# Homework Help: F=ma Qtn

1. Feb 22, 2014

### Lee80

If a car is travelling towards a bridge at a constant speed of 20ms-1. The bridge takes 30s to cross entirely. The car creates a force of 3000N. Calculate the mass of the car when it is on the bridge.

a=(v2-v1)/t
F=ma

I figured that a = 0 due to the fact that the car is travelling at constant speed.
If a = 0 then isnt it impossible for the car to create a force of 3000N? I am very confused?

2. Feb 22, 2014

First,the speed is expressed as m/s not ms-1.
When does the car create a force of 3000N?On the start of bridge?

Edit:Oh I forgot.Welcome to PF

Last edited: Feb 22, 2014
3. Feb 22, 2014

### Staff: Mentor

Hi Lee80! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

I'm a little confused by the question, too. Perhaps the bridge is curved, maybe semicircular? It is not clear whether this 3000N is a vector sum of multiple forces, including also that force known as the vehicle's weight.

Are you sure you have quoted the question details in full?

Last edited by a moderator: May 6, 2017
4. Feb 22, 2014

### Lee80

Thanks for the welcome! :)
I have written the question exactly the way it is on the page including the ms-1, which i know is supposed to be superscript but isn't it interchangable with m/s? Anyway the only extra info given was a second part to the question which asks "The bridge can support a mass of 5 tonne or less safely. Is it safe for the car to travel across the bridge?"
I don't think the question is correct? I think it needs more info. The question is supposed to be aimed at year 11 physics.

5. Feb 22, 2014

### Staff: Mentor

Pity, because it sounded like the basis of a good question: "If the bridge is of level height but curves around the coastline in a semicircular arc, determine that radius of curvature, and the length of the bridge ..."

Sure is.

There is either too much information provided, or else not enough. But something's amiss.

6. Feb 22, 2014

### SammyS

Staff Emeritus
Yes, if the -1 is a superscript then they're interchangeable.

Use the X2 icon above the message box for superscripts.

m/s is ms-1 .