# Basic Momentum and Energy and time

• Ronaldo21
In summary: How do you find that? What unit's are you using?In summary, the conversation discusses a 5.36 g bullet with a velocity of 681.8 m/s that penetrates a tree trunk to a depth of 3.79 cm. Using work and energy considerations, the magnitude of the force that stops the bullet is found to be 32870.74731 N. In determining the time it takes for the bullet to stop, the incorrect equation vf^2=vi^2+2at is suggested, and it is corrected to v2=u2+2as. It is then suggested to use F=ma to find the acceleration and use a=(change in v)/time.
Ronaldo21
Basic Momentum and Energy! and time!

1. A 5.36 g bullet moving at 681.8 m/s penetrates a tree trunk to a depth of 3.79 cm.
a) Use work and energy considerations to find the magnitude of the force that stops the

for this i did 1/2mv^2=w=fd
so 1/2(.00536)(681.8)^2=w=fd
1245.801323=fd
1245.801323=f(.0379)
and i solved for f and got 32870.74731

2. (part 2 of 2) b) Assuming that the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment the bullets stops moving.

for this one i just did vf^2=vi^2+2at. is that correct??

THANK YOU!

Hi Ronaldo21!

(try using the X2 and X2 tags just above the Reply box )
Ronaldo21 said:
… for this one i just did vf^2=vi^2+2at. is that correct??

No! That is not one of the standard constant acceleration equations.

(You're thinking of v2 = u2 + 2as)

Try again!

hmm. i don't get it :(

any help? I am just so confused!

You got the first one right. tiny tim was just pointing out that vf^2=vi^2+2at is a wrong formula. Why don't you use F=ma to find the acceleration and use a=(change in v)/time?

that'll just give me 32,870.74672 as an answer. it looks big.

It is big if it's in seconds. That's like about 9 hours. In fact, that's the same numerical answer you gave for the force. Why don't you show your intermediate steps and put units on your numbers so I've got some clue what you are talking about? You've already got that the force is 32871N. The mass is 5.36g. What's the acceleration?

Last edited:

## What is momentum?

Momentum is a physical quantity that measures an object's tendency to keep moving in the same direction at the same speed. It is calculated by multiplying an object's mass by its velocity.

## What is the law of conservation of momentum?

The law of conservation of momentum states that the total momentum of a closed system (one with no external forces acting on it) remains constant over time. This means that the total momentum before an event must equal the total momentum after the event.

## What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is calculated by multiplying an object's mass by the square of its velocity, divided by two.

## What is potential energy?

Potential energy is the energy an object has due to its position or state. It can be stored energy, such as in a compressed spring, or energy due to an object's position in a gravitational field.

## What is the relationship between time and energy?

Time and energy are closely related because energy is often measured in units of joules per second, or watts. This is because watt is equal to one joule per second. In other words, the rate at which energy is transferred or used is measured in units of time.

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