P3c(i) Forces and Motion You will find out about How a force speeds up or slows down an object How Force, Mass and Acceleration are linked How to use the F=ma equation

Forces all around us To make this car move faster all the driver has to do is press harder on the accelerator pedal. This causes an increase in PULL of the engine that provides that FORWARD force. Without that pull force there would be no forward force. To calculate force you need to use the equation: Force = mass x acceleration F = m x a The unit is the Newton, N, named after Isaac Newton. Example: What force is needed to move a car of 1,500kg to give it an acceleration of 5m/s 2 ? F = ma F = 1,500kg x 5m/s 2 = 7,500N I wonder if Isaac Newton ever said “Let the force be with you…”!! F mX a REMEMBER: You need to be able to re-arrange the F=ma equation so use the Triangle rule to help.

F=ma rules F = m a B has a larger mass than A. To accelerate at the same amount B requires more Force. B has a larger mass than A. If the forces acting on A and B are the same then A will have a higher acceleration. A B Contextualising: A catapult is stretched back as far as it can go. Therefore the forces acting on A and B are equal. A travels further because it has a smaller mass. A B

Balancing of Forces 50N In this instance the object was not moving to begin with, so it started at rest. If 50N push the object from the left and 50N push the object form the right then the RESULTANT (overall) force is: 50N – 50N = 0N (The object is still stationary) 50N In this instance the object was already moving at a steady constant speed with a forward force of 50N. If a push force of 50N acted in the opposite direction then the object could not accelerate so would reach a steady constant speed. It would not be stationary as it started with a steady constant speed provided by the forward force. The RESULTANT force would still be: 50N – 50N = 0N (The object is moving at a constant speed) So if two forces acting on an object are BALANCED then that object would either be stationary (if it started at rest) or it would move at a constant speed (if it was already moving).

Unbalanced Forces The box and arrow diagram represents a car moving on a road. The engine provides the forward force of 12,000N. The road causes friction which works against the forward force. This OPPOSITIONAL force is 3,000N. The UNBALANCED Force is equal to: Forward Force – Backward Force. If the RESULTANT Force is positive then the car would accelerate FORWARD. If the RESULTANT Force is negative then the car would accelerate BACKWARD (or decelerate). So our UNBALANCED Force for above: Forward Force – Backward Force 12,000N - 3,000N = 9,000N Forward (Recall this has magnitude AND direction so is a VECTOR quantity) Note that the arrows are to scale; 12,000N is 4x larger than 3,000N so the arrow must be 4x larger too. 12,000N 3,000N Positive so accelerating FORWARD

Questions 1.John plays badminton. He hits the shuttle with a higher Force. What happens to its motion? 2.A football and a tennis ball are kicked with equal Force. Which goes further? Why? 3.A bus has a mass of 32,000kg. What Force is required to accelerate it to 4m/s 2 ? 4.Why do skydivers use parachutes as they land? 5.A tank of mass 50,000kg has a resultant forward force acting on it of 25,000N. What is its acceleration? 6.Why do you think there are rules about the maximum mass of footballs in competitions? 7.Olivia is driving on a road at 45m/s. She removes her foot from the accelerator pedal and slows down to 15m/s in 10 seconds. Calculate her deceleration. If the mass of her car was 1,800kg what was the size of the resistive force acting on the car?

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