1
HW 1 FORCES
Answer Section
1.
The force exerted on the car by the brakes causes it to slow down, but your body continues to move forward with the same
velocity (due to inertia) until the seat belt exerts a force on it to decrease its speed.
The thrust form the engines accelerates the plane. For your body to have the same acceleration a force must act on it; this is
the force you feel as the seat pushes on your back.
When the magnitude of the force exerted on the girl by the rope equals the magnitude of her weight, the net force acting on
her is zero. As a result she moves with constant velocity.
As you brake harder you car has a greater acceleration. The greater the acceleration of the car the greater the force required
to give the flat of strawberries the same acceleration. When the required force exceeds the minimum force of static friction
the strawberries begin to slide.
a.
w = mg = (45.0 kg)(10.0 m/s2) = 450. N
b.
w = mg = (45.0 kg)(25.0 m/s2) = 1130 N
a) they would be heavier on the Sun
b) W = mg = 6.30x104 N
F = ma = 45000 N
Because of the large mass, the elephant would have difficulty in zigzagging - in changing its motion.
W = mg = 400 N
10.Fne t = mg – Fdrag = 600 N
11.F upward = 1000 N
m = 50 kg
W = mg = 1300 N
13. It doubles.
14. 5 N downward; 10 N downward;
zero
2.
3.
4.
5.
6.
7.
8.
9.
12.
HW2 - FORCES
Answer Section
1.
2.
3.
10.
4. 2,000 N
5. 6.0 N
6. 3.0 m/s2
0.17 N
18 N
57 N
7. 1.0 m
8. 7.5 m/s2
9. 4.0 m/s2
You, the coin, and the air are all moving horizontally at the same speed. When you flip a coin into the air, it will continue
moving horizontally at that speed (Newton's first law).
11. 30.0 g = 0.0300 kg
8.00 cm 0.0800 m
Next, find the acceleration of the arrow before finding the force.
a = (vf2 – vi2)/2d = - 1720 m/s2
The negative sign before the answer implies that the apple was causing the arrow to slow down. Now solve for the force exerted
by the apple.
F = ma = (0.0300 kg)(—1720 m/s2) = —51.6 N
This is the force that the apple exerts on the arrow. It is negative because its direction is opposite to the arrow’s direction of
motion.
12. In order to find the force of sliding friction, you need to know the normal force, or the force the ground exerts upward on Brian.
On a horizontal surface, this normal force is equivalent to the object’s weight, which in this case is 420 N
Solve:
Ffr = Fn = (0.040)(420 N) = 17 N
13. W = mg = 820 N
Fn = 820 N
14. Fnet = ma
Fnet = Fapp - Ffr
Ffr =
k Fn
= k mg
k =
 = Ffr/Fn = 0.39
Ffr = Fapp - Fnet = Fapp – ma = 1.0 x 10 N
Ffr /mg
k =
0.20
15. a.  = Ffr / Fn = Ffr / mg = 0.20
b. higher
16.
17.
18.
19.
20.
 Ffr /Fn = 6.0 N/30. N = 0.20
They keep the body and head moving together when the car is suddenly accelerated forward.
Surface irregularities; opposes motion.
Twice the force acts on twice the mass.
a = Fnet /m = (100N-20N)/(25kg) = 3.2 m/s2
d = 1/2 a t2 = 40 m
2
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
7.5 m/s
2500 N
2 m/s
2.0 m/s2
0.05
0.062
C
D
C
A
E
I.
e
II.
2.5 103 m/s2
A
36. E
37. m = 0.05 kg
vi = 1.15 m/s
vf = 0 m/s
d = 0.840 m
F net = Ffr = Fn = mg = 0.5 
Fnet = ma = m(vf2 - vi2 )/2d = 0.5 
m(vf2 - vi2 )/2d = 0.5   = 0.157
38. Ffr = Fn = mg = (0.060)(40.0 kg)(10.0 m/s2) = 24 N