Question

answer the problems using GRESA method:

1. A 53kg skateboarder pushes on the ground with -351N of force to set himself moving. How fast will the skateboarder accelerate?

2. A responsible driver, driving at a rate of 15m/s, sees the traffic light red. She presses on the brakes and after 3.20s, comes to a stop. The mass is 5400 g. How much force was exerted on the car?

Answer 1

1) Given:

$m=53\ kg, F=351\ N.$

Required:

Acceleration of the skateboarder.

Equation:

$F=ma.$

Solution:

$a=\dfrac{F}{m}=\dfrac{351\ N}{53\ kg}=6.62\ \dfrac{m}{s^2}.$

Answer:

$a=6.62\ \dfrac{m}{s^2}.$

2) Given:

$v_i=3.2\ \dfrac{m}{s}, v_f=0, t=3.2\ s, m=5.4\ kg.$

Required:

Force exerted on the car.

Equation:

$F\Delta t=m(v_f-v_i).$

Solution:

$$F=\dfrac{m(v_f-v_i)}{\Delta t},$$ $$F=\dfrac{5.4\ kg\cdot(0-15\ \dfrac{m}{s})}{3.2\ s}=-25.31\ N.$$

Answer:

$F=-25.31\ N.$