Unbalanced Forces A force is a push or a pull that causes an object to either: - increase or decrease its speed - change its direction - change its shape. If any of these things happen then a net force must be acting on the object in question. A net force is the overall force acting on an object. If the net force acting on an object is not zero then we say that unbalanced forces are acting on the object. If the net force is zero then there may be no force acting on the object or all the forces acting are balanced. When a rocket accelerates skyward a net force is acting to push it skyward. The forces of gravity and thrust, which act in opposite directions, are not balanced. A hovering stationary aircraft is experiencing zero net force, hence its speed, shape and direction remain unchanged. The forces acting on this aircraft are said to be balanced. This F111 Fighter plane is accelerating skyward. Since it is accelerating, its speed is increasing, a force must be acting on it. Click to find out more about acceleration. The forces acting on it include drag, lift, gravity and thrust. Obviously the plane is acted on by unbalanced forces. Since the plane is climbing, the force of lift is greater than the force of gravity and thrust is greater than drag. The racing car on the right accelerates along a horizontal road surface. Once again unbalanced forces are acting on this vehicle. The force acting on this vehicle include drag, friction, thrust from the engine, gravity and the push upwards by the road surface. Since the car accelerates the thrust produced by the engine is greater than the drag and friction forces acting on the vehicle. The road acts to push the car upwards with a force equal and opposite to the force of gravity pushing down. The Tomcat pictured on the left is accelerating horizontally close to the surface of the water. What forces are acting on this plane? What are the balanced forces? What are the unbalanced forces? What happens to the force of drag as the plane slows down? Solution The skydiver is descending at a slow constant speed. The drag forces are maximised by the parachute and oppose the force of gravity pushing down. In what direction are the drag and gravitational forces acting? Are they balanced? What would happen if both of these forces were equal and acted in opposite directions? Solution Click to see a 120kb movie of a goldfish swimming around in a bowl. Explain why it is obvious that unbalanced forces are acting The vertical take-off jet shown on the right accelerates skyward from a standing start and then falls back to earth again. What type of forces are acting on the aircraft? unbalanced forces balanced forces no forces Which force can be controlled by the pilot to lower the plane or raise it off the ground? thrust drag gravity What forces are acting on the plane when it is on the ground with its engines shut down? thrust only drag and thrust gravity pushing down and the ground pushing upwards none of the above are acting on the plane. . What can you say about these forces? they are unbalanced equal in magnitude and act in opposite directions unequal and act in the same direction equal in magnitude and act in opposite directions . Consider the jet pictured on the right taking off down a runway. Which statement is true? Force of lift is greater than the gravitational force pushing the plane down Gravitational force is always greater than the force of lift. Gravitational forces are not present Drag is greater than thrust A car is travelling on a straight road, its speed vs time graph is shown on the right. When answering the questions below keep in mind that a net force is acting when the speed of an object changes. a) Over which period of time is a force applied to the car to increase its speed? 7-8 hrs only 0-2 hrs and 6-7 hrs 3-6 hrs only 2-3 hrs only 2-3 hrs and 6-9 hrs 2-3 and 7-11 hrs only b) Over which period of time is there no net force acting on the car? 7-8 hrs only 0-2 hrs and 6-7 hrs 3-6 hrs only 2-3 hrs only 2-3 hrs and 6-9 hrs 2-3 and 7-11 hrs only c) Over which period of time are frictional forces larger than thrust created by the engine? 7-8 hrs only 0-2 hrs and 6-7 hrs 3-6 hrs only 2-3 hrs only 2-3 hrs and 6-9 hrs 2-3 and 7-11 hrs only d) When is the force of thrust created by the engine greater than the drag and frictional forces acting on the car? 7-8 hrs only 0-2 hrs and 6-7 hrs 3-6 hrs only 2-3 hrs only 2-3 hrs and 6-9 hrs 2-3 and 7-11 hrs only What force is maximised by the parachute? thrust drag gravity lift The Shuttle is slowing down. Which statement is true? Forces only act on the Shuttle if it is accelerating There are no forces acting on the Shuttle The forces acting on the Shuttle are balanced Unbalanced forces are acting on the Shuttle During the Olympics, on windy days, records that are smashed may not be recorded. The wind can assist athletes to jump to record heights or lengths. The net force acting on athletes is increased by wind, as shown on the right. a) How does a runner generate thrust? By pushing off the track with their legs By pushing through the wind By using the wind for extra power. b) Why is drag acting in the opposite direction to thrust? Because drag is smaller then thrust Drag acts to slow the forward movement Drag promotes the forward movement of the athlete. c) If an athlete is able to complete a Long Jump which of the following is true? Drag is greater than thrust Thrust was greater than drag Both drag and thrust were balanced d) It is important for an athlete performing the Long Jump not to carry too much weight . This is to minimise thrust maximise drag forces minimise the gravitational forces acting on the athlete e) When is wind capable of increasing the net force ? when it is blowing in the opposite direction to the athlete's direction of movement when it is blowing in the same direction as the athlete is travelling f) When is wind capable of decreasing the net force ? g) What is the net force acting on the athlete as he jumps off the runway, if he jumps with a force of 15N and with a tail wind of 3N acting while the drag force as he moves through the air is 1N? Solution. What are the forces acting on the rocket as it ascends skyward? drag only drag and gravity gravity and thrust only drag, gravity and thrust thrust only Are there any forces that impede the rocket from ascending? gravity only drag only thrust only drag and gravity A rocket that is orbiting Earth but out of the atmosphere will most likely not experience gravity only drag only thrust only drag and gravity What other force should be included in this diagram ? In what direction should it be acting? up down sideways What force is capable of overcoming this force? thrust drag gravity magnetic How do we know that a net force acted upon this vehicle? it has sped up it has changed direction it has slowed down it has changed shape it has changed colour Continue with the forces of yachting