# Which Brick Lands with the Highest Speed and Other Physics Enigmas?

• lollol
In summary: Therefore, the answer to the question is yes, the total mechanical energy can be negative if the potential energy is greater than the kinetic energy. In summary, potential energy can be negative and can affect the total mechanical energy, which can also be negative. The harmful factor in a car crash depends on the acceleration, which is affected by the collision type.
lollol
1) Three bricks are thrown from the roof of a building with the same speed but with different launch angles: one above the horizontal, one below the horizontal and one horizontal. Assuming no air resistance, which brick has the highest speed by the time it reaches the ground?

My answer: Simple conservation of energy... mgh = 0.5mv^2. They all have the same velocity... is this correct? Just wanted to confirm

2) If 2 cars crash, which would you expect to be more harmful to the passengers: if the cars crash and stay together, or if the cars collide and rebound backward? Explain.

My answer: Honestly, I am not sure at all. Momentum is conserved in both, thus impulse is 0. Only thing is... energy is lost in a sticky collision, so I'm going with an inelastic collision. However, I cannot explain it.

3) Can the total mechanical energy be negative? E = KE + PE
Explain + provide an example.

My answer: I always learned that energy is positive. But I'm thinking this can depend on the point of reference. If my reference point is 1000m above a cliff, the potential energy of a rock on the cliff is negative and has no kinetic energy... so E is -ive.

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For 2) what's harmful to the passengers is the acceleration. Assuming both collisions happen in the same time t at the same velocity v, then in one case they decelerate from v to 0 in time t. In the other they decelerate from v to 0 and then up to the rebound velocity in the same time. In which case is the magnitude of the acceleration larger? 1) looks fine. And yes for 3). KE is positive but PE depends on a choice of origin and can certainly be negative.

Dick said:
For 2) what's harmful to the passengers is the acceleration. Assuming both collisions happen in the same time t at the same velocity v, then in one case they decelerate from v to 0 in time t. In the other they decelerate from v to 0 and then up to the rebound velocity in the same time. In which case is the magnitude of the acceleration larger? 1) looks fine. And yes for 3). KE is positive but PE depends on a choice of origin and can certainly be negative.

So in fact, the total mechanical energy can be negative then... it all depends on the magnitude of the values?

lollol said:
So in fact, the total mechanical energy can be negative then... it all depends on the magnitude of the values?

Sure. PE can be negative, so if it's magnitude is greater than KE, the total is negative.

## 1. What is a collision?

A collision is an event in which two or more objects come into contact with each other and transfer energy.

## 2. What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is calculated by the formula KE = 1/2 * mass * velocity^2.

## 3. How is energy conserved in a collision?

In a collision, the total energy before and after the collision remains the same. This is known as the law of conservation of energy.

## 4. What is an elastic collision?

An elastic collision is a collision in which the total kinetic energy of the objects remains the same before and after the collision. This means that no energy is lost or converted into other forms of energy.

## 5. How does the mass and velocity of objects affect the energy in a collision?

The kinetic energy of an object is directly proportional to its mass and the square of its velocity. This means that an object with a larger mass or a higher velocity will have more energy in a collision.

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