# Issac physics question about momentum

An alpha particle (mass equals,= 6.7x 10^-27 kg) is fired at the nucleus in a gold atom with a speed of 3.5x10^6 (m s^-1). It bounces off at the same speed in the opposite direction. If the collision takes 10^-19 seconds, what is the magnitude of the average force? Give your answer to 2 significant figures.

Sounds like youre close, what did you do?
(Original post by mqb2766)Sounds like youre close, what did you do?

I got the momentum of the atom by multiplying the mass and velocity, and used the equation force=momentum/time to put the momentum and the time that I got given from the question, then I got the force eventually.
Original post by lheeseung_05
(Original post by mqb2766)Sounds like youre close, what did you do?

I got the momentum of the atom by multiplying the mass and velocity, and used the equation force=momentum/time to put the momentum and the time that I got given from the question, then I got the force eventually.

The impulse will be equal to the change in momentum.
Original post by mqb2766
The impulse will be equal to the change in momentum.

It said the atom bounces off in the same speed, and wouldn't this mean the momentum of atom before and after collision would be the same as well?? Therefore the change in momentum will be zero.
Original post by lheeseung_05
It said the atom bounces off in the same speed, and wouldn't this mean the momentum of atom before and after collision would be the same as well?? Therefore the change in momentum will be zero.

Momentum is signed, like velocity.
Original post by mqb2766
Momentum is signed, like velocity.

What does it mean by signed??
Original post by lheeseung_05
What does it mean by signed??

Has direction, so a positve velocity means its moving in one direction and a negative velocity would mean its moving in the opposite direction.
(Original post by mqb2766)Has direction, so a positve velocity means its moving in one direction and a negative velocity would mean its moving in the opposite direction.

Original post by mqb2766
Has direction, so a positve velocity means its moving in one direction and a negative velocity would mean its moving in the opposite direction.

Omg I got the answer now!! Thank you so much!!!