# A 1 ton shark swims toward a beach. In which direction is the reaction force in relation to the shark's weight?

jgregerson | Certified Educator

As with any object on Earth, weight is a force that acts perpendicular to the Earth's surface. Weight is different from mass in that it depends on the gravity of where the object is located. Weight can be calculated using the formula w = mg where is w is weight in Newtons, m is mass in kg, and g is the acceleration due to gravity. With a shark swimming through the water, the direction of force from weight will be straight down.

The reaction force is a result of the shark using its tail to propel itself through the water. Newton's 3rd law states that for every action there is an equal and opposite reaction. In other words, if a force is exerted on an object, an equal and opposite force will be exerted by that object. In the case of a shark moving through the water, the tail fin is working to push water backward. Therefore the water will also exert a force on the shark, propelling it forward. So the reaction force will be in the direction in which the shark is traveling.