Solve Perplexing Physics Problems with These Helpful Tips and Examples"

[Pseudo Statistic]

Hi.
I'm having problems with some Physics problems; among them:

9. At the top of a cliff a 100 m high, a rock is thrown upward with velocity 15 m/s. How much later should a second rock be dropped from rest so both stones arrive simultaneously at the bottom of the cliff?

a. 5.05 s

b. 3.76 s

c. 2.67 s

d. 1.78 s

2. A 50-gram superball traveling at 25 m/s is bounced off a brick wall and rebounds at 22 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.5 ms, what is the average acceleration of the ball during this time interval?

a. 13,428 m/s2

b. 6,715 m/s2

c. 857 m/s2

d. 20 m/s2

For 9), I thought I'd use v = u + at and find 2t after solving:
0 = 15 - 9.8t for t, and that would give me the answer... but I'm wrong.
Anyone know how I can go about this one?
And for 2), I really don't know what to do...

Thanks for any help.

 

[StatusX]

For the first one, your answer was wrong because the first rock will have a downward speed of 15 m/s when it passes you again, and the other rock is dropped from rest. You need to drop the second rock before this point, and the only way to figure out when is to find the total time it will take each rock to hit the ground.

For the second, acceleration is the change in velocity over time. This problem is easier then it seems.

 

[quicknote]

I would suggest approaching these problems by first identifying the known variables and then using relevant equations to solve for the unknowns. For problem 9), the known variables are the initial velocity (15 m/s), the acceleration due to gravity (9.8 m/s^2), and the height (100 m). The unknown variable is the time (t) it takes for the first rock to reach the bottom of the cliff. Using the equation v = u + at, we can solve for t by rearranging the equation to t = (v-u)/a. Plugging in the known values, we get t = (0-15)/(-9.8) = 1.53 s. Since we want both rocks to arrive at the bottom at the same time, the second rock should be dropped 1.53 seconds after the first rock is thrown, leading to a total time of 3.06 s. This is closest to option d) 1.78 s.

For problem 2), the known variables are the mass of the ball (50 g), the initial velocity (25 m/s), the final velocity (22 m/s), and the time in contact with the wall (3.5 ms). The unknown variable is the acceleration (a) of the ball during this time interval. Using the equation a = (v-u)/t, we can solve for a by plugging in the known values: a = (22-25)/(3.5/1000) = -857 m/s^2. This matches option c).

In general, when solving physics problems, it is important to carefully identify the known and unknown variables and use relevant equations to solve for the unknowns. It may also be helpful to draw diagrams or make tables to organize the information and make the problem more visually understandable. Additionally, double-checking the units and making sure they are consistent throughout the problem can help catch any errors. With practice and careful attention to detail, you can become more confident in solving perplexing physics problems.