The acceleration of an object is always in the same direction as the net force. In using the formula for Newton's second law, it is helpful to realize that the units N/kg and m/s2 are equivalent. The Math Skills box reinforces this relationship.
Note that Newton's second law also applies when a net force acts in the direction opposite to the object's motion. In this case, the force produces a deceleration that reduces the speed. This is the principle used by automobile seat belts. In a collision, the seat belt applies a force that opposes a passenger's forward motion. This force decelerates the passenger in order to prevent serious injury.
Math Skills
Newton's Second Law
An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car's acceleration?
Read and Understand
What information are you given?
Plan and Solve
What unknown are you trying to calculate?
What formula contains the given quantities and the unknown?
Replace each variable with its known value and solve.
Look Back and Check
Is your answer reasonable?
Powerful sports cars can accelerate at 6 m/s2 or more. Thus, a smaller acceleration of 4 m/s2 seems reasonable.
Math Practice
A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?
What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?
An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.)
A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair? (Hint: Solve Newton's second law for mass.)
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