚

The tension in the string is given caused by the centripetal force, 𝐹𝑐

𝑣2

𝐹𝑐 = 𝑚

𝑟

By reducing 𝑟 we increase the tension in the string.

The only force that is responsible for doing work if 𝐹𝑐 since the motion is in a horizontal

circle.

𝐹𝑐 = 0.97 ∗

212

= 23.765 𝑁

18

The tension increases by a factor of four, therefore,

𝐹𝑛𝑒𝑤 = 23.765 ∗ 4 = 95.06 𝑁

𝑟𝑛𝑒𝑤 = 11 𝑚

Work if ...

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