Children sometimes like to race toy cars down ramps to see which car will move faster. Cars move more quickly down ramps than they do when pushed across a level playing surface such as a floor. This is because a ramp is an example of an inclined plane, in which unbalanced forces acting on the car pull it quickly toward the bottom of the plane. The greater the angle of incline, the faster the car will go. In order to understand why this happens, you need to grasp the basics of how motion works.
Three Laws of Motion
A child playing with a toy car illustrates Newton's three laws of motion. These three basic laws come into play whether the child is pushing the car across the floor or sliding it down a ramp.
The basic laws of motion are:
1) An object at rest will stay at rest unless a direct force is exerted upon it.
2) Force equals mass times acceleration. The greater the acceleration required, the greater the force needed to act upon it.
3) Every action has an equal and opposite reaction. When a child pushes a toy car, it pushes his body back in the opposite direction.
It is easy to see how these laws apply when a child pushes a car across a level surface such as a floor. The car is initially at rest until the child exerts force on it by pushing it. The child's body tends to move back slightly as her hand pushes forward on the car. The car then accelerates as it moves across the floor. How much it accelerates depends on how hard the child pushed it. Once it begins moving, other forces, such as friction, act to slow it down until it stops altogether.
A ramp is not a level surface like a floor. Instead, it is an example of an inclined plane, a surface which is tilted. Objects placed on inclined planes tend to slide or roll down them because the forces acting upon the object are unbalanced. For this same reason, objects move faster down an inclined plane set at a greater angle.
When a toy car is placed on top of a ramp, there are two components of the gravitational force pulling on the car. In addition, there is a force perpendicular to the plane of the ramp pushing the bottom of the car up.
First, gravity pulls the toy car down, perpendicular to the ramp. This force stops it from floating in the air when at rest. Another component of gravity pulls the car parallel to the incline, toward the bottom of the ramp.
If these were the only two forces acting on the car, the ramp would sag because of the downward pull of gravity, and as the car moved, the ramp would be pulled farther and farther down until it broke. However, there is a third force acting on the car, called the normal force. The normal force is exerted on an object by the surface it rests upon. It is always perpendicular to the surface. On an inclined plane, the normal force counteracts the component of gravity pulling the car down perpendicular to the ramp.
The car accelerates as it comes down the ramp because of the effect of the parallel component of gravity on the car. This force pulls it down toward the bottom of the ramp. On Earth, objects in free fall normally accelerate at a rate of about 9.4 m/s^2.
The greater the angle of the ramp, the stronger the parallel force of gravity is, and the faster the car will accelerate. Other forces, such as friction, can act to slow the car down. This is why cars will accelerate faster down a waxed ramp.
- Photo Credit toy car image by ann triling from Fotolia.com
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