Memes and cartoons have been using the magnet-pulled truck for years now. Working under the theory that the magnet attached to an arm in front of the vehicle could attract the plat and pull the truck forward, producing perpetual motion without fuel. However, it isn't so simple in real life.
So if Wile E. Coyote could defy physics, why can't we?
Unraveling Forces in Nature
Force describes the constant interactions between objects. When standing in a room, the Earth is pulling you down using force we know as gravity. If a magnet is placed near an iron piece, the attraction between the molecules of the metal and the iron pulls them closer. This force is due to the magnetic interaction between the objects.
When looking at the interactions between two generic objects. Object A pushes object B; object B also pushes object A with the same strength but in different directions. This phenomenon is referred to as Newton's third law: In every action, there is an equal and opposite force, according to NASA.
According to Newton's third law, the one-dimensional equation of the net force equalling the product of the object's mass and acceleration. Hence, if the net force of the object is zero, the acceleration, or its velocity, must also be zero. If the truck is at rest with zero acceleration, it will stay at rest unless an external force is acted upon.
The Science of Magnet Truck
The first step to testing the popular meme is recreating the phenomenon using a real-world model. Wired tested the phenomenon by using a large neodymium magnet attached to a lego-based crane connected to a cart with tape. The experiment aims to see if the magnet's force on the iron is enough to get the miniature truck moving.
Surely enough, it fails. The cart doesn't accelerate, nor does it move. However, if the magnet is attached to a different cart that already has acceleration, then it can move thanks to magnetic forces.
Ignoring friction, there are three forces at work on the cart. A downward pull due to gravity, the upward push contact interaction of the track on the cart, and the interaction between the magnet and the cart with nothing opposing it.
The only difference is that the magnet and crane are added to the equation. The force of the crane pushing on the cart still equates to a net force of zero. Since the magnet pulls the immobile cart to the left, there is an equal force of the cart pulling the magnet to the right. This has the same magnitude as the force of the magnet pulling the cart. All of the objects concerned still have a net force of zero. With a zero net force, there will be zero acceleration. The objects that start with a velocity of zero will stay put. This is why a magnet-pulled truck can't work.
Although it is a popular cartoon schtick, it is far from the truth.
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