Calculating the Force of a 1500kg Car on the Road

[kthejohnster]

Homework Statement

a car of mass 1500kg and decelerating 15ms^-2. what is the total force of car on road from all tires?

Homework Equations

W=mg
F=ma

The Attempt at a Solution

I'm not sure if the force is simply mg or if it is ma in this case. There is also a frictional force but I don't know if it applies here.
mg=15000?
ma=22500?

 

[Redbelly98]

Try drawing a free body diagram of the car. What are the forces that are present, and in what direction are they?

Also, in what direction (horizontal or vertical) does the car decelerate? Which force or forces are acting in that direction?

 

[kthejohnster]

Try drawing a free body diagram of the car. What are the forces that are present, and in what direction are they?

Also, in what direction (horizontal or vertical) does the car decelerate? Which force or forces are acting in that direction?

well there's the weight of the car downwards, the normal force from ground probably equal to that since car isn't moving upwards. then in the horizontal there is each wheel friction in direction of car moving

 

[Redbelly98]

Yes: gravity downwards, normal force from ground upwards, and friction force horizontally. And two of those forces are exerted by the ground, so the car exerts equal-but-opposite forces on the ground.

 

[kthejohnster]

Yes: gravity downwards, normal force from ground upwards, and friction force horizontally. And two of those forces are exerted by the ground, so the car exerts equal-but-opposite forces on the ground.

so would that be mg+ma because opposite of gravity would be mg and the force opposite friction seems to be ma or is there a separate y force and x force

 

[cmb]

a car of mass 1500kg and decelerating 15ms^-2. what is the total force of car on road from all tires?

Difficult to tell. I expect this is not the answer the question is looking for, but it means you are in the process of having an accident so your tyres might not be generating any force at all!

I do not believe your tyres can generate 1.5g (I think the limit for road cars is around 1.1g with latest tech), unless you are in a race car with aerodynamic downforce, but then the car wouldn't weight more than 605kg. (due to Formula 1 construction rules 605kg or Formula 3 540kg.)

 

[Redbelly98]

so would that be mg+ma because opposite of gravity would be mg and the force opposite friction seems to be ma or is there a separate y force and x force

The two forces act in different directions, x or y, so we can't simply add them. But yes, they turn out to be equivalent to mg and ma -- I guess this is what you were saying in your first post. But technically you need to say in what direction those forces are acting. So for the 15,000 N force you calculated from mg, in what direction does it act? Likewise for the 22,500 N force you calculated from ma.

Difficult to tell. I expect this is not the answer the question is looking for, but it means you are in the process of having an accident so your tyres might not be generating any force at all!

I do not believe your tyres can generate 1.5g (I think the limit for road cars is around 1.1g with latest tech), unless you are in a race car with aerodynamic downforce, but then the car wouldn't weight more than 605kg. (due to Formula 1 construction rules 605kg or Formula 3 540kg.)

While you bring up a valid point, since this is intro physics it's probably best to keep things simple; neglect air resistance, and use the numbers given. It's likely that the question author was not thinking through about the implications of a 1.5g acceleration due to friction -- or the 15 m/s² was a simple typo.