Question

Give the direction of the net force acting on each of the following objects. If the net force is zero, state "zero."

Part A.

A car accelerating northward from a stoplight.

northward

southward

westward

eastward

zero

Part B

A car traveling southward and slowing down.

eastward

westward

northward

southward

zero

Part C

A car traveling westward with constant speed.

southward

eastward

northward

westward

zero

Part D

A skydiver parachuting downward with constant speed.

downward

upward

zero

Part E

A baseball during its flight from pitcher to catcher (ignoring air resistance).

upward

downward

zero

Answer 1

From Newton's 2nd law:

F_net = m*a

So direction of force will be in same direction as the direction of acceleration.

Part A.

Car is accelerating towards North, So

Direction of Force = Northward

Part B.

Car is traveling is south but slowing down, So acceleration will be in opposite direction of motion, which means

direction of acceleration = North

Direction of force = Northwards

Part C.

A car is traveling north with constant speed.

Since a = dV/dt

V = constant, So dV/dt = a = 0

F_net = m*a = m*0 = 0

Force is zero,

Part D,

Again speed is constant, So acceleration is zero, which means

Force is zero.

Part E,

In this case since we are ignoring air resistance, So there is only acceleration in vertical direction due to gravity, So

acceleration = directed downwards

So direction of force = downwards

Let me know if you've any query.