A. You are standing on the back of a pickup truck that is at rest, you are throwing banana forwards with a velocity of 10 m/s.

Question

A. You are standing on the back of a pickup truck that is at rest, you are throwing banana forwards with a velocity of 10 m/s.

1. (Refer to situation A for your answer) If your friend is standing on the sidewalk, how fast would he say the bananas are moving? Are you in the same frame of reference?

B. Now the truck starts to move forward at a velocity of 20 m/s. You are still throwing bananas forward with the velocity of 10 m/s.

2. (Refer to situation B for your answer) If you are not paying attention to what’s going on around you, how fast would you say the bananas are moving?

3. How fast does your friend on the sidewalk say the bananas are moving?

C. Now you turn around and start throwing the bananas (10 m/s) from the rear of the truck. (For items 4 to 7 refer to the situation C)

4. If you are ask, how fast the banana is moving?

5. How fast does your friend on the sidewalk say the banana is moving?

6. Do you think you and your friend would have the same answers?

7. From the given situations above, how will you define inertial frame of reference?

Answer 1

Hey Anna,

Rather than answer all these questions individually, I'll say this:

1. If you throw something from a moving object, to you, to you, it will appear to be just a normal throw, just like if you threw the object while standing still. HOWEVER, to someone who isn't moving with you, the velocity of the object will depend on the speed of your motion when you threw the object. The velocities will add together (remember that velocities can be positive or negative).
2. If you are moving at 5m/s and throw something at 5m/s forward, people on the ground will observe that object to be going 10 m/s
3. In the same situation, if you threw it 5m/s BACKWARDS, people on the ground will see basically a motionless banana (in the horizontal direction at least. Vertically, it will just be falling straight to the ground.)
4. To show your work, it helps to call one direction positive and the other direction negative. Then write your given velocities as either positive or negative numbers before adding them together.

I guess the gist is that all motion is relative to the observer. Typically, we speak in terms of standing still on the Earth being "motionless" and everything else is relative to that. But really, we're whipping around space pretty fast and Earth is spinning pretty fast too, so there's no such thing as motionless.