Question

What is the magnitude of the force needed to keep a 60-newton rubber block moving across level, dry asphalt in a straight line at a constant speed of 20 meters per second?

Answer 1

A force of `mu_k*60 N` is needed to keep a 60-newton rubber block moving across level, dry asphalt in a straight line at a constant speed.

Explanation:

Since the block is to be moving, this involves kinetic friction. To keep the block moving at constant velocity, a force equal and opposite the force of friction is required. (Therefore, the net force on the block is zero -- meaning that the acceleration is zero.) The formula for kinetic friction, `F_"kf"`, in such a case is

`F_"kf" = mu_k*N`

Thus, the 2 factors in the friction formula are:

• `mu_k` which is a constant that varies according to the materials involved
• and N, the normal force pressing them together.

In the statement of this problem, the 60 N is the value of N. However, no value is given for `mu_k`, the coefficient of kinetic friction. Members of the police that analyse motor vehicle accidents may have an approximate value that could be used.

I hope this helps,
Steve

Answer 2

Coefficient of friction between rubber block and dry asphalt is

Kinetic `" "` Static
`mu_k=0.67 and mu_s=0.85`

As the block is moving and is to be kept moving with constant speed, net force on the block must be zero. We need to use coefficient of kinetic friction.

Force of friction `F=mu_kN`,
where `N` is normal reaction and is `=60\ N`.

Force needed to keep the rubber block moving is equal to force due to kinetic friction`=mu_kN`

`=0.67xx60=40.2\ N`