Newton's First and Second Laws

Question 1

According to Newton’s First Law what is needed to make an object accelerate?

Answer 1

A non-zero resultant force acting on the object.

Question 2

What is the equation that best describes Newton’s Second Law?

Answer 2

F = m x a or force = mass x acceleration

Question 3

A golf cart is reversing over flat ground at a constant speed. The driving force produced by the motor is 120 N. What is the total magnitude of the resistive forces acting on the cart?

Answer 3

120 N
The cart is travelling at a constant speed, so you know the resultant force is zero. The driving force is 120 N, so the resistive force must also be 120 N

Question 4

Martha throws two stones with the same force. One stone has a mass of 30g and the other a mass of 60 g. Explain why the 30g stone will have the greater initial acceleration.

Answer 4

Acceleration is inversely proportional to mass, for a fixed resultant force. Martha throws the stones with the same force, so the lighter stone will have a greater acceleration.

Question 5

Explain why it can be dangerous for people to experience large decelerations

Answer 5

Large decelerations require large forces because F = m x a, and large forces can cause serious injuries.

Question 6

A car travelling at 20 m/s hit a lamp post and stopped quickly. Estimate the magnitude of the resultant force on the car.

Answer 6

Around 20,000 N
The car comes to a stop in approximately 1 s.
a = (v-u) / t = (0-20) / 1 = -20m/s^2
Mass of a car is approximately 1000 kg
F = m x a = 1000 x -20 = -20000 N
So, the magnitude of the resultant force on the car is around 20000N