# Homework Help: Calculate how much force you would have to apply to crate?

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1. Oct 19, 2015

### HaileyY

1. The problem statement, all variables and given/known data
Calculate how much force you would have to apply to the crate to get it to go at constant speed up a 30 degree ramp.

2. Relevant equations
Using kinetic friction for equation.

3. The attempt at a solution
I couldn't find the formula for kinetic friction which also includes calculating force with an angle. Anyone know the correct formula/equation to use?

2. Oct 19, 2015

### Staff: Mentor

Hi HaileyY, Welcome to Physics Forums.

Is that the entire problem statement? No other information given? If it is then it doesn't look like you need to consider friction. Just assume that the ramp is smooth and frictionless.

3. Oct 19, 2015

### ogg

There's two different "types" of friction: static (also known as "standing") friction and kinetic (aka "moving") friction. Since your problem says to use kinetic friction (which is almost always less than static friction) then there must be some way for you to look that up - maybe a table in your book or an earlier problem. The force required is to counteract the friction. Pushing a crate up an incline requires you to do the work of raising the crate, mgh, and of countering the normal force of friction. Friction is computed using the component of force perpendicular to the inclined surface, so you have to do a force diagram. (Look at it this way: if the incline was 90°degrees then there would be 0 force required to overcome friction (since gravity, the only force involved by assumption, CAN'T act perpendicular to the downward direction, so there's no friction) while if the inclined surface was 0° (which means flat, horizontal) the force would be 100% of the crate's weight times the friction coefficient. So, what trig function is zero at 90° and 1 at 0°? The formula has to include weight * function_of_(angle) * friction_coefficient. As is, (originally), your problem doesn't have enough information..hint: the trig function is going to be sine or cosine. You need to know how both behave at 0, 90°, 180° and 270° since picking which to use will be a common enough problem...memorizing formulas will only get you so far (but perhaps far as you want to go, haha). so anyway there will be two forces to counter-act: the force of gravity (the weight) and the force of friction (which is related to both weight and surface friction). But you should know how a wedge helps you lift objects, so that force should be easy. And friction is always considered to be normal (perpendicular) to the surface on which the object is moving (in this case the surface is inclined). So you always (when friction is involved) need to be able to compute that normal force given the inclination to the Earth (and the weight of the object (and the coefficient of friction)).

Last edited: Oct 19, 2015