Ride of his life: BMX-bike superstar Mat Hoffman uses physics to break world records.

Pro biker Mat Hoffman speeds up the ramp and rockets into the sky. He lets go of his bike, does a flip in midair, and then quickly grabs on again before plummeting back to Earth. A 10-time world champion BMX biker, Hoffman is known for his high-flying stunts off vertical half-pipes (U-shaped ramps).

In 1991, Hoffman broke the world record for the biggest air (height reached above a ramp). He soared two stories above a 6.4 meter (21 foot)-high ramp. "I doubled the record," Hoffman boasts. Still not satisfied, he built a ramp that was 1 m (3 ft) taller. He recently smashed his own record by blasting 8 m (26.5 ft) off the ramp. "I was just over 50 feet off the ground," he says.

Whether he's just getting air or pulling one of the many extreme stunts he's invented--like the "no-handed 900," a midair two-and-a-halftime rotation--Hoffman relies on an ally: He uses the forces of physics to dominate the ramps.

TAKING OFF

What's Hoffman's biggest challenge in making sky-high jumps? Fighting against the force (push or pull) of gravity (force that pulls two objects together). To beat gravity and get air, he builds up vertical velocity, or speed in one direction. "How high he jumps depends on the final speed he reaches before leaving the ramp," says Jusak Tandean, a physicist at Southern Methodist University, in Dallas, Texas.

Hoffman can pedal only so fast. So for his record-breaking jumps, he was towed by a fast-moving motorcycle. As they reached the bottom edge of a quarter-pipe (half of a vertical half-pipe with a horizontal ramp leading up to it), they separated. Then Hoffman blasted solo up the ramp at 97 kilometers (60 miles) per hour.

FLYING HIGH

For most stunts, Hoffman isn't towed. Instead, he gets air with the help of physics--a motor-free speed builder. Hoffman begins a ride by dangling off the top of one side of a half-pipe. There, he has gravitational potential energy (stored energy due to height). When he drops off, gravity pulls him down the ramp and his potential energy converts to kinetic energy, or the energy of motion. With little pedaling, he can easily climb the opposite side of the half-pipe and get air.

What if Hoffman wants even more air? "The higher he starts, the more speed he'll get on the way down," says Steven Pollock, a physicist at the University of Colorado in Boulder. By adjusting his starting height, Hoffman can begin with more potential energy. That means he'll gain more kinetic energy--and fly higher.

Because average half-pipe ramps are only 3 m (11 ft) high, daredevil HoffMan built a taller model off the side of a building. Hoffman rolled into the half-pipe from one side of the ramp that towered at almost five stories from the ground. The added potential energy paid off: "The drop gave me a speed of about 80 km (50 mi) an hour," he says. Result: Hoffman flew about 5 m (18 ft) off the ramp into the air.

SMOOTH RIDE

Once Hoffman builds speed, he doesn't want anything slowing him down. But as iris bike rolls across the half-pipe, friction (resistance to movement when two surfaces rub together) between his tires and the ramp acts like a brake. Since rough surfaces create more friction, Hoffman builds his half-pipes with the smoothest material possible. His favorite surface is a new material called Skatelite. Made of tightly packed wood fibers mixed with plastic, it boasts a smooth-as-glass riding surface. Added bonus: Less friction means fewer scrapes and bruises if he crashes.

CRASH AND BURN

He may put speed limits to the test, but even Hoffman has his boundaries. "The main thing I am concerned about is the G-force," he says. As he accelerates (changes velocity over time) up the curve of a half-pipe, the acceleration creates a force, called a G-force, on his body. The force is just like the feeling of being smashed into your seat at the bottom of a roller-coaster valley.

If he goes too fast, Hoffman may not be able to stay on his bike. "The faster he's going through the half-pipe, the more acceleration there is, and the bigger force he feels," explains Pollock.

Gravity pulls down with 1 G. Hoffman felt about 2.5 G's when he went screaming up the ramp for his first world record. Hitting a half-pipe much faster could spell trouble. "I would just crash into the ramp like it was a wall," he explains.

That's why he considers physics before trying new stunts. "Whenever you're entering the world of the unknown, you only have science and math to prepare you," he says. "That's your crutch."

Hoffman's High-Tech Bike

Hoffman's BMX bike is specially built to hold up to the forces of half-pipe riding.

MINI WHEEL:

BMX-bike wheels are 50 centimeters (20 inches) across--smaller, but stronger, than standard bike wheels.

NO FLATS:

Hoffman's tires are designed to hold more air than standard tires. That makes them harder, which creates less friction with the ramp.

SPINY:

BMX wheels can have 36 or 48 spokes. Hoffman's bike sports 48-spoke heels for added strength.

FRAMED:

Hoffman's bike frame is made of a very strong material called Chrome-Moly Steel (steel made of the elements chromium and molybdenum).

TRICKED:

Hoffman wows crowds by grabbing the pegs (steel tubes in the center of both wheels) while flying in midair.

It's Your Choice

1. Hoffman could increase--to get more air.

A The G-force strength

B His vertical velocity

C Friction

D Gravity

2. When Hoffman drops down a ramp, his potential energy

A is destroyed

B helps him pedal faster

C increases

D converts to kinetic energy

3. Which of the following is NOT true about friction?

A It slows Hoffman's ride,

B Low friction results in faster rides.

C Friction is higher on smoother surfaces.

D There is more friction on rough surfaces.

Answer

1.b 2. d 3. c

NUTS BOLTS

To what does Hoffman owe his record jumps? The law of conservation of energy. The rule: Energy can't be created or destroyed--it can only be changed to another energy form. For instance, the potential energy Hoffman has stored at the ramp's top transforms to kinetic energy as he speeds down the ramp.

The downside? Some of Hoffman's energy gets transformed into thermal energy (heat) from the friction between his tires and the ramp. That energy change slows him down.

DID YOU KNOW?

* Bicycle Motor Cross (BMX) is believed to have originated in California in the early 1970s when a bunch of kids modified their bicycles to "get more air."

* Wonder why zooming down a bike ramp or a roller coaster makes you feel as if your stomach is about to come out of your throat? When you go downhill, your body battles between inertia (the force of resistance to change)--which wants your stomach to stay in place--and the G-force, which wants to pull your body down.

CRITICAL THINKING:

* Mat Hoffman stated, "Whenever you're entering the world of the unknown, you only have science and math to prepare you." What does he mean? Name an activity that reties on the properties of science and math. Then discuss how.

CROSS-CURRICULAR CONNECTIONS:

HEALTH: Research BMX safety. Then, create a human-body diagram highlighting the biking gear used to protect each body part.

RESOURCES

* Grolier search term: Laws of Motion 

* Use this site to calculate gravitational potential energy: http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html

* To learn about BMX competition rules, visit the Web site of the National Bicycle League: www.nbl.org/

DIRECTIONS: On a separate piece of paper, answer the following questions in complete sentences.

1. What's Mat Hoffman's biggest challenge in making sky-high jumps?

2. What is kinetic energy?

3. How can Hoffman increase his gravitational potential energy?

4. Why does Hoffman build his ramps with Skatelite? Which force does the material help counter?

5. What is the law of conservation of energy?

Answer

1. His biggest challenge is fighting the force of gravity.

2. Kinetic energy is the energy of motion.

3. Hoffman can increase his gravitational potential energy by starting at a higher point on the ramp.

4. Hoffman uses Skatelite because it's very smooth. This material helps reduce friction.

5. The law of conservation of energy states that energy can't be created or destroyed. It can only be changed to another form of energy.