Newton’s third law Conceptual Questions
1. Does a baseball bat slow down when it hits a ball? Draw a diagram and explain.
2. Why does a rope climber pull downward on a rope to move upward? Draw a diagram and explain
3. You push a heavy car by hand. The car, in turn, pushes back with an opposite but equal force on you. Doesn’t this
mean the forces cancel one another, making acceleration impossible? Draw a diagram and explain
4. Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is
released. How fast does the heavier cart roll compared to the lighter cart?
5. You find yourself in the middle of a frozen lake with a surface so slippery that you cannot walk. However, you happen
to have several rocks in your pocket. The ice is extremely hard. It cannot be chipped, and the rocks slip on it just as much
as your feet do. Can you think of a way to get to shore? Use pictures, forces and Newton’s Laws to explain your
reasoning.
6. How do you paddle a canoe in a forward direction? Explain. Your explanation should include pictures showing forces
on the water and forces on the paddle.
Newton’s Third Law
1. Block A is pushed across a horizontal surface at a constant speed by a hand that exerts
force F hand on A. The surface has friction.
(a) Draw two free-body diagrams, one for the hand and the other for the block. On these
diagrams:
 Show only the horizontal forces
 Label force vectors using the form F on___ by___
 Connect action/reaction pairs with dotted lines
 On the hand diagram show only F on hand by A. Don’t include F on hand by body
 Make sure vector lengths correctly portray the relative magnitudes of the forces
(b) Rank in order, from largest to smallest, the magnitudes of all of the horizontal forces you showed in part a.
Order:
Explanation:
c. Repeat both part a and b for the case that the block is speeding up
Order:
Explanation:
2. A second block B is placed in front of Block A. B is more massive than A: mB > mA. The blocks
are speeding up.
(a) Consider a frictionless surface. Draw separate free-body diagrams for A, B and the hand.
Show only the horizontal forces. Label forces in the form F ___on____ Use dashed lines to connect
action/reaction pairs.
b. By applying the second law to each block and the third law to each action reaction pair, rank in order all the horizontal
forces from largest to smallest.
Order:
Explanation
3. Blocks A and B are held on the palm of your outstretched hand as you lift them straight up at constant speed. Assume
mB > mA and that mHand = 0
(a) Draw separate free-body diagrams for A, B, and your hand.
 Show all vertical forces, including the gravitational forces on the blocks.
 Make sure vector lengths indicate the relative sizes of all the forces
 Label the forces in the form F ___on_____
 Connect action/reaction pairs with dashed lines
Rank in order, from largest to smallest, all of the vertical forces. Explain your reasoning
Order:
Explanation:
The horse and the Cart Problem
A horse is harnessed to a cart. If the horse tries to pull the cart, the horse
must exert a force on the cart; the horse must exert a force on the cart. By
Newton’s third law the cart must then exert an equal and opposite force on
the horse. Newton’s second law tells us that acceleration is equal to the net
force divided by the mass of the system. (F = ma, so a = F/m) Since the two forces are equal and opposite, they must add
to zero, so Newton’s second law tells us that the acceleration of the system must be zero. If it doesn’t accelerate, and it
started at rest, it must remain at rest (by the definition of acceleration), and therefore no matter how hard the horse
pulls, it can never move the cart.
List all the physical errors and mistakes in the above paragraph and explain why they are wrong. Show a free body
diagram of the horse and cart, identifying all relevant forces, and then write a short paragraph describing this situation
correctly