You are Here: Home >< Physics

# A few (VERY BASIC) multiple choice physics questions I need help on Watch

1. 1 Which of the following pairs of quantities must be conserved in a particle decay?
A charge and mass
B charge and momentum
C energy and mass
D energy and momentum
the bolded option is the answer. however I wrote down C - any ideas why this is wrong?

2 A football falls to a level surface with a momentum 1.5Ns. After impact it rebounds with a momentum of 0.3Ns. The balls' change in momentum is
A 1.8Ns away from the ground
B 1.8Ns towards the ground
C 1.2Ns away from the ground
D 1.2Ns towards the ground
again, bold = correct answer. I didn't really understand this question, can anybody tell me why it's A?

3 The charge-to-mass ratio of an electron (e/m) is equal to 1.76 x 10^11 Ckg^-1 at low electron speeds. At speeds approaching the speed of light, e/m is
A > 1.76x10^11 Ckg^-1 because the charge increases
B > 1.76x10^11 Ckg^-1 because the mass decreases
C < 1.76x10^11 Ckg^-1 because the charge decreases
D < 1.76x10^11 Ckg^-1 because the mass increases
Ok so apparently it's D, but I wrote B since I thought that mass will decrease the closer something gets to the speed of light?

hope I'm not asking for too much (need to get an A in physicsto meet my offer!). any guidance is much appreciated, thanks in advance
2. (Original post by ucasisannoying)
1 Which of the following pairs of quantities must be conserved in a particle decay?
A charge and mass
B charge and momentum
C energy and mass
D energy and momentum
the bolded option is the answer. however I wrote down C - any ideas why this is wrong?

2 A football falls to a level surface with a momentum 1.5Ns. After impact it rebounds with a momentum of 0.3Ns. The balls' change in momentum is
A 1.8Ns away from the ground
B 1.8Ns towards the ground
C 1.2Ns away from the ground
D 1.2Ns towards the ground
again, bold = correct answer. I didn't really understand this question, can anybody tell me why it's A?

3 The charge-to-mass ratio of an electron (e/m) is equal to 1.76 x 10^11 Ckg^-1 at low electron speeds. At speeds approaching the speed of light, e/m is
A > 1.76x10^11 Ckg^-1 because the charge increases
B > 1.76x10^11 Ckg^-1 because the mass decreases
C < 1.76x10^11 Ckg^-1 because the charge decreases
D < 1.76x10^11 Ckg^-1 because the mass increases
Ok so apparently it's D, but I wrote B since I thought that mass will decrease the closer something gets to the speed of light?

hope I'm not asking for too much (need to get an A in physicsto meet my offer!). any guidance is much appreciated, thanks in advance
1)Charge cannot be destroyed. Mass can be changed into energy and vice versa. Momentum must be conserved unless an external force is acting on an object. There's no external force in particle decay. It can be likened to an explosion.

2)The change in momentum equals the final momentum minus the initial momentum. If you take the upward direction as positive, then it gains 0.3Ns of momentum and loses -1.5Ns of momentum which would the total momentum change as 0.3-(-1.5) which equals 1.8Ns.

3)The faster an object goes, the more kinetic energy it has (if it has mass). Because energy and mass are interchangable (E=mc^2) then an increase in energy equals an increase in mass. As an object approaches the speed of light, its mass increases considerably due to the large gain of kinetic energy.

Hope this helps.
3. 1 Because momentum is conserved? Law of conservation of momentum

2 Say down is -ve and up is +ve (since momentum is a vector)

3 Mass does increase when an object approaches speed of light

EDIT: Gemini has a better explanation
4. thank you both! much appreciated

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: November 29, 2010
Today on TSR

### Oxford interview invitations

When can you expect yours?

### Official Cambridge interview invite list

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.