Newton's Laws (Concept Test)

4.1a Newton's First Law I
A book is lying at rest on a table. The book will remain there at rest because…

• there is a net force but the book has too much inertia

• there are no forces acting on it at all

• it does move, but too slowly to be seen

• there is no net force on the book

• there is a net force, but the book is too heavy to move

Answer explanation

There are forces acting on the book, but the only forces acting are in the y-direction. Gravity acts downward, but the table exerts an upward force that is equally strong, so the two forces cancel, leaving no net force.

4.1b Newton's First Law II
A hockey puck slides on ice at constant velocity. What is the net force acting on the puck?

• more than its weight

• equal to its weight

• less than its weight but more than zero

• depends on the speed of the puck

• zero

Answer explanation

The puck is moving at a constant velocity, and therefore it is not accelerating. Thus, there must be no net force acting on the puck.

4.1c Newton's First Law III
You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why?

• a net force acted on it

• no net force acted on it

• it remained at rest

• it did not move, but only seemed to

• gravity briefly stopped acting on it

Answer explanation

The book was initially moving forward (since it was on a moving bus). When the bus stopped, the book continued moving forward, which was its initial state of motion, and therefore it slid forward off the seat.

4.1d Newton's First Law IV
You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. You conclude that…

• the force pushing the stone forward finally stopped pushing on it

• no net force acted on the stone

• a net force acted on it all along

• the stone simply ran out of steam

• the stone has a natural tendency to be at rest

Answer explanation

After the stone was kicked, no force was pushing it along! However, there must have been some force acting on the stone to slow it down and stop it. This would be friction!!

4.2a Cart on Track I
Consider a cart on a horizontal frictionless table. Once the cart has been given a push and released, what will happen to the cart?

• slowly come to a stop

• continue with constant acceleration

• continue with decreasing acceleration

• continue with constant velocity

• immediately come to a stop

Answer explanation

After the cart is released, there is no longer a force in the x-direction. This does not mean that the cart stops moving!! It simply means that the cart will continue moving with the same velocity it had at the moment of release. The initial push got the cart moving, but that force is not needed to keep the cart in motion.

4.2b Cart on Track II
We just decided that the cart continues with constant velocity. What would have to be done in order to have the cart continue with constant acceleration?

• push the cart harder before release

• push the cart longer before release

• push the cart continuously

• change the mass of the cart

• it is impossible to do that

Answer explanation

In order to achieve a non-zero acceleration, it is necessary to maintain the applied force. The only way to do this would be to continue pushing the cart as it moves down the track. This will lead us to a discussion of Newton's Second Law.

4.3 Truck on Frozen Lake
A very large truck sits on a frozen lake. Assume there is no friction between the tires and the ice. A fly suddenly smashes against the front window. What will happen to the truck?

• it is too heavy, so it just sits there

• it moves backward at const. speed

• it accelerates backward

• it moves forward at const. speed

• it accelerates forward

Answer explanation

When the fly hit the truck, it exerted a force on the truck (only for a fraction of a second). So, in this time period, the truck accelerated (backwards) up to some speed. After the fly was squashed, it no longer exerted a force, and the truck simply continued moving at constant speed.

4.4a Off to the Races I
From rest, we step on the gas of our Ferrari, providing a force F for 4 secs, speeding it up to a final speed v. If the applied force were only 1/2 F, how long would it have to be applied to reach the same final speed?

• 16 s

• 8 s

• 4 s

• 2 s

• 1 s

Answer explanation

In the first case, the acceleration acts over time T = 4 s to give velocity v = at. In the second case, the force is half, therefore the acceleration is also half, so to achieve the same final speed, the time must be doubled.

4.4b Off to the Races II
From rest, we step on the gas of our Ferrari, providing a force F for 4 secs. During this time, the car moves 50 m. If the same force would be applied for 8 secs, how much would the car have traveled during this time?

• 250 m

• 200 m

• 150 m

• 100 m

• 50 m

Answer explanation

In the first case, the acceleration acts over time t = 4 s, to give a distance of x = ¹/₂ at² (why is there no v₀t term?). In the 2nd case, the time is doubled, so the distance is quadrupled because it goes as the square of the time.

4.4c Off to the Races III
We step on the brakes of our Ferrari, providing a force F for 4 secs. During this time, the car moves 25 m, but does not stop. If the same force would be applied for 8 secs, how far would the car have traveled during this time?

• 100 m

• 50 m < x < 100 m

• 50 m

• 25 m < x < 50 m

• 25 m

Answer explanation

In the first 4 secs, the car has still moved 25 m. However, since the car is slowing down, in the next 4 secs, it must cover less distance. Therefore, the total distance must be more than 25 m but less than 50 m.