#### Transcript Year 12 11th - Hinchingbrooke

For today’s lesson all questions will assume that the acceleration due to gravity, g, will be 10ms-2 Quick Starter Fred Lemming (mass 80 kg) goes on holiday to Blackpool so that he can throw himself from the top of Blackpool tower (158m tall). How long will he take him to hit the floor if we consider air resistance to be negligible? Lesson Objective: 1) To use Newton’s first law to find missing forces when an object is in equilibrium 2) To use Newton’s first law to find missing forces when two or more objects are connected in equilibrium Bodies and Connected Bodies in Equilibrium Newton’s first law: Every body continues in a state of rest or in uniform motion in a straight line unless acted on by a resultant force In short: A particle/body is in equilibrium if it is at rest or travelling at constant velocity in a straight line Eg A 60kg crate rests on a rough horizontal surface. Fred Lemming pulls it using a rope that is parallel to the floor with a force of 15N and Gertrude Lemming pushes it from behind using a force of 18N, but it does not move. Find: a) The Normal contact force between the crate and the floor b) The Resistance force between the crate and the floor Eg A 50kg crate rests on a rough horizontal surface. Fred Lemming pushes it along at a constant speed using a force of 20N. If air resistance is 1N, find: a) The Normal contact force b) The resistance force between the crate and the floor Connected Bodies in equilibrium YOU CAN EITHER: Consider the bodies separately (This is usual for working out internal forces between the bodies inside the system) OR Ignore the internal forces and consider the system as a whole Look at the system of masses below: 1 kg 2 kg 6 kg If the system is in equilibrium, what can you say about the table top? Calculate the tension in each string Calculate the magnitude of the friction force Fred Lemming parachutes from a plane. Fred Lemming has a parachute and air resistance on his body is 20N. He is supported by a harness that connects to the parachute with 4 equal light inextensible ropes. The parachute weighs 10kg. If he is travelling at constant speed: a)Find the Tension in each rope b)Find the Resistance Force on the parachute The diagram shows a train, consisting of an engine of mass 50,000kg pulling two trucks, A and B, each of mass 10,000kg.. The force of resistance on the engine is 2000N and that on each of the trucks 200N. The train is travelling at a constant speed. a) By considering the train as a single system find the driving force it produces. b) By considering the forces on each truck find the Tension forces in each coupling. Eg Look at the diagram 4 kg a) What is the friction force on the 4kg block? b) What is the tension in the string between the 6kg and 3kg block? 6 kg 3 kg Sheet Key points when solving problems involving forces: 1) Draw a clear diagram 2) Mark on all the forces 3) Write down equations in two perpendicular directions 4) Solve the equations to find the missing forces Probably need a calculator tomorrow!