You are driving in your car with a sack of groceries in the seat ...

[fbq8i]

You are driving in your car with a sack of groceries in the seat next to you. You see a light change and you slow down and the sack remains on the seat. Suddenly the car in front of you slams on the brakes, and you are forced to brake harder.
 
  The groceries slide off the seat. Explain what happened.

Question 2

A 330-g air track cart on a horizontal air track is attached to a string that goes over a pulley with a moment of inertia of 5.00 x 10-6 kgm2 and a radius of 1.35 cm. A 255-g mass hangs from the other end of the string and is released.
 
  What is the acceleration of the cart? A) 9.72 m/s2
  B) 7.14 m/s2
  C) 6.53 m/s2
  D) 4.74 m/s2
  E) 4.08 m/s2

[skipfourms123]

Answer to Question 1

At the lower deceleration the frictional force required to decelerate the sack of groceries was below the maximum static friction between the sack and the seat, but at the higher deceleration the frictional force was not sufficient, so the sack slid.

Answer to Question 2

E

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TYSM

[xiaomengxian]

Thanks for the timely response, appreciate it