When a moderate amount of force is applied to my rim brakes, the wheels will lock up and will not rotate. This causes a problem especially when I am going down hills and the front wheel locks up when the brakes are applied. The momentum throws me over the handlebars and I get injured. Can someone tell me how to fix this? thanks.

There are several factors possibly involved, and not possible to tell without much more info which apply. The first thing we need to know, as with most problems, is when the problem started and if any maintenance or incident (accident, etc.) occurred just before the problem started. If it's always been present we need to know how old the bike is.The make and model of the bike would also be helpful, and if it's know if the brakes and wheels are original equipment.

As for possible causes:

• Contamination on the rims or pads can cause them to grab, as can bulges on the rim from previous impacts, especially if the tires are not kept inflated.
• A brake lever with too much mechanical advantage for the calipers could make it difficult to properly modulate the braking.
• You could just be grabbing the brakes too hard, and both brakes need to be used. If the rear starts skidding you need to let up on the front.
• It's advisable to shift back on the saddle during hard braking to increase the braking of the rear wheel and to avoid pitching over.
• You have too much time and too little intelligence, so you thought you'd waste our time. But guess what - the above info may be useful to someone else.

<guess>? Bent/ crud/rust jamming the cable in the housing is stopping brake from returning to open position.

Please post video for our entertainment

Please tell me what kind of brakes you have, I would like to have that much stopping power.

How many times have you been thrown over the handlebars?

why does that matter? once is too many!

There can be several reasons for lock up. Crud on the rims. Old soft sticky brake pads altho most just get hard. Binding brake cables.

There have been discussion on which brake pads wear out quicker, the front or the rear. In my case it is the rear, since I do most of my braking on the rear, which is much safer. Maybe you should try to do the same. With a locked up front wheel you are going to go down, no doubt about it.

https://www.youtube.com/watch?v=WsqzjV9fybk

Don't squeeze the brake lever so hard.

well, I did say "moderate". beside they aren't suppose to work that way.

Assuming you aren't a troll, why don't you answer the legitimate questions that were asked?

Major rim damage from the same jump that broke the handlebar?

Why hasn't this thread been given a merciful death many posts ago?

I deleted the "off-topic" comments. Please keep the insults out of the conversation.

Unless you ride straight into an immovable object, there are at least two entirely different things cooperating in throwing you over the bars, braking force and body positioning.
You don't need wheel lock to get thrown over the bar if you're poorly positioned and braced.
Many who claim that "the brakes locked up" just weren't doing it right, and folded at the elbows or lost hold of the bar as the bike slowed down and momentum carried them forward. If you fail to brace yourself you will get pitched forward when braking even if the wheels are still turning.
Before trying even "moderate" braking, you need to understand what's going to happen and be ready to counter the forces that will be generated.

You need to have a good grip of the bar, elbows and shoulders braced, and for hard braking, and have your body shifted back-and-low before clamping down on the brakes.
Otherwise you might as well blame the car's brakes for thowing you into the seatbelt/dash.
And I don't hear many people deliberately dulling their car brakes down.

Huge assumption

people dont detune the brakes on their cars
because for the last twenty years or so
manufacturers have been doing it for us
through what is known as
antilock brakes
which automatically release the brakes momentarily
to avoid loss of control when the limit of braking traction is reached

as for the ops specific concerns
moderate is a relative word
and brakes should be able to lock up the wheels
if the rider so desires
and it is generally up to the rider to practice braking so that it can be done safely

if that is impossible
there are devices available that limit braking power
and were semi common on hybrid bikes in the 1990s
by basically adding a spring between the cable housing and the housing stop on the brake
but these were only designed for v brakes I think
and op has not given any details about his ride
so we don't know what brakes he has
so cannot offer any suggestions to fix the problem

+1