Explore

You know that momentum is based on the mass of an object and its velocity. You also know that momentum tends to be conserved. Momentum can also be changed. You can use this information to help you make decisions about road safety.

In this activity, you will learn that momentum can be affected by applying a force for a period of time.

Imagine you are rollerblading, skating, biking, snowboarding, windsurfing, or driving. If you want to slow down, what do you do? Obviously, you brake. No matter what activity you are doing, you can decrease your momentum by braking — applying a force that slows your vehicle. Even a snowboarder has to learn braking quickly!

You may remember learning about friction previously. When two objects press against each other, the force the two objects exert on each other is called friction. Think of the way you stop when you are skating. You turn your skate blades slightly and press against the ice. The friction between your blades and the ice reduces your momentum and causes you to slow down.

In a similar way, vehicles use friction when braking. In the case of bikes, cars, and trucks, two sources of friction help to reduce the vehicle's momentum. Friction occurs between the wheel and the road, and between the brake pads and the rotors (or brake drums) attached to the wheels. If you stop accelerating, the vehicle coasts to a stop. If you apply the brakes, you could stop very quickly.

Shown above is a vehicle's braking system.

Even on water, friction slows the momentum of boats, canoes, kayaks, and other watercraft. When you stop paddling or turn off your motor, you will gradually come to a stop due to the friction between the boat and the water.

Applying a force affects momentum. The change in momentum can vary depending on how much force is applied and for how long the force is applied.

If you are driving in town at the speed limit of 50 km/h and you approach a school zone, you apply your brakes in order to slow your momentum to the speed limit of 30 km/h. Then, you can continue at that speed through the school zone.

If a child suddenly ran out into the road, you would quickly apply your brakes with as much force as possible to bring your car to a stop as soon as possible.

This combination of a force being applied over time is called impulse. The bigger the impulse, the greater the change in momentum. Impulse is calculated using this formula:

Impulse = Force x Time

Images courtesy of www.imagesgoogle.com