We learned in sixth-grade science class that a body at rest tends to stay at rest and a body in motion tends to stay in motion. Roll a bowling ball down a lane at a tetractys of bowling pins at the far end, and you will get an easily understood demonstration of Newton’s Laws of Motion, 1 and 2. You can also experience these laws every time you apply the brakes on your car. But what exactly happens to the forces that regulate motion?
Where does momentum go when braking? Momentum dissipates into the atmosphere, although it does not have spatial dimensions. Since a body in motion tends to stay in motion unless acted up by an outside force, the car would theoretically roll forward forever. Braking becomes the outside force that acts upon the car, and it uses friction to counteract and dissipate the momentum.
Momentum, then, doesn’t really “go” anywhere. It simply ceases to exist. We are all subject to the invisible, inaudible, otherwise undetectable laws of motion and energy. Gravity, momentum, inertia, attraction, friction, repulsion, and other laws are called laws because they are inescapable. They affect all of us universally.
How Does an Automobile Brake Work?
Friction is the antithesis of momentum. Friction is created when two surfaces come in contact with one another and impede one another. Rubbing your hands together creates friction. The sole of your basketball sneaker screeching on the gym floor creates friction, and friction counteracts momentum. In other words, it slows you down.
In an automobile brake, the brake interacts with the wheel, creating friction. The surface of the brake is rough and sandpaper-like, which creates multiple points of contact and, therefore multiple opportunities to create friction. When the brakes become so worn that this surface is too smooth, the effectiveness of the brake is reduced.
Most automobiles have two types of brakes – disk brakes on the front two wheels and drum brakes on the rear wheels. All of the brakes are operated by the driver using the same pedal, but the manner in which the brakes contact the wheel differ by brake type.
The disk brake makes contact with the wheel from the sides, parallel to the axle. The drum brake makes contact with the wheel on the perimeter, at a right angle to the wheel. The brakes are activated by the pressure of fluid in a cylinder, the result of someone stepping on the brake pedal.
Disk Brakes
When the driver presses on the brake pedal, fluid is ejected from the master cylinder and into a caliper, and then into a piston that forces two disk pads on opposing sides of the wheel to press against a rotor, which rotates with the wheel. This creates friction and dissipates the momentum of the rotating wheel. The faster the wheel rotates, the longer it takes for the momentum to be eliminated.
The disks are lined with asbestos, which serves two purposes. One, it provides more friction – and hence, more momentum dissipation – and two, it mitigates the heat that is generated by the friction.
Disk brakes on a car are similar to the brakes on a bicycle, except that the actual closing of the brake pads is accomplished through hydraulics rather than a mechanical action. (The bicycle brake uses steel cables to force the brakes onto the wheels.)
Some cars have disk brakes on all four wheels, and while they’re generally regarded as being superior to drum brakes, they are more expensive. To save costs, most car manufacturers use drum brakes on the rear wheels. The two technologies act together to bring your car to a stop, but clearly the disk brakes do most of the work.
Drum Brakes
Drum brakes represent an older technology than disk brakes, but it’s a technology that has stood the test of time. They differ from disk brakes in that they engage the wheel (drum, actually) along its perimeter, rather than the sides.
Their components essentially consist of a backing plate, replaceable brake shoes, and a drum. There is also a cylinder for brake fluid. The brake shoe has friction material on its face to increase friction and stopping power. The brake shoe is replaceable, and when the term “brake job” is used, they’re primarily talking about replacing the brake shoe.
The backing plate holds it all together. It attaches to the axle and serves as a place upon which the cylinder, brake shoes, and hardware can be mounted. The brake drum is the point of contact for the brake shoe and has a machined surface that adds friction to the braking process. Brake drums can wear out also, and they’re frequently replaced when the wear is significant enough.
Everything in a Moving Car Has Momentum
Applying the brakes merely starts the process of slowing down or stopping a car. The wheels are the first things to experience the loss of momentum. But everything in the car, or attached to the car has the same momentum as the car – even the air within it. All the additional elements do not lose momentum at the same rate as the wheels. So while the wheels are slowing down in their rotation, the forward motion of the car still wants to proceed forward.
There is a delay in the actual slowing down of the car. For things solidly attached to the car – like the seats – the delay is very short. For things loosely resting within the car – like a sack of groceries, a basketball in the back seat, and you and your passengers – the delay is a bit longer. So, even if the car’s body is slowing down, you are not. The momentum that is projecting you forward hasn’t broken down yet.
In the case of automobile collisions, this is a serious issue. The car’s momentum is broken down rapidly because it has hit something. Yet everything within the car is still moving forward at roughly the same speed as before the impact. And as the law of momentum states, they will continue moving forward unless acted upon by another object or force.
It could be the steering wheel, dashboard windshield, front seat (for back seat passengers), or even the highway pavement (if the passenger was unrestrained by an airbag or seatbelt.)
Momentum Miscellania
- If you’re riding in a car going 45 miles per hour and toss a rubber ball in the air, why wouldn’t it rush straight back, as the car “caught up to it?” It’s because the car and everything in it shares the same momentum – even the air.
- Objects in deep space encounter nothing capable of producing friction, so there’s nothing to slow down their motion. They can be affected by the gravitational pull of planets and collisions with other objects, but theoretically, the moving object will continue in motion forever.
- Seat belts protect automobile passengers by locking them into the frame of the car and therefore allowing the passengers’ momentum to be the same as that of the car, even when the car’s momentum is in a state of change.
- Airbags supplement the seatbelts in providing a cushioning action for that split second that a seat-belted passenger continues to move forward faster than the car. Airbags without seatbelts are greatly compromised in their ability to protect passengers.
Momentum can be transferred from one moving object to another. An object at rest tends to stay at rest unless acted upon by another object or force. A stationary billiard ball will remain stationary until it is struck by the cue ball. Then the momentum exhibited by the cue ball is transferred to the target ball.