advertisement

Calculations Practice Acceleration vf - vi a t Acceleration the rate of change of velocity change in speed or direction a v f vi t a: vf: v i: t: acceleration final velocity initial velocity time Calculations You are traveling in a car that is moving at a velocity of 20 m/s. Suddenly, a car 10 meters in front of you slams on it’s brakes. At that moment, you also slam on your breaks and slow to 5 m/s. Calculate the acceleration if it took 2 seconds to slow your car down. GIVEN: vi = 20 m/s t=2s vf = 5 m/s vf - vi a=? a t WORK: a = (vf - vi) ÷ t a = (5m/s - 20m/s) ÷ (2s) a = -15 m/s ÷ 2 s a = -7.5 m/s2 Newton’s Second Law F a m F = ma F a m F: force (N) m: mass (kg) a: accel (m/s2) 1 N = 1 kg ·m/s2 Calculations A dock worker needs to stop a box of goods that is rolling across a manual conveyer belt. The box has a mass of 8.35kg and is accelerating at .75m/s2. How much force will he need to apply to the box in order to stop it?? GIVEN: F=? m = 8.35 kg a = .75 m/s2 F m a WORK: F = ma F = (8.35 kg)(.75 m/s2) F = 6.26 N Gravity Weight the force of gravity on an object W = mg MASS always the same (kg) W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) WEIGHT depends on gravity (N) Calculations Ms. Stayner’s morbidly obese aunt Flossie weighs 2695 N. What is her mass? GIVEN: F(W) = 2695 N m=? a(g) = 9.8 m/s2 F m a WORK: m=F÷a m = (2695 N)÷(9.8 m/s2) m = 275 kg Momentum Momentum quantity of motion • Use the unit kg·m/s p m v p: m: v: p = mv momentum (kg·m/s) mass (kg) velocity (m/s) Momentum A cannon fires a 40.5kg shell toward a target and the shell moves with a velocity of 120m/s. Calculate the shell's momentum. GIVEN: p=? m = 40.5 kg v = 120 m/s p m v WORK: p = mv p = (40.5 kg)(120 m/s) p = 4860 kg·m/s