Solving Word Problems AP - Kinematics

AP Physics — Word Problems, Solving With The GUESS Method

Ever See an Object Roll UP Hill?

Ex 1

A car produces a 92 m skid mark with a deceleration of 7.00 m/s². What speed was the car travelling just before it applied the brakes?

Given

V_i = ? a = −7.00 m/s² V_f = 0 Δd = 92 m

↓

V_f² = V_i² + 2aΔd

0 = V_i² + 2(−7.00 m/s²)(92 m)

V_i² = 1288

V_i = 36 m/s

Ex 2

A car accelerates uniformly from rest at a rate of +10 m/s². How far will the car travel in 4 seconds?

Step 2 — Turn all your information into letters

Given

V_i = 0 (starts from rest) a = +10 m/s² t = 4 sec d = ?

↓

Step 3 — Substitute and Solve

Δd = V_it + ½aΔt²

= 0 m/s(4 sec) + ½(10 m/s²)(4 sec)²

Δd = 0 + 5(16)

Δd = 80 m

Ex 3

If a car traveling 20 mph can stop in about 40 ft, if the same car was traveling twice as fast, how far would it take to stop?

↓

V_f² = V_i² + 2aΔd

0 = V_i² + 2aΔd → −V_i² = 2aΔd

Doubling V_i quadruples Δd

4× further — 160 ft