Presentation on theme: "Momentum and Energy PHYS 1090 Unit 3. Question If a car collides with a bug, which experiences the greatest force? A.The car. B.The bug. C.It’s a tie."— Presentation transcript:
Newton’s Third Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first, along the same line of interaction. To every action there is always an opposed equal reaction. If object A exerts force F on object B, object B exerts force –F on object A. F A→B = −F B→A
Bug + Windshield Small car: 1250 kg From the same force, the bug accelerates a lot more! Large insect: 0.00025 kg Forces are the same—the accelerations are different.
Interaction Forces All forces are interaction forces! –gravity –wind –jumping –everything! This means: whenever something accelerates, something else accelerates in the opposite direction! Whoa!
Accelerations are in opposite directions More massive cart accelerates the least Equal-mass carts have equal accelerations (in opposite directions) Total Mass·Velocity the same before and after collisions
Momentum Changes and Newton’s Third Law At any instant: p = (mv)= m v= ma t= m(F/m) t= F t For interacting objects, F A = −F B, so: p A = F A t p B = F B t = −F A t p A = − p B
Conservation of Momentum Momentum can be transferred between objects, but the total momentum can never change. p 1 + p 2 = 0
The lighter the meter, the farther it drags. The heavier the ball, the farther it drags a meter. The higher the ramp, the farther a ball drags a meter. The higher the ramp, the faster a ball rolls at the bottom. The mass of the ball has no influence on how fast it rolls at the bottom.
Some Forms of Energy Potential Energy Kinetic energy
Work Against Gravity Force = –w = mg distance = h work = mgh Source: Griffith, The Physics of Everyday Phenomena
Get It Back? Gravity exerts force mg as object drops distance h. work = mgh Source: Griffith, The Physics of Everyday Phenomena
Potential Energy The energy of relative position of two objects gravity springs electric charges chemical bonds
Potential Energy Gravitational potential energy = the work done by gravity in lowering an object – or – the work to raise an object to a height Gravitational PE = mgh
A Moving Object Can Do Work Source: Griffith, The Physics of Everyday Phenomena
Kinetic Energy the work that a moving object does in stopping – or – the work to bring a motionless object to speed KE = 1 2 mv 2
Rollerball Energy Conversions work Potential energy Kinetic energy work The more work you put in, the more work you get out!