1 PHYSICS FOR ENGINEERS We’ll use the book from P.A. Tipler... and some more resources from the web

2 Second law Second law. A net force F acting on a body gives it an acceleration a which is in the direction of the force and has magnitude inversely proportional to the mass m of the body. NEWTON’S LAWS First law First law. An object at rest stays at rest unless acted on by an external force. An object in motion continues to travel with constant velocity unless acted on by an external force. Remark: Also called the "law of inertia," Newton's first law states that a body at rest remains at rest and a body in motion continues to move at a constant velocity unless acted upon by an external force. Momentum: for a single particle of mass m with velocity v, the momentum is defined as From Newton's second law, a force F produces a change in momentum Which are the SI units of force and momentum? "SI" stands for "System International" and is the set of physical units agreed upon by international convention "SI" stands for "System International" and is the set of physical units agreed upon by international convention nd weekDynamics

3 Example. How much horizontal net force is required to accelerate a 1200 kg car at 2 m·s -2 from the rest on a horizontal plane? nd week 1200 kg 2 m·s -2 Dynamics To do yourself: Example 4-1 (5 min) To do yourself: Example 4-2 (5 min) To do yourself: Example 4-3 (5 min) For you to read: The Force Due to Gravity: Weight (15 min) Exemple. A box on a lift W0W0 Let W 0 be the weight of the box The lift accelerates upwards, let a be its acceleration Which is then the apparent weight of the box? The box is inside a lift Which is the apparent weight of the box in case the lift moves up with a constant velocity?

4 Also related with Newton’s second law: impulse. An impulse is an instantaneous change in momentum which can be found by integrating a force F over a characteristic time t, giving nd weekDynamics Which S.I. units do we use for impulse? Can you pose some exemples on impulse? Third law Third law. Whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former. This is known as the law of action and reaction. A well knowing horseTo do yourself: Exemple 4-11 (5 min) Also related with Newton’s second law: friction forces.

nd weekDynamics FRICTION FORCES StaticKinetic

nd weekDynamics FREE-BODY DIAGRAMS A diagram that shows schematically all forces acting on a system, is called free-body diagram. It is called free-body diagram because the body is drawn free from (without) its surroundings. To do yourself: Example 4-6 (10 min)

7 MORE PROBLEMS To do yourself: Example 4-9 (10 min) To do yourself: Example 4-10 (10 min) To do yourself: Example 4-12 (10 min) nd weekDynamics Find out the acceleration: Atwood’s Machine (consider an ideal pulley) m2m2 m1m1 m1gm1g T m2gm2g T +) What about the tension of the rope? The block m 2 has an inhabitant:it has a hollow, which is the home of a bug. Let m b be the mass of the bug. What about its weight when it is at home? Try it yourself!

8 (a) more (b) less (c) the same An astronaut on Earth kicks a bowling ball and hurts his foot. A year later, the same astronaut kicks the same bowling ball on the moon with the same force. Question: His foot hurts... The masses of both the bowling ball and the astronaut remain the same, so his foot will feel the same resistance and hurt the same as before. Question: Where (the Earth or the Moon) is easier for the astronaut to pick up the bowling ball from the floor? The weights of the bowling ball and the astronaut are less on the Moon... Thus it would be easier for the astronaut to pick up the bowling ball on the Moon than on the Earth. SOME MORE QUESTIONS & PROBLEMS nd weekDynamics