I'm kinda new here, can someone help me with physics?

Question

1.What does a 138N rock weigh if it is accelerating a)upward at 12 m/s^2 b) downward at 3.5m/s^2?

2. Three identical 6.40 kg masses are hung by three identical springs. Each spring has a force constant of 7.80kN/m and was 12 cm long before any masses were attached to it. a)how long is each spring when hanging?

3.a 0.145 kg baseball leaves a pitcher's hand at a speed of 32m/s. if air drag is negligible, how much work has the pitcher done on the ball by throwing it?

4. A 25kg child plays on a swing having support ropes that are 2.20 m long. A friend pulls her back until the ropes are 42 degrees from the vertical and releases her from rest a)what is the potential energy just as she is released, compared to the PE at the bottom of the swing? How fast will she be moving at the bottom of the swing? c) how much work does the tension in the ropes do as the child swings from the initial position to the bottom?

I'm a chem major, I kinda hate physics so I will be coming here for help from time to time.

thanks in advance :)

Answer 1

1.What does a 138N rock weigh if it is accelerating

a)upward at 12 m/s^2

138N rock has a mass of 138/9.8 = 14.1 kg

F=ma

f = 14.1 kg x 12 m/s² = 169 N

upwards, so the forces add, and net is 138+169 = 307 N

b) downward at 3.5m/s^2?

f = 14.1 kg x 3.5 m/s² = 49 N

upwards, so the forces subtract, and net is 138 - 49 = 89 N

3.a 0.145 kg baseball leaves a pitcher's hand at a speed of 32m/s. if air drag is negligible, how much work has the pitcher done on the ball by throwing it?

Work = energy imparted to the ball in the form of kinetic energy

KE = ½mV²

KE = ½(0.145)(32)² = 74.2 Joules

.

Answer 2

1.What does a 138N rock weigh if it is accelerating a)upward at 12 m/s^2 b) downward at 3.5m/s^2?

This is kind of a trick question; the weight of the rock depends only on the force of gravity on it, NOT the speed and direction in which it is traveling.  In both cases, the rock weighs 138 N.

Also, keep in mind that a kilogram is NOT a measure of weight; it is a measure of inertia (or mass).

2. Three identical 6.40 kg masses are hung by three identical springs. Each spring has a force constant of 7.80kN/m and was 12 cm long before any masses were attached to it. a)how long is each spring when hanging?

6.40 kg mass weighs 64 Newtons, or 0.064 kN.

(0.064 kN) / (7.8 kN /m ) = x

100*x+12 will give you the length of the spring (be wary of units... need to convert x into centimeters)

3.a 0.145 kg baseball leaves a pitcher's hand at a speed of 32m/s. if air drag is negligible, how much work has the pitcher done on the ball by throwing it?

Work = change in kinetic energy.

KE = 1/2 mv^2 = 1/2 (0.145 kg)(32 m/2)^2 joules.   sorry, i don't have a calculator handy :-P

4. A 25kg child plays on a swing having support ropes that are 2.20 m long. A friend pulls her back until the ropes are 42 degrees from the vertical and releases her from rest a)what is the potential energy just as she is released, compared to the PE at the bottom of the swing? How fast will she be moving at the bottom of the swing? c) how much work does the tension in the ropes do as the child swings from the initial position to the bottom?

a)

potential energy = mgh = (25 kg)(10 m/s^2)(2.20 * cos(42 degrees))

you know that her potential energy will all be converted to kinetic energy at the bottom of the swing, so set the above value equal to 1/2mv^2 and solve for v.