1. D. Roberts PHYS 121 University of Maryland Physic² 121: Fundament°ls of Phy²ics I September 25, 2006

2. D. Roberts PHYS 121 University of Maryland Gravity as a Force Question: Does gravity affect all objects the same? –How do you know? –If not the same, what does it depend on? –(book, paper) Experimentally, it has been observed that all objects experience the same acceleration due to gravity near the surface of the earth, when gravity is the only force involved (i.e. neglecting air resistance) –Galileo dropped objects of different mass from the Leaning Tower of Pisa to show this –Test… The acceleration is: –g = 9.8 m/s 2 –Note: units of acceleration (not force) –If we call “up” positive, then the acceleration is: a  g

3. D. Roberts PHYS 121 University of Maryland Gravity as Force g = 9.8 m/s 2 Gravitational Force –F grav = -mg (from N2, F = ma with a = -g ) –Be careful of sign! g is a positive number! –Value of g would change if you weren’t on surface of earth (on the moon, for example) –Value of mass doesn’t change Weight is magnitude (absolute value) of grav force, mg –Unit of weight is Newton (just like force)

4. D. Roberts PHYS 121 University of Maryland Some Action-Reaction Pairs – is the normal force, the force the table exerts on the TV – is always perpendicular to the surface – is the reaction – the TV on the table – normal is math term meaning perpendicular

5. D. Roberts PHYS 121 University of Maryland More Action-Reaction pairs – is the force the Earth exerts on the object – is the force the object exerts on the earth –

6. D. Roberts PHYS 121 University of Maryland Example Problem Bob pushes two crates across a frozen river. The crates slide without friction (Bob is wearing spiked shoes so he can get traction on the ice). The masses of the crates are 150 kg (A) and 50 kg (B). Bob’s hands exert a 100 N force on block A. How much force does block A exert on block B? A B

7. ? If you drop an object in the absence of air resistance, it accelerates downward at 9.8 m/s 2. If instead you throw it downward, its downward acceleration after release is: 12345 1.Less than 9.8 m/s 2 2.9.8 m/s 2 3.More than 9.8 m/s 2

8. ? You are throwing a ball straight up in the air. At the highest point, the ball’s: 12345 1.Velocity and acceleration are zero 2.Velocity is non-zero, but its acceleration is zero 3.Acceleration is non-zero, but its velocity is zero 4.Velocity and acceleration are both non-zero

9. ? A person standing at the edge of a cliff throws a ball straight up and another ball straight down at the same initial speed. Neglecting air resistance, the ball to hit the ground below the cliff with the greater speed is the one initially thrown: 12345 1.Upward 2.Downward 3.Neither, they both hit at the same speed